A FEW of the earlier physicists have treated the subject of respiration. But in regard to the purpose which it subserves in the animal organism, some of them have given no explanation whatever, and others, although they have discussed it, have been wrong in their statements and have lacked empirical knowledge of the facts. Furthermore, they declare that all animals respire. This, however, is untrue. It will be necessary, therefore to return to these points, in order that we be not thought to make unfounded charges against writers who are no longer living. It is plain that all animals with lungs breathe. But amongst these, the animals that have a spongy, anaemic lung need respiration less than the others. Consequently they can remain, owing to their physical strength, a considerable time under water. All oviparous animals have a spongy lung, as is the case in frogs. Again, water-tortoises and land-tortoises can remain a long time under water. In these animals the lung has little heat, because it has little blood. Consequently when it has been once inflated, it effects refrigeration by its motion and enables the animal to continue a long time under water without breathing. Even in these cases, however, when the animal is forced to hold its breath too long, it is suffocated. For none of these animals can inhale water, as fishes do. All animals, on the other hand, whose lung is full of blood, have greater need of respiration, because of their greater heat. As to the animals that have no lung at all, they have no respiration at all.
Democritus of Abdera and certain other writers on the subject of respiration have not spoken definitely about the animals last named, but they appear to assert that all animals breathe. Anaxagoras, however, and Diogenes make the statement that all animals respire, and they say that fishes and oysters are endowed with a sort of respiration. Anaxagoras declares that when fishes discharge water through their gills, they inhale the air that is developed in the mouth (for a vacuum does not exist), and so respire. Diogenes, on the other hand, says that when fishes discharge water through their gills, they inhale air by the action of the vacuum formed in the mouth, out of the water which surrounds the mouth, on the theory that water contains air. These views, however, are untenable. For, in the first place, they leave out of account half of the truth, because their entire statement refers only to one aspect of the case. For by respiration one understands partly inspiration and partly expiration, and they have nothing to say in explanation of how the latter takes place in lungless animals. And it is impossible for them to give any explanation. For when inspiration takes place, expiration must also follow by the same channel as that employed in inspiration, and these two things must succeed each other in constant alternation. The consequence is that exhalation must take place at the same moment that water is being received into the mouth, and in that case the one must impede the other by meeting. Secondly, if they exhale by the mouth or gills at the moment when they discharge water, the consequence will be that inspiration and expiration will be simultaneous, and, according to the above assertion, this is the moment in which animals inspire. But simultaneous inspiration and expiration is an impossibility. Consequently, if it is true that respiration involves both inspiration and expiration, and if it is further true that aquatic animals are not capable of expiration, it is clear that they are also incapable of respiration.
Again, the assertion that they inhale air from the mouth, or from the water through the mouth, is impossible. For aquatic animals have no windpipe, because they have no lung, but the stomach is immediately adjacent to the mouth, and consequently the stomach would necessarily be the organ of inspiration. But if this were true here, the stomach would have this power in other animals also. As a matter of fact, it does not have this power. Further, if aquatic animals were removed from the water they would then clearly show this capacity to respire; but they do not show it. Furthermore, we observe in those animals that respire and inhale air a certain movement in the organ of inhalation. This is not observable in fishes. They appear to move no organ about the stomach other than the gills, whether they are in the water or are thrown gasping on the dry shore. Again, when any respiring animal dies from suffocation in the water, its breath, as it forcibly leaves the body, is formed into bubbles, as one sees in the case of tortoises or frogs, or other animals of this sort, when they are forcibly drowned. With fishes, however, this is not the case, whatever method one may use, because they contain no inhaled air. According to the explanation of respiration above mentioned, it would be possible also for men to respire when in water. For if fish inhale air from water by means of their mouth, why should not men and other animals do the same thing? They should inhale air from the mouth quite as much as fishes. If the latter have this power, the former should have it also. But as this is not true in the one case, it evidently does not hold good in the other. Furthermore, if fishes respire, why is it that we see them die in the air and gasp as if suffocated? It is not owing to lack of food. The explanation given by Diogenes is foolish. He says that fishes, when in the air, inhale too much air, and this is why they die, whereas in the water they inhale a moderate amount. But this should then be possible for land animals also. In point of fact, no land animal is suffocated by excessive inhalation. Further, if all animals respire, insects must evidently respire also. Many of them, however, seem to live when they are divided, not only when divided into two parts, but into several, as in the case of the centipedes. How or by what organ is it possible for these parts to breathe? The chief cause of the error of these writers was their ignorance of the internal organs, and also the fact that they did not grasp the truth of design in nature. For by asking to what end animals are endowed with respiration, and by making a test of their theory on the organs themselves, as, e.g., on the gills and lungs, they would soon have discovered the real explanation.
Democritus makes the statement, it is true, that respiration produces certain effects in the respiring animal, viz. it prevents the soul from being expelled from the body. He by no means says, however, that nature in creating this function did so with this end in view. For he is, on the whole, like the other physicists and makes no application of any such causality. He maintains that the soul and heat have one and the same nature, viz. they are elemental spherical atoms. Consequently, when these are compressed by the force of the surrounding air, inhalation comes to their assistance. For in the air there is a large number of the atoms which he calls mind and soul. In the act of inhalation, then, and along with the entrance of the air, these atoms also enter, and, by counteracting the pressure, prevent the expulsion of the soul that resides in the animal body. It is for this reason that life and death depend upon inspiration and expiration. For when the surrounding medium by its pressure gains control and the outer air is no longer able to enter and counteract this control, respiration in the animal becomes impossible and death ensues. For by death one means the departure of these psychical atoms from the body due to expulsion by the surrounding medium. The reason, however, why death necessarily comes at all to every animal, and why it does not come at any chance period, but in the course of nature only in old age,—a violent death is contrary to nature,—he has not in the least explained. And yet, because this phenomenon occurs evidently at one period and not at another, it behoved him to explain whether it is due to an external or to an internal cause. Further, he has not a word to say regarding the origin of respiration, whether its cause is external or internal. And yet it is evidently not the external mind that comes to the rescue here, but the principle of respiration and of respiratory movement is due to an internal cause, and we are not to suppose that the force of the surrounding medium is any explanation. It is also absurd to think that the surrounding medium has at once the effect of extinguishing by compression, and on its entrance the opposite effect. The foregoing, in content and manner of statement, conforms closely to the theory of Democritus. If one is to regard as true what was said a while ago, viz. that not all animals respire, then we must regard the Democritean explanation of death as not universally applicable, but only to those cases where animals breathe. But even to these cases it does not well apply, as is evident from facts observed by all of us. For in warm weather, when we are more than usually heated, we have greater need of respiration and we all breathe more rapidly. When, however, our environment is cool and contracts and chills the body, we hold our breath. This is the very moment, however, that the air from without should enter and prevent the soul’s expulsion. In point of fact it is the opposite that takes place. For when excessive heat is accumulated, owing to its not being exhaled, that is the moment we need respiration, and inhalation is necessary to this. The truth is, men breathe rapidly when they are hot, because respiration has a cooling effect, at the very moment when they would be, to use a proverb, ‘adding fire to fire.’
The theory of circular push described in the Timaeus gives no explanation whatever of the way in which heat is maintained in animals other than man, whether the preservation of heat in the various animals is due to the same or different causes. For if the phenomenon of respiration is found in land animals alone, we must explain why they alone breathe. If, however, it occurs in other animals also, but by a different process, assuming that they can all respire, we must find an explanation for the difference in process. Furthermore, the whole manner of explaining the phenomenon is fanciful. For Plato says that by the issuance of hot air from the mouth, the surrounding air is pushed forward and is transmitted through the pores of the flesh, and rests at the point from which the internal hot air issued. These elements thus effect a complementary displacement, owing to the fact that a vacuum is impossible. In the same way the inhaled air in turn, when heated, is discharged and the warm air within, issuing out through the mouth, continues this ‘circular push.’ And so this process, which is inspiration and expiration, goes constantly on. The logical consequence of the theory is that expiration precedes inspiration, whereas the opposite is the fact, as the following proves. The two things are correlated phenomena. Now man’s last act is expiration, consequently inspiration must form the beginning. Further, the end which these processes (I mean inspiration and expiration) subserve in the animal body is not taken into account at all by the philosophers who advocate this theory. They treat them merely as unessential phenomena. We see, however, that they are the master-factors in life and death. For when a breathing animal is unable to respire, at that moment death ensues. Further, it is absurd to suppose that the issue of hot air through the mouth and the entrance of air again by the mouth should be observable by us, whereas the entrance of the breath into the thorax and its discharge should not be observable. It is also strange that respiration should mean the introduction of heat. Observation shows the contrary, for expired air is hot, whereas inspired air is cool. And when the atmosphere is warm animals pant in respiring and they draw their breath frequently, because the entering air does not adequately cool them.
We must also reject the theory that the purpose of respiration is nutrition, which presupposes the feeding of internal heat by means of the breath. According to this view, inspiration is similar to throwing fuel on a fire, and expiration follows when the fire is fed. We again urge the same objections to this theory as we did to the theories enumerated above. The same process, or something analogous to it, should be found in all animals, for they all have vital heat. In the next place, the advocates of this theory should explain how heat is generated out of the breath. The whole view is fanciful. According to our observation generation of heat is due much rather to food. A further consequence of their theory is that food is received and excrement discharged at the same orifice, which is not seen in any other instance.
Empedocles also has a theory of respiration, although he does not explain the purpose of respiration, nor does he say definitely whether all animals are endowed with respiration or not. In treating of respiration through the nostrils, he fancies he is dealing with the main factor in this process. He is here mistaken, for there is respiration through the windpipe, which leads from the chest, as well as respiration through the nostrils, and without a windpipe the nostrils themselves could not respire at all. Animals may even be deprived of respiration through the nostrils and suffer no harm, but if the use of the windpipe is shut off they die. In certain animals, indeed, respiration through the nostrils is employed by nature for the secondary function of smell. Although almost all animals are endowed with the sense of smell, they do not all employ the same organ for this purpose. On this subject, however, we have spoken elsewhere more in detail.
Empedocles asserts that inspiration and expiration take place through particular veins, in which there is blood, although they are not entirely filled with blood, and that these veins are provided with channels that lead into the outer air, channels which are too minute for the admission of crass matter, but large enough for air. Now, the blood is so constituted as to move up and down, and after its downward motion the air streams in and inspiration takes place; on its upward motion expiration into the outer air ensues—a process which resembles what we observe in the Clepsydra:
Thus all things breathe and breathe out air again. Long bloodless tubes the body’s surface reach, And at their close-packed vents are nostrils fixed Pierced through; and so a passage way is cut For air, while yet the blood is hidden held. When yielding blood along these channels ebbs, Then bursts the surging air with tempest’s wave Within. But when the blood rebounds, the air Is then expired again, as one may see A child with smooth bronze water-clock at play. Upon her comely hand she sets the tube, And dips it in the yielding water’s sheen, Of which no drop slips in the vessel’s form. Upon the close-packed vents the air doth press Within, until the maid her hand removes And frees the urgent stream, which entrance makes, Whose even flow drives back the yielding air. So, too, when e’er the waters full free flow Hath filled the deep bronze tube, and maiden hand The passage firm hath blocked, then doth the air, The eager outer air, the vents make fast And hold in its restraint the inner stream Whose waters at the narrow gates complain, Until the maiden lifts her hand. And now Is true the converse of what was before: The air flows in—the water’s equal stream Flows out. Thus also ‘tis with fluent blood That coursing through our limbs now hurries back To inner depths, and straightway air pours in With surging swell. Again the blood returns From its retreat; then forthwith yields the air, Exhaled once more, in nature’s even course.
These are his words on the subject of respiration. As we have already said, animals that visibly respire do so by means of the windpipe as well as by means of the mouth and nostrils. Now, if Empedocles is speaking of respiration in this sense, we must inquire how far his explanation harmonizes with the facts. Apparently the facts contradict his theory. For in inspiration the receptacle is expanded like a brazier’s bellows. Expansion, however, is naturally explained by heat and by blood which takes the place of heat. In expiration, on the other hand, contraction and collapse take place, as in the bellows, excepting that the cases are not quite parallel in this respect, viz. the bellows do not admit and discharge air by the same orifice, whereas in inspiration and expiration the same orifice is used. If, however, he is here referring merely to respiration through the nostrils, he is quite wrong. For respiration is not a function which is peculiar to the nostrils; on the contrary, along the passage near the uvula, at the extreme end of the roof of the mouth, part of the air passes here through the openings of the nostrils and part of it through the mouth, and this applies equally to inspiration and expiration.
It was said above that life and the possession of soul are accompanied by a certain degree of heat. For even the process of concoction, by which food is prepared for animal life, cannot be accomplished without soul and heat; all this is effected by fire. Consequently, such a fundamental process as this must be situated in the primary region of the body and in the primary organ of this region, and here it is that we must look for this elementary nutritive soul. This is the middle region between the orifice for admitting food and that for discharging excrement. In bloodless animals the primary organ has no name, in sanguineous animals it is the heart.
The food out of which animal members are generated is the blood. The blood and blood-vessels must have the same starting-point. For the one, as vessel and receptacle exists for the other. The originating point for these vessels in sanguineous animals is the heart. They do not traverse the heart; they all issue from it and are attached to it, as is evident from dissection. Now, the other functions of the soul cannot be performed independently of the nutritive principle, and the nutritive principle in turn cannot subsist without natural heat. For it is through natural heat that nature has endowed the nutritive principle with warmth. Fire may be destroyed, as we said before, in two ways: by extinction and by exhaustion. Extinction is effected by opposing forces. Consequently even when the fire is massed it may be extinguished by environing cold, and when scattered it is more easily quenched. This extinction by external force applies to animal heat as well as to inanimate fire. For animals die when dismembered by instruments or when congealed by excessive cold. Exhaustion, on the other hand, follows from excessive heat. For if the surrounding heat is great, and the internal supply of fuel is not maintained, the fire ceases to burn, not from extinction by cold, but from exhaustion. Consequently there must be some cooling process, if survival is to be attained; for this comes to the rescue and prevents extinction.
Some animals are aquatic and others have their existence on the dry land. In the case of the very small and bloodless specimens of both classes, the cooling produced by their surroundings, whether air or water, is adequate to protect them against the above-mentioned extinction. For being endowed with little heat they need little protection. Animals of this kind are, consequently, in the rule short-lived, for a slight change on one side or the other destroys the balance. The longer lived insects (which, like all insects, are bloodless) have a fissure just below the middle part in order that cooling may be effected through the membrane, which at this point is very thin. For inasmuch as they have more heat they have more need of cooling. Bees, for example, some of which live as long as seven years, and the other insects that hum, such as wasps, cockchafers, and locusts, belong to this class. They produce this noise by their breath, as if by panting. As the natural breathing within rises and falls, it produces friction against the membrane in the middle region. For insects keep this region in motion just as animals that breathe the outer air maintain motion by their lungs or fishes by their gills. This motion is similar to what would take place if one should suffocate a respiring animal by holding its mouth; for then this swelling movement would be produced by the lungs. In the latter case, however, such motion is inadequate for cooling, although it is adequate in the case of insects. By means of friction against a membrane they produce a humming noise, as we said, in much the same way as children make a noise through a perforated reed after stretching a thin membrane in it. And it is in this way, too, that the singing locusts produce their song. They possess greater heat than other varieties, and have a fissure in the middle region. In the songless locusts this fissure is lacking. Sanguineous animals endowed with lungs that contain little blood and are spongy, can live a long time without respiration, because the lungs are capable of great expansion, containing as they do little blood or fluid. Consequently, their own peculiar motion is adequate for cooling through a considerable period. Finally, however, it is unable to continue this, and without respiration it suffocates, as we said before. That form of exhaustion which consists in destruction through lack of cooling is called suffocation, and animals that die in this way are said to be suffocated. We have already remarked that insects do not respire. One can observe this plainly in the case of small insects, such as flies and bees, for they can swim a long time in a liquid, provided it is not too hot or too cold. And yet animals which have less strength require more frequent respiration. They are destroyed, however, and are suffocated, as we say, when the belly is filled with water and the heat of the middle region quenched. From this we can understand how it is that such insects get up again after being covered for some time with warm ashes. We also observe that bloodless aquatic animals live in the air longer than do sanguineous animals that take in sea-water, as the fishes. For the former have little heat, and consequently the air is adequate to cool them for a considerable time, as is the case with crustacea and polyps. And yet it is finally inadequate for life, because they possess little heat; for even fishes are often dug out of the earth and found to be living, although motionless. Animals that are endowed either with no lungs at all or with lungs containing little blood, need the least frequent respiration.
In regard to bloodless animals we have said that some of them owe their survival to the surrounding air, others to the water. In the case of animals that have blood and a heart, all those that are provided with lungs take in air and effect cooling by means of inspiration and expiration. Now, viviparous animals are provided with lungs, not those, however, that bear their living young outside of themselves (for the cartilaginous fishes are viviparous, but not within their own bodies), and amongst oviparous animals those that have wings are provided with lungs, as, e.g. birds, and further, such animals as have scales, like the tortoises, lizards, and snakes. Viviparous animals have a lung well filled with blood, whereas most of the oviparous animals have spongy lungs. Therefore, as we said before, the latter need less frequent respiration. All of them, however, do breathe, even those that live and maintain their existence in the water, such as hydras, frogs, crocodiles, fresh-water tortoises, tortoises of the sea and land, and seals. For all of these animals, and others similar to them, bear their young on the land, and sleep either on land or in the water with their mouths above the surface for respiration. Animals, on the other hand, that have gills, are cooled by taking in water. To this class belong the cartilaginous fishes and other apodous animals, including all fishes. For their organs of locomotion are after the analogy of wings ([rather than feet]). Amongst animals that have feet, only one, so far as has been observed, has gills, viz. the tadpole, as we call it. But no case has ever been seen of the possession of lungs and gills together. The reason is that the lungs are designed for cooling by the admission of air (even the name πνεύμων, ‘lungs,’ seems to have been derived from their reception of πνεῦμα, ‘air ‘), and gills are designed for cooling by the admission of water. But only one organ is used for one purpose, and one method of cooling is adequate for each animal. And so, since we know that nature makes nothing in vain, and since one of these two organs would be useless, some animals are provided with gills and others with lungs, but no animal with both.
Since every animal needs food for its subsistence and cooling for its persistence, nature employs for these two purposes one organ. And as in some animals the tongue is employed for the double purpose of tasting and communicating thought, so in those which are provided with lungs, the mouth serves for the mastication of food as well as for inspiration and expiration of air. In those, on the other hand, that have no lungs and do not respire, the mouth serves for the mastication of food, but gills are provided for cooling where cooling is needed. In what way the functioning of the aforesaid organs effects cooling, we shall explain later. In order not to hinder the admission of food, a similar method is employed by respiring animals and by those that take in water. For in the former case they avoid respiring and swallowing their food at the same instant, otherwise they would choke by admitting liquid or solid food into the lungs through the windpipe. For the windpipe lies in front of the oesophagus, through which food finds its way into the stomach. In the sanguineous quadrupeds the windpipe is provided with a sort of lid called the epiglottis. In birds and oviparous quadrupeds, on the contrary, there is no such lid, but they attain the same end by contracting the windpipe. When food is being swallowed, the ovipara contract the windpipe, whereas the vivipara close the epiglottis. And after the food has passed, in the one case the windpipe is expanded, and in the other the epiglottis is opened, and air is admitted for the purpose of cooling. In regard to those animals that are provided with gills, they discharge the water through these and then admit food through the mouth. They have no windpipe, so that they can suffer no harm by the wrong discharge of water into it, but only by the entrance of water into the stomach. For this reason, the discharge of water and the swallowing of food is done rapidly, and their teeth are sharp, and in almost all instances are serrated, for they cannot chew their food.
Regarding the cetaceous aquatic animals, such as dolphins, whales, and such others as have what is known as a spout-organ, one might feel some doubt, yet even these conform to our theory. For they are apodous, and although they have lungs they take in sea-water. The ground for this apparent exception is given in the foregoing explanation; for the end to which they take in water is not cooling. This is produced in their case by means of respiration, for they have lungs. Consequently, they sleep with their mouths above the water’s surface, and dolphins, it is certain, snore. Again, when they are caught in nets, they soon suffocate from lack of respiration. It is in order to breathe, then, that we observe them lying on the sea’s surface. Since, however, they are forced to take their food in the water, they must on swallowing discharge the water, and for this reason they are all provided with a spout-organ. When they have taken in water they discharge it through this spout-organ, just as fishes do through their gills. A proof of this fact is the position of the spout-organ. It does not lead to any of the blood-filled organs, but is situated in front of the brain and discharges the water here. For the same reason the molluscs and crustaceans admit and discharge water. I mean the sea-crayfish and crabs, as we call them. They make no use of it for cooling, for they are endowed with only a small amount of heat and are in every case bloodless, so that they are kept cool enough by the surrounding water. But it is discharged on account of their food, viz. in order that the water may not enter at the moment of swallowing. The crustaceans, such as the sea-crayfish and crabs, discharge the water through the plaited folds along their shaggy covering; the purple fish and polyps discharge it through the hollow passage above the head. These questions have been treated with greater detail in the History of Animals. Concerning the phenomenon of the admission and discharge of water, we have said that it is due, in certain cases, to the need of cooling, and in others to the fact that aquatic animals are obliged to swallow their food in the water.
We must next describe the method by which cooling is effected in respiring animals and in those provided with gills. We have already said that animals which have lungs respire. As to the reason why some animals have this organ and why those that have it need respiration, it is because the higher order of animals are endowed with greater heat. At the same time it must be that they are endowed with a higher order of soul. For such beings are of a higher order than plants. Consequently, animals whose lungs are more abundantly supplied with blood and heat are of greater bodily dimensions than others; and the animal that is supplied with the purest and most abundant blood, i.e. man, is the most erect of all animals, and his upper structure points to the upper region of the universe—true of him alone—because he has lungs constituted as we have described. The essential character both of man and of other animals must, therefore, be ascribed as much to this as to any other organ. This, then, is the purpose of the lungs. One must suppose that the material conditions and moving cause have constructed these animals in this way, as they have also operated to produce other animals with a different constitution. For some are composed chiefly of earth, like plants, others chiefly of water, like aquatic animals. And amongst the winged and terrestrial animals, the one class is composed chiefly of air and the other of fire. And they severally have their place in regions akin to their own natures.
Empedocles was wrong in saying that the aquatic animals are warmest and contain most fire, and, being defective in cold and fluid, they seek refuge from constitutional excess of heat in a medium to which their nature is opposed. For water is cooler than air. It is, however, altogether unintelligible how animals born on dry land can change their place of abode to water. For they are, in almost all cases, apodous. And yet, when speaking of their primary constitution, he asserts they are born on the dry land and later leave this and migrate to the water. Again, our observation shows that they are not warmer than land animals; for some of them are absolutely bloodless, while others are almost so. But what kind of animals we should call warm and what kind cold, is a subject itself that requires investigation. Regarding the explanation given by Empedocles, his contention is, in a certain sense, correct, although what he says is not entirely true. For it is true that regions and seasons which exhibit characteristics opposed to abnormal conditions in animals tend to preserve them, and yet their normal nature is best preserved in a place of abode similar to their own constitution. For the matter out of which animals are severally constituted must not be confounded with the varying states and conditions of this matter. I mean, e.g. if a thing were formed of wax or ice, its preservation would not be secured by placing it in a hot environment. For, owing to the opposed nature of its surroundings, it would be quickly destroyed, for heat melts that which consists of the contrary nature. Again, if a thing were composed of salt or nitre, nature would not carry it and set it down in a wet environment, for water dissolves substances of a warm, dry constitution. If, therefore, the fluid and solid constitute the matter out of which all bodies are formed, it is reasonable to suppose that fluid and cold structures will be found in a moist environment; solid structures, on the other hand, in a solid environment. Consequently, trees do not grow in water, but in the earth; although, according to this same theory of Empedocles, they should migrate to the water, because of their being predominantly dry, or, to use his expression, “predominantly fiery.” This migration would be to water not because it is cold, but because it is fluid. The natural constitution of matter, therefore, conforms to the environment in which it is found—the moist, e.g. is found in water, the warm in the air. Acquired conditions, however, are better regulated through an opposite environment, excessive heat through cold surroundings, and excessive cold through warm surroundings. For the environment reduces the excess in these conditions and brings them to an equable mean. This reduction is to be sought in an environment adapted to the particular constitution of the thing and in the variations of ordinary climate. For acquired conditions may be opposed to the place of abode, but this is impossible in the case of the original constitution.
Touching the theory of Empedocles that animals are divided into aquatic and land animals on the basis of differences in natural heat, and touching the explanation of the phenomenon that the one class has lungs and the other not, let the foregoing discussion suffice.
The reason why animals with lungs can take in air and respire, especially such as have lungs well filled with blood, is to be found in the fact that the lungs are porous and filled with tubes. The lungs contain more blood than any other organ in what we call the viscera. Animals whose lungs are abundantly supplied with blood need rapid refrigeration, because of the delicate balancing of the natural heat, and because the cooling process must penetrate through the entire interior, owing to the great supply of blood and heat. Air can easily meet both these demands. For owing to its rarity, it rapidly penetrates everywhere, and effects cooling. This is not true of water. It is also plain from this why it is that animals which have lungs well filled with blood breathe best. It is due to the fact that the warmer the nature the greater is the need of cooling, and at the same time that the air fills the lungs, it passes readily to the original source of animal heat in the heart.
The way in which a passage is made between heart and lungs must be studied through dissection, and in the History of Animals. Animal nature, in general, needs cooling, because of the vital fire in the heart. This is accomplished by means of respiration, excepting in those cases where animals are provided with a heart only but no lungs. When they have a heart but no lungs, as is the case in fishes, whose natural abode is water, cooling is attained by water through the use of the gills. In regard to the relative positions of heart and gills, one must study them ocularly in dissection and their nicer philosophy in the History of Animals. To give a summary description, however, the case is as follows: One might suppose that the position of the heart in land and aquatic animals was different; as a matter of fact, it is the same in both. For the direction in which the animal’s head naturally inclines is the direction in which the heart’s apex is turned. But inasmuch as the heads of land animals do not incline in the same direction as those of aquatic animals, the heart’s apex in the latter case is turned towards the mouth. A sinewy vein-like tube extends from the extremity of the heart to a central point, where all the gills are united. This is the largest of all the tubes, but there are others on each side of the heart which extend to the several extremities of the gills, whereby cooling is produced and transmitted to the heart, the water being constantly piped through the gills. The rapid swelling and falling motion of the thorax in inhaling and exhaling air serves the same purpose in respiring animals that the movement of gills does in fishes. Respiring animals suffocate in a small quantity of air that remains unchanged; for each medium soon becomes hot, and contact with the blood heats them. When, however, the blood becomes hot, the process of cooling is impeded. Also when respiring animals become unable to inflate their lungs, or aquatic animals to move their gills, whether owing to disease or to the weakness of old age, their end must be at hand.
Birth and death are phenomena common to all animals, although there are specific differences in their modes of occurrence. Death is not everywhere the same, although in its varied forms there is a common element. Death ensues from violence or from the ordinary course of nature. Death is violent when due to an external cause, natural when due to internal processes. The latter conforms to the original organic structure, and is not an adventitious condition. In plants this process is called decay; in animals, senility. Death and decay attach to all organisms alike that are complete, and to the incomplete also, but in a different way. Under incomplete, I understand such things as eggs, and seeds of plants which as yet have not taken root. Death is caused in all things by lack of heat; in complete organisms by its failure in that part where the vital principle is lodged. This principle is lodged, as we said above, in the middle region, where the upper and lower parts are conjoined. In plants it is the point at which stem and root unite; in sanguineous animals, it is the heart; in bloodless animals, in an organ analogous to the heart. In some of the bloodless animals we find many vital centres potentially, though not actually. For this reason certain insects, when divided, continue to live, and such sanguineous animals as are not highly organized live a considerable time after the removal of the heart, as is true of tortoises. Tortoises continue to move their feet so long as their shell is not removed, because their organization is of a lower order, resembling in this respect the insects. The vital principle succumbs in its possessor when the heat which is its accompaniment is not reduced by cooling. For otherwise, as we have often remarked, it is consumed by its own agency. When, therefore, the lungs or gills respectively become hardened, or dried up and earthy through lapse of time, it is impossible for these organs to function, to dilate and contract. And finally, when a further demand is made upon them, the fire of life is extinguished. Consequently, death quickly ensues in old age, even on the appearance of trivial ailments. This is due to the fact that there is little heat left in old age, most of it having been exhaled in a long life, and if any extra strain is put upon the lungs, life is speedily quenched. For the fire within, being now but a tiny feeble flame, is extinguished by a slight movement. That is also the reason why death in old age is painless, for death comes to the aged with no element of violence in it, rather the dissolution of the soul occurs quite without their feeling it. Diseases which make the lungs hard, whether by tubercles or deposits or by excessive morbid heat, as in fevers, produce an acceleration of the breathing, because the lungs are incapable of full expansion and contraction. And finally, when motion is no longer possible, men exhale their breath and die.
Birth, then, is the original suffusion of the nutritive soul with heat, and life is the maintenance of this heat. Youth is commensurate with the growth of the primary organ of cooling, old age with the wasting of the organ, and the prime of life with the middle period between the two. Death and violent destruction mean respectively the exhaustion and extinction of the vital heat (for it is destroyed from both causes); exhaustion is given in the nature of the thing itself, and is caused by lapse of time and by the completion of a normal term of life. In plants this is called decay; in animals, death. Death in old age is due to the exhaustion of the organism that comes from senile inability to effect cooling. We have now explained the meaning of birth, life, and death, and have treated the causes of these phenomena in animals.
From these considerations one may clearly see why it is that respiring animals are suffocated in water, while fishes are suffocated in the air. For in one case cooling is effected by the medium of water; in the other by that of air, and both of them are deprived of this by the change in their place of abode. We have further to explain the movement in gills and lungs respectively,—exhalation and inhalation in the one case, and the admission and discharge of water in the other. We have also to explain the structure of the organ of respiration in what follows.
There are three phenomena regarding the heart, which might be supposed to have the same nature, but are different, viz. palpitation, pulsation, and respiration. Now, palpitation is a compression of heat in the heart, owing to cooling in other parts of the body produced by excretion or waste, such as we see in the disease called palpitation of the heart, as well as in other diseases, and in fear also. In fear the upper regions of the body are cold, and their heat is discharged and collected in the heart, where palpitation is caused, and the heat being thus compressed into a small space, it sometimes happens that animals are suffocated and die from fear and its morbid conditions. The phenomenon of pulsation, however, that occurs in the heart, and which, as we see, is a constant process, is similar to the throbbing in an abscess. In the latter the movement is painful owing to abnormal change in the blood. This process continues to a point where the blood is concocted and converted into pus. The condition is analogous to boiling. For boiling takes place when water is evaporated by heat, and it bubbles up owing to its increase in volume. The development of abscesses is arrested when the pus is not evaporated and the liquid becomes very thick; the process in boiling is arrested when the confining vessel is overflowed. The supply of moisture derived from food and its expansion through heat produces pulsation in the heart,—the expansion extending to the heart’s outer covering. And this is a constant process, for the flow of fluid to the heart, out of which the blood is generated, is constant. It is in the heart that blood is first formed. One can observe this plainly in the growth of an embryo. For before the veins are distinguishable the heart is seen to contain blood. Pulsation, for this reason, is more marked in youth than in old age. For the process of evaporation is stronger in youth. The blood vessels all pulsate, and they do so simultaneously, for they are all connected with the heart and originate in it. The heart, however, is in constant motion. So, too, the blood vessels are in constant motion, and simultaneously with each other, as long as the heart moves. Palpitation, then, is a reaction in the heart due to the compression of heat by the cooling of other parts of the body; pulsation is the evaporation of the moist element as it becomes heated.
Respiration is due to the increase of the heated element, in which the nutritive principle is lodged. As all other bodily elements need maintenance, so does this element of vital heat, and even in a greater degree than the others, for it is the source of maintenance for the other elements. When it is increased, it necessarily expands the organ in which it is found. One must conceive the structure of this organ to resemble a brazier’s bellows. For neither lungs nor heart differ very much from a form such as is illustrated by a bellows. Both are double. The nutritive principle must be situated in the centre of the vital power. The lungs then increase and expand, and, by expanding, the part in which they are lodged must also expand. We see this when we respire. For the thorax is then expanded, because the inherent principle in this part is expanded. Owing to this expansion, as one sees in the bellows, cold air must be introduced from without and, by its cooling effect, the excess of internal heat is lowered. But just as the organ was expanded owing to the increase of heat, so now it necessarily contracts when the heat is diminished, and by contracting, the air which was inhaled is again discharged—air that was cold when admitted, but warm when discharged owing to contact with the heat inherent in the organ, especially in the case of animals whose lungs are well-filled with blood. The air enters through a mass of pipes, canals as it were, with which the lungs are provided, and blood vessels extend alongside each of these pipes, so that the entire lung appears to be filled with blood. The admission of air is termed inspiration, and its discharge, expiration. The process of respiration is continuous, so long as life and this organic motion continue. Life, therefore, is given in the processes of inspiration and expiration. The movement of the gills in fishes is produced in the same way. For by the expansion of the blood’s heat in its course through the members, the gills are lifted and water passes through. When, on the other hand, the heat retreats to the heart through the channels and cooling is effected, the gills are lowered and the water passes out. The expansion of the heart’s heat is constant and its re-admission when cooled is constant. And so in animals provided with lungs, life and death are ultimately conditioned by respiration, and in fishes by the admission of water.
This, then, is a statement of our views of life and death and of almost all the questions germane to them. It is the province not only of the physician, but also of the natural philosopher, up to a certain point, to discuss questions of health and disease. We must not, however, forget how these two classes of men differ and how they regard a subject from different points of view, although experience shows that both professions are, to a certain extent at least, conterminous. For the better educated and more painstaking physicians are conversant with the laws of nature and deem it correct to derive their principles of practice from this source, while the best trained philosophers of nature almost always conclude with a discussion of the principles of medicine.