Herman LeRoy Fairchild

“ The harmony and mutual dependence between all parts of an animal's body are not better shown than in the adaptation of the teeth of vertebrates to the kind of food, and their consequent agreement with the habits, structure, and form of the individual. A single tooth frequently reveals the entire character of its possessor. Indeed, no single organ is of greater zoölogical value. Not only does the student of living forms use the teeth as indicative of character and a means of classification, but their superior durability makes them of the very greatest importance in the study of ancient life. ”

“ Indeed, the complex functions of prehension, mastication, digestion, and circulation are only subdivisions of nutrition which begins in the lowest life as a single act. The present purpose, however, is not to trace the evolution of specialization of the digestive function further than to illustrate its general principles and methods, and present some of its peculiar and interesting features.
The tape-worm has no digestive organs whatever, having no use for them. A robber subsisting on the labors of its victim, it takes food in the same manner as a plant, by absorption from the outside. ”

“ In proportion as digestion and absorption of food become specialized and restricted to certain parts, circulation becomes more important in order to convey that food to the tissues, and carry from the tissues the worn-out material. To maintain the character of the fluid, it must itself undergo constant change, and hence the excretory processes—respiration being the most urgent—which increase the necessity for movement of the fluid. Circulation of the nutritive fluid is the immediate function for upbuilding and repairing the body. ”

“ Organs of Respiration.—As the function of respiration is so simple in principle, being a single physical action, it can be conducted almost anywhere in the body, or wherever the blood can be conveniently exposed to the surrounding medium. The nature of an animal, as regards other less easily modified functions, and its peculiar circumstances, may therefore greatly determine the character of its breathing organs. So it results that these organs, in the various groups of animals, are exceedingly diverse in form, structure, and position. ”

“ We can otherwise gain an idea of the power of Nature's enginery, by observing what the heart actually performs. The quantity of blood in the human body is at least six quarts. In its course it has to traverse many feet of tubing and two sets of capillaries, and, notwithstanding the friction and loss of power, all the blood completes the circulation in about thirty-two heart-beats. We should further observe that the heart never rests, but is ceaseless from birth to death. ”

“ The amœba, sponge, and tapeworm have no blood; they have no necessity for it, as they are destitute of digestive organs, their food being in immediate contact with all parts of the body: or, we might regard their blood as simply the water or fluid in which the animal is immersed. In animals possessing the simplest digestive cavities, as the jelly-fish and sea-anemone, the blood is merely the dissolved food, corresponding to the chyme of higher animals. In the starfish, sea-urchin, and other invertebrates, having a complete and distinct stomach, the blood is chyle ”

“ Its purpose is threefold—that of a reservoir, as animals can not always procure their proper food and can not, like plants, be ever eating; a liquefier, as all food, both for plants and animals, must be in the fluid state; and, thirdly, a chemical laboratory, as nourishment must be like the body in composition. Plants have no need of such an organ, because the food is always at hand in proper condition. Consequently, food is not in the animal body proper when in the stomach. It is within the body, not of the body. ”

“ The food must be forced through the connecting tube, known as the œsophagus or gullet. We do not find such in the lowest animals. The whole of nutrition is a single process in the tape-worm; with the amoeba, grasping of food is not distinct from digestion; while in the anemone and jelly-fish the mouth opens directly into the stomach cavity.
If the stomach were always beneath the mouth, as in man and birds, food might with some difficulty reach the former by gravity. Birds do help the descent of food by jerking the head, and most birds in drinking lift the head each time the beak is filled. ”

“ ANIMAL COMBUSTION.—Within every living organism there are two opposing forces. The "vital force," which produces all the phenomena of life, holds the material elements in unstable relations—against their will, so to speak—and it is antagonized by the natural chemical affinities of the elements, which tend to break down the organic compounds and rearrange the elements in more stable form. This decomposition takes place in some degree during the life of every organism, and when life ends, or when the vital force ceases to act, it rapidly destroys the structure. ”

“ Many animals of higher groups breathe mainly or entirely by the skin. Among articulates, the leech and earth-worm are examples. In these a network of minute blood-vessels is spread beneath the delicate skin, thus bringing the blood into proximity with the water or air. The lowest crustaceans, and some sea-snails, and sea-spiders also, respire by the skin alone.
Most amphibians use the skin largely in respiration. In cold water, frogs will breathe entirely by the skin, and can not be killed by forced immersion, so long as they are provided with food. Even at ordinary ”

“ The parts of the food-tract are always clearly marked, and the stomach is frequently divided. In all animals digestion is more prolonged in proportion as the food is unlike animal substance. With carnivorous mammals the process is simple. The whole length of the food-tract in members of the cat tribe is only about three times the length of the body. Man employs a mixed diet, and has the canal six times the length of the body. No food is less like flesh than herbage, consequently we find with such food the most difficult digestion. ”

“ A sticky saliva causes the insect to adhere.
Poison-fangs of serpents are helps to procure food as well as weapons; and the power of charming is a very strange and effective way of obtaining food.
The alligator will approach a large animal which may be standing at the water's edge, and, by a quick blow of its powerful tail, knock the unsuspecting creature into the water. In deep water it is at the mercy of the reptile, which kills it by drowning. The nostrils of the alligator and crocodile are so placed as to be out of water, while the prey held in the jaws is beneath the surface. ”

“ From what has already been stated as the purpose of circulation, we should not expect to find any circulation in those animals which are destitute of a separate digestive cavity; and in such we do not discover a true blood-circulation. But it would appear that no animal is entirely without the power of distributing its food to the parts of the body. ”

“ As the opposite of the enormous throat of the snake, the bulky whalebone whale has the smallest throat, proportionate to its size, of any animal just large enough to admit the tiny creatures which are its food. We see in this a fine example of Nature's economy. ”

“ In the adult state they are air-breathers, and, if their circulation were complete, they would be "warm-blooded." But the temperature is subdued by imperfect circulation, which results from the arrangement of the heart-chambers. There is but one ventricle for the two auricles, hence the pure blood from the breathing organs and the impure blood from the body are mingled, so that, besides the venous and arterial, they have a mixed blood. The blood which goes to the lungs is never wholly impure, and that which goes to the body is never entirely pure. ”

“ While the people of the tropics subsist chiefly on vegetable food, which supplies little fuel, but on the contrary much fluid to cool the body by evaporation, the inhabitants of frigid regions use carbonaceous foods affording the greatest proportion of fuel.
Experiments tend to prove that human respiration is, as would theoretically be expected, less rapid in the tropics than in the cold regions. Every traveler knows that a less amount of food is required. ”

“ When a nutritious particle comes in contact with the body, the surface at that point begins to depress or fall in, and so continues until the surrounding surfaces meet and unite. In other words, the animal wraps itself around the particle, and the bit of food is enveloped in the albuminous body-mass. The nutritive matter is absorbed, and any undigested or waste matter is expelled by a reverse process. Briefly, the amoeba extemporizes a stomach upon the place and at the time it is needed, and is not troubled with that uneasy organ when it is not needed. ”

“ Whatever food one digests serves to nourish the whole colony. They are absolute communists.
At this point should be presented the fact that in all animals the lining or secreting membrane of the food-canal is essentially but a continuation of the skin. That such is true of the amœba is evident, for what was the outside of the body-mass becomes when food is enveloped the lining of the new cavity. The cup-shaped body of the hydra can be turned inside out without interference with the business of digestion. The skin in these cases must have a chemical digestive power ”

“ The method of its accomplishment is one of the most admirable functions of the animal economy. To remove the effete matter in the natural liquid or solid state would be very exhausting; consequently it is burned, and the gaseous products of its union with oxygen are then easily carried away. Literally speaking, this makes a furnace of the body of every animal; and the most pressing and ceaseless demand of the system is for oxygen to support its fires. Respiration is hence an absorbing and excreting process, whereby oxygen is received and carbonic acid and water removed. ”

“ In variety of prehensile means mammals far surpass any equal group of the animal kingdom. Owing to our familiarity, however, these peculiarities seem less interesting; but, if we could divest ourselves of preknowledge, or see them with a new vision, we should be astonished and delighted by the various contrivances and the curious adaptations. The tongue is the chief or only prehensile organ of many animals. ”

“ As the mollusks are mostly so sluggish that their circulation has little aid from the movements of the body, they require a more powerful pump. In the higher mollusks the heart has generally two cavities—an auricle for receiving the blood and a ventricle for propelling it. The bivalve mollusks have generally two auricles. In the mollusks we discover a well-developed capillary system, but the venous or return circulation is still partly lacunar. The heart of invertebrates is always systemic—it forces the blood to the body, not to the breathing-organs. ”

“ The pure and impure blood are never mingled, and all the blood has to pass through the lungs and be oxygenated every time it makes the complete circuit. This perfect circulation, with aerial respiration, produces more rapid chemical changes in the blood and tissues, and consequently the higher temperature of the "warm-blooded" animals. In the embryonic stages of the heart, the septum dividing the auricles is slowly formed, and an aperture exists for a time, called the foramen ovale. Cases rarely occur of human subjects in which the opening persists. ”

“ They are firm, elastic tubes, which expand under the pressure from each heart-contraction, and then by their own elasticity contract and help the onward flow of the blood. In the smaller arteries the flow loses the intermittent character it possesses in the larger arteries, and becomes a steady stream. The elasticity of the arteries serves precisely the same purpose as the air-chamber of any force-pump, that of equalizing the flow, and so increasing the amount delivered. The whole force is derived from the heart; the arteries cause the force to act continuously. ”

“ The temperature of the blood depends upon the rapidity with which the blood is oxidized; and, within natural orders, it has a relation to the activity of the animal. For example, that of the swallow is 111°, hen 109°, gull 100°; among mammals, squirrel 105°, cat 101°, dog 99°, man 98°. The animals called cold-blooded are only relatively so, for fishes and reptiles have a temperature somewhat above that of the water or air in which they live. Even the lower creatures are slightly warmer than the surrounding medium. ”

“ A relative of the dragon-flies, the ant-lion, when in the larval state, digs a conical pitfall, at the bottom of which it lies buried, with only its strong jaws projecting. Here it patiently waits for an ant or some small creature to tumble in. If the struggling animal seems likely to clamber out, the ant-lion hurls sand at it to bring it within reach of the fatal jaws, where it is surely lost. ”

“ Fig. 6.—Ant milking an Aphis.
With few exceptions fishes depend upon their numerous teeth for seizing food. The sturgeon is toothless, and draws in its food by suction. A single long tooth is used by the hag-fish to transfix its victim, while it bores the flesh with a long, spiny tongue. The angler lies half concealed by sand and weeds, with its enormous mouth agape ready to seize any small fish which may incautiously venture near. Upon its nose is a long, flexible spine, with shiny tip, which, waving in the water, decoys small fry to their death. ”

“ If its stomach becomes troublesome from indigestion or other cause, it simply ejects it through the mouth, along with its other internal organs. Then it quietly awaits the growth of a new set—certainly a very happy and efficient method. Many human dyspeptics would rejoice in the same power. This animal is said to reject its viscera when it is injured or alarmed. This is interesting, as showing in the low animals that which is well known in the highest, the immediate effect of fear and pain upon the internal organs, or the close dependence of the nutritive organs upon the nervous system. ”

“ The velocity in the large arteries of man can hardly be over twenty inches per second, but varies greatly at different times. The length of the capillaries is about one half of a millimetre, and the blood passes through them in about one second. In the human retina the corpuscles travel at the rate of 75 millimetre per second. The small arteries pulsate within one sixth of a second after the main trunks; but the rate of flow is much slower than the wave-progression.
In vertebrates, the rapidity of the circulation is generally proportionate to the activity of the animal. ”

“ Insects, the most active of all creatures, have a very sluggish and imperfect circulation, for in this class the air is so freely admitted into the body as to obviate the necessity of great movement of the blood.
The human pulse is somewhat more rapid in childhood, and again in old age; slightly faster in the evening than in the morning, in summer than in winter, and probably increases with geographical altitude. In fever the circulation is very greatly and mysteriously quickened. ”

“ These are various in structure, sometimes folded like gills, or in other cases cellular, like a rudimental lung in structure.
Among mollusks only the true snails are air-breathing, and some of these inhabit water. They have organs resembling those of the spider, concentrated into a single sac. This is lined with blood-capillaries and constitutes a true external lung. The orifice opens on the right side of the neck, and can be closed by a circular muscle.
In the pond-snails we have a clear illustration of the identity in principle of air and water respiration. ”

“ Digestion is not more perfect, however, in the latter than in the former. The simple nutritive act of the amoeba is as perfect for itself as the differentiated process of the highest animals is for them. In the lowest animals, the function is single, and so simple that no special organ is necessary. As we rise in the animal scale, the function is divided into secondary functions, which require for their performance a corresponding number of special organs. ”