THE ORGANS OF ANIMAL BODIES— THEIR FUNCTIONS. 



If these arteries be irritated in an unusual degree, they act more forcibly 

 and propel a gi^ater quantity of blood; the ner.vous fluid becoming more 

 abundant, increases the local sensibility; and, reacting upon the irritabi- 

 lity of the arteries, carries their mutual action to a high degree. This is 

 called nervous excitement, or orgasm; when it becomes painful and per- 

 manent, it is termed inflmmnatiov . 



This mutual influence of the nerves and muscular fibres, whether in 

 the intestinal or arterial systems, is the true source of those involuntary 

 actions, common both to plants and animals. 



Each internal organ is susceptible of irritation only from its pecah'ar ir- 

 ritant, to which it is in a manner especially adapted, just as an exter- 

 nal sense can be affected only by its particular objects. Thus mercury 

 irritates the salivary glands, and cantharides, the vesica. These agents 

 have been called specifics. 



As the nervous system is continuous and of uniform structure, local 

 irritations, and frequently repeated sensations, fatigue it throughout the 

 whole extent; so that any function, when excessively exercised, may en- 

 feeble all the others. Thus, too much food impedes the action of the 

 intellectual powers, and long protracted study impairs the powers of di- 

 gestion. 



An excessive local irritation may affect the whole body, just as if all 

 the vital energies were concentrated upon one single point. But a se- 

 cond irritation, in another place, will diminish the first, or, as it has been 

 called, determine the first into another part; such is the effect of blisters, 

 laxatives, and other counter-irritants. 



We have thus shown, in the above brief sketch, that it is possible to 

 account for all the phenomena of physical life; if we merely assume hy- 

 pothetically the existence of a nervous fluid, possessed of certain proper- 

 ties, which are deduced from generalizing the phenomena of the vital 

 system. 



SECT. VI THE ORGANS OF ANIMAL BODIES, THEIR APPROPRIATE FUNCTIONS, 



WITH THEIR. VARIOUS DEGREES OF COMPLICATION. 



Sensation Touch — Ta.-^te — Smell — Sight — Hearing — Muscular Sense — Head — 



Brain Voluntary/ Motion — Muscles — Bones — Tendons — Ligament'^ — NutTition 



Stomach — Gastric Juice — Chyme — Chgle — Lacteals — Arteries — Veins — 



Lymphatics — Respiration — Lungs — Gills — Trachea— Capillary Vessels — *S"e- 

 cretory Glands — Generation. 



After having considered the organic elements of the animal body, the 

 chemical elements of its composition, and the active forces which prevail 

 in it, nothing now remains to complete a general view of the animal sys- 

 tem, excepting a summary account of the several functions of which life 

 is composed, with a description of their appropriate organs. 



The functions of the animal body may be divided into two classes — the 

 animal functions, which are peculiar to animals — and the vital or vegeta- 

 tive functions ; common to animals and plants. The former comprise sen- 

 sation and voluntary motion, the latter nutrition and generation. 



We shall commence with sensation, which resides in the nervous 

 system. 



The sense of touch is the most extensively diffused of all the external 

 senses. It is seated in the skin, a membrane enveloping the entire body, 

 and traversed in every part by nerves. Their extreme fibres are ex- 

 panded at the surface of the skin into minute papillce, or small project- 

 ing filaments, where they are protected by the outer skin, and by other 

 insensible coverings, such as hair or scales. 



The degree of perfection in which different animals possess this sense varies consi- 

 derably; but its exercise, in a high degree, is always accompanied by certain conditions. 

 The organ must be supplied with numerous nerves and papillae, under a very fine cu- 

 ticle; with a soft cellidar substance, like a cushion; and with a hard resisting base. 

 It must also be endowed with a considerable degree of flexibility, as a close contact 

 with the surfaces of bodies is indispensable. Most animals are possessed of soijie par- 

 ticular organ, in which the sense of touch is developed in a high degree. In the hand 

 of man, and particularly at the extremities of the fingers, we find all the necessary 

 requisites of this sense, combined in their most perfect form. The proboscis, or trunk, 

 of the elephant seems to rank next to the human hand; and, among the higher orders 

 of animals, either the snout or the lip is endowed with much sensibility. This 

 quahty is particularly observable in the nose of the tapir, and of the hog, in the 

 lips of the mole, and in the upper lip of the rhinoceros. The seal, and animals of the 

 cat kind, such as the lion and tiger, have whiskers, possessed, near their roots, of consi- 

 derable delicacy, which renders them of important use to these animals as feelers. Cer- 

 tain species of monkeys have deUcate prehensile tails, which they use with surprising 

 agility. In birds, the nerves of touch seem chiefly developed in the feet and toes, 

 and most of the aquatic species are endowed with bills of considerable feehng. Ser- 

 pents use their slender tongues as instruments of touch ; and the great flexibility of 



their bodies renders them well adapted for the exercise of this sense. The snouts 

 of lishes have some nicety; but, with this exception, these animals seem nearly desti- 

 tute of delicate sensation. Insects feel chiefly by means of their antennaj; and the 

 several tribes of annelida, actinix, and polj^i, by their tentacula. Several animals 

 arc covered with a dense integument, in many of their parts, which are thus wholly 

 unfitted for this sense. The thick hides of the elephant and rhinoceros, the feathers 

 of birds, the scales, horny coverings, and shells of the lower animals, are evidently 

 inconsistent with the necessary conditions of tou^h. Bats are enabled to fly in the 

 darkest places, by the extreme acuteness of their tactual nerves. 



Taste and smell are merely more delicate modifications of this sense, 

 for the exercise of which the membranes of the tongue and nostrils are 

 spociall)'" organized. 



In most of the lower animals the sense of taste is very imperfect, or it is altogether 

 wanting. The tongue of man is supplied ^nth numerous papillee, of a conical form 

 and spongy texture, projecting in a manner visible even to the naked eye. Taste 

 seems in him to attain its most perfect state; and he not only enjoys the natural va- 

 rieties of an omnivorous animal, but also a number of acquired tastes, which other 

 species are wholly denied. The tongues of birds, of reptiles, and of fishes, are often 

 covered with a hard and horny cuticle, which renders them altogether unfit for the 

 delicate exercise of this sense. IMany animals swallow their food vi-ithout mastication ; 

 anJ they must be thereby efTectually deprived of the enjoyments of taste, as a certain 

 degree of contact between the food and the organ is essentially necessary for its exer- 

 cise. 



The sense of smell resides in an organ, rendered susceptible by the ex- 

 treme delicacy and extent of its ever humid surface. 



Very minute particles of an odoriferous substance are darted forth in every direc- 

 tion, and are received upon the extensive and complex membrane, which lines the in- 

 ternal parts of the nasal cavity. Matter is thus perceived, when in a state of great 

 subdivision, w*ith a degree of acuteness far surpassing any of the other senses. The 

 extreme minuteness of these particles may be inferred from the fact, that musk, and many 

 other substances, will exhale odour for several years, and yet no loss in their weight 

 can be detected, even by the most delicate balances. Carnivorous animals, in general, 

 possess a more acute sense of smell than those living upon vegetable food ; and the struc- 

 ture of their nasal cavities is consequently much more intricate. This power was ob- 

 viously given to facihtate the discovery of their food. In man the sense of smell seems 

 best adapted for vegetable effluvia. A dog, though surpassing him in detecting the 

 most minute effluvium of another animal, will derive no pleasure from the finest 

 vegetable odours. M. Audubon is of opinion tliat birds of prey are not endowed 

 with an acute sense of smell. The degree in which this sense is enjoyed by the lower 

 tribes of animals has not yet been completely determined, but it is observed to exist in 

 bees and snails. 



The beauty of the eye, and the unbounded sphere which it exposes to observation, 

 give to the sense of sight a decided pre-eminence. Light, when emitted from the sun 

 or any luminous body, strikes upon the external covering of the eyeball. By means 

 of the crystalline lens, it is then refracted or bent from its original direction to a focal 

 point, from which the rays of light are again distributed on the expanded extremity of 

 the optic nerve, prepared to receive them. The size of the eyes in quadrupeds, and 

 tlifi intensity of their vision, bear a constant relation to the nature of their food. Her- 

 bivorous animals, such as the elephant and the rhinoceros, have very small eyes in 

 comparison with their entire bulk. The eyes of the whale, when viewed singly, arc 

 very large ; but they seem disproportionately small, if we contrast them with the 

 enormous mass of the entire body. But quadrupeds and birds feeding on flesh, require 

 powevs of vision of very great intensity. In these animals we accordingly find the 

 organ large, and highly developed, so as commonly to impart a peculiar expression of 

 ferocity to their countenances. The animals which are the objects of pm'suit are fre- 

 quently supplied with acute vision, thereby enabling them to escape or avoid danger; 

 and this is particularly exemphfied in the squirrel, the rat, the deer, and the hare. 

 Animals which burrow under ground, as the mole and the shrew-mouse, have, 

 in general, exceedingly small eyes; while in some they have been found nearly want- 

 ing, as in the bhnd rat (3/«5 typhlus^ Linn.) The cat, the lemur, and other animals 

 which pursue their prey during the night, are peculiarly adapted, by the construction 

 of their eyes, for acutely perceiving objects, when illuminated by a very small quantity 

 of light. The eyes of reptiles and fish are accommodated to the medium in which 

 they reside. The chameleon can move one eye with rapidity, and in various direc- 

 tions, while the other remains fixed. Reptiles residing generally in the water, also 

 fish, and the cetacea, such as the dolphin and seal, have their eyes covered with a 

 dense skin, and the lens is more convex than in other animals. The arachnides, or 

 spiders, possess generally eight eyes, arranged upon the upper part of the head in a 

 symmetrical form ; and there are not less than twenty-eight in the common millepede 

 (Julus terrestris.) The insect tribes enjoy great variety and intensity in their visual 

 organs; but the precise limits of this sense among the lowest animaJs iji the scale of 

 creation is not yet clearly ascertained. 



The organ of hearing is excited by vibrations or undulations of air, of water, or 

 of some solid medium, recurring at intervals, with different degrees of frequency. 

 These impulses are received upon the tympanum or ear-drum ; thence they are com- 

 municated to the acoustic nerve, and are finally transmitted to the brain. When the 

 vibrations are not performed in equal times, or do not occur more frequently than 

 seven or eight in a second, there is heard merely a noise. But when they rise much 

 above this velocity, a very low or grave musical note is first heai"d. By an increase 

 of velocity, the note becomes higher or more acute, and the ear is finally capable of 

 perceiving sounds resulting from 31,000 impulses in a second. There is a regular 

 gradation among animals in the perfection of the organ of hearing, but none of them 

 can rival the delicacy with which the practised ear in man perceives minute changes of 

 tone, alterations in the quality or expression of sound, and varieties in its intensity and 

 loudness. Feeble and timid quadrupeds generally have their ears directed backwards, 

 to warn them of approaching danger; while, in the predaceous tribes, the eax-s are 



