Rudiments of animal physiology

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Section I

ORGANISATION— LIFE. CLASSIFICATION OF ANIMALS.

1. All natural objects are divided into two great classes, called the Organic and the Inorganic, the distinctive properties of each of which are in general readily recognised, but not easily explained. When the structure of animals and plants is attentively examined, parts are found to be included, to each of which some function or office has been assigned. To these parts the term organs has been given, and the whole structure is consequently said to be organised. Thus the heart and stomach of an animal are called organs, their functions being to circulate the blood and to digest the food. Animals and plants are hence said to be Organic Bodies. Inorganic bodies are such as rocks, air, water, &c., which do not possess a structure of the kind mentioned.

2. Organic bodies both possess different, qualities from the inorganic, and fulfil different purposes in the economy of the world. Animals and plants are of certain determinate kinds, each kind having certain peculiarities, and each individual of each kind passing through a certain routine of existence, from what may be called its birth to its death. In the first place, life is indispensable to the existence of an animal or a plant. Of this quality we know little more than that it is one which appeal’s essential to organisation, and that, while it is present, organic bodies arc able, apparently through its means, to resist the action of various agents which would alter or decompose them if dead. Being in possession of the quality called lite, a plant or animal commences the routine of existence — takes in nourishment from food and air, by virtue of which it grows to maturity — is afterwards supported for a certain space by the same means — and, finally, when its purposes in the world have been fulfilled, and it has reached the term allotted to its species, it ceases to live, and is resolved into the elements of which it was formed. In addition to these peculiarities, which attend plants and animals in common, animals possess parts, which give them what is called sensibility, and which enable them to fulfil certain purposes of a character quite apart from those ful-

ASIMA.Ii PHYSIOLOGY.

filled by plants. In organic bodies, wherever there is sensation or voluntary motion, we have an animal ; where these are wanting, a vegetable. Bichat, an eminent French anatomist and physiologist, has shown still further that this distinction forms a natural division of the complex parts combined in the animal system. Such parts as the heart, the intestines, &c., which act in general independently of our will, and without our consciousness, belong to what he calls the vegetative or organic life. The senses and parts that bring us into relation with our fellow beings and the external world, he calls the animal life. The division is so natural and comprehensive, that it has been adopted by almost all the best writers since the time of Bichat.

3. Animals, as well as plants, are distinguished from each other by peculiarities in their structure, some being of very simple forms, and possessing few organs or parts, while others are complicated in their figures and structure, and exercise a greater number of functions. Something like a regular progress or gradation has been observed from the most simple up to the most complicated, and the distinctions observed between different groups of animals have given rise to classification , or an arrangement for scientific purposes. The best existing classification is that formed by Baron Cuvier, usually called from its author the Cuvierian System, of which the following is an outline : —

4. The Cuvierian System supposes four Divisions of the Animal Kingdom, the first and simplest being the Radiata, so called because some of the more remarkable creatures embraced by it have a rayed or branched figure. At the time, though it is still recent, when Cuvier formed his system, the animals placed by him in this division had not been very attentively observed ; and it is probable that, as they become better known, the propriety of classifying them otherwise will be acknowledged. Cuvier represented them as composed of a simple homogeneous pulp, moveable and sensible, without any apparent apparatus for the senses ; whereas parts different from pulp, and parts which some naturalists consider an apparatus for the senses, have since been in many cases ascertained.

6. The first of the so-called Radiata demanding attention are the Animalcules, or microscopic animals, so named as being only observable by means of the microscope. Apparently the simplest of these, and of all animals, is one termed the Monad, which seems to consist of merely a small round speck of animated matter, but is nevertheless found to possess at least organs of nutrition. It is one of a large class of animalcules, found in water in which decaying vegetable or animal matter

CLASSIFICATION OF ANIMALS.

has been infused, and thence called Infusoria, or Infusory Animals. Some of these are small to a degree which the mind cannot conceive, being only the 24,000th part of an inch in diameter, and yet possess organs for feeding, breathing, and volition or will. Many hundreds of varieties have been described and classified by Professor Ehrenberg of Berlin, who has also discovered that large strata of siliceous or flinty rock under the earth’s surface are composed of the hard parts of these minute creatures, no doubt deposited in remote ages from large bodies of water filled with them.

6. The Hydatid (Fig. 1), a parasitic worm, found in the human and other bodies, is larger, but has also a very simple structure.

It has a head with four suckers, and a neck communicating with a bag, which forms its stomach. This sac, when distended with fluid, nearly obliterates the neck, and the animal then forms simply a globular bag.

Fig. l. Hydatid. One species of tnis creature is some-

times found in the brain of the sheep, and gives rise to the disease called by shepherds sturdy or staggers. These bodies are so exceedingly simple, that it has been doubted whether they really are animals. An opportunity was afforded to many gentlemen in Edinburgh, a few years ago, of seeing a girl with one about the size of a pea within the ball of her eye, m which spontaneous motion appeared manifest. When at rest, it was nearly globular, but every few seconds it elongated itself into something like the form of a bottle of India-rubber.

7. Of a structure a little more complex is the fresh-water

Polype, of which two specimens, perched upon a stalk, are presented in Figure 2. It consists merely of a tube, with arms or tentacula at one extremity, with which it seizes small worms or insects, and conveys them into its interior to be digested ; but of so little importance is the surface used

g. 2. Polype. for this purpose, that the ani-

mal may be turned inside out, like the finger of a glove, without apparently suffering the slightest inconvenience. It may also be cut into numerous pieces, when each separate piece will become a distinct animal ; or parts of one polype may he grafted on the body of another.

ANIMAL PUYSIOLOGV.

8. The Star-fish (Fig. 3), a well-known creature, often found on the beach when the sea has receded, is among the highest of the Radiata.

In it we find a stomach distinct from the mass of the body, and teeth surrounding its entrance. We find, likewise, for the first time, parts having the form of feet, used in progression. In the five rays there are no less than 1520 feet of a very Fig. a star-fiah. curious construction.

9. The next division is denominated Mollusca (molluscous or pulpy animals), in which we find, gradually more and more developed, organs used for progression, a stomach and intestinal canal, a heart, and organs for breathing, ns well as several of the senses. Indeed, in some respects the higher orders of this division are superior to the articulated animals which have been placed above them. In their senses and instincts, however, they are generally much inferior. The Oyster, Mussel, and different kinds of snails ; the Clio Borealis, a small animal found in multitudes in the northern seas, and which forms the principal food of the Greenland whale ; the Nautilus (Fig. 4) ; s i / I \f all belong to this division. The Sepia or

it — — \ 7\ Cuttle-fish (from which the paint of this

t \ f name is got) is one of the highest of the

Fig. 4. Nautilus. molluscous division, being possessed of vision

and several other senses, as well as a heart ; besides parts that serve either as powerful amis for seizing its prey, or feet for walking with.

10. The third division includes the Articulated or Jointed Animals (Articulata), which, like the Mollusca, may be said to be intermediate between the Radiated animals and the highest division, or Vertebrata. The Radiata and Mollusca are generally aquatic, and limited in their powers of motion; but the Articulata have often a very complete motive apparatus.

Among them also are found all the senses, and, for the first time, a symmetrical form of the body, or that ^form in which two similar halves appea period of their growth, pear to have been, and really were, at one , joined together. This form likewise obtains in man and all the higher classes of animals. In the Articulata, the solid parts or skeleton are always placed exter-

CLASSIFICATION OF ANIMALS.

nal to the rest of the body. The Pontobdella, a species of leech (Fig. 5), is a good example of a class of the Articulata, whose bodies Fig. s. Pontobdciia. consist of a succession

of rings. The Crab (Fig. 6), the Spider, the Bee, Beetle, and Butterfly, are other specimens of this division.

11. The fourth and last division includes a vast series of animals, among which are seen the most elaborate exertions of creative power. The members composing it vary exceed- Fig. a Crab. ingly in their instincts, appearances,

and other peculiarities, but all agree in possessing an internal skeleton. In the simplest form in which this skeleton ever appears, the vertebral column, or back-bone, as it is usually termed, is always present ; and hence this, by far the most interesting and best defined division, has been called the Vertebrata. In such fishes as the lamprey, the back-bone is merely a continuous soft tube, with slight divisions marked upon it ; but as we ascend in the scale, these divisions become more decided, until we arrive at the separate and solid vertebra: of man and the higher animals.

12. Fishes are placed at the bottom of the series of Vertebrata — Reptiles next — then Birds — and finally Mammalia, or suck-giving animals, at the head of which is the human race. Although less highly organised in other respects, fishes have a skeleton of superior character to some of the class of reptiles, which is in some instances very slightly developed. The skeleton of the serpent, one of the class of reptiles, is of a simple structure, though comparatively of great length. It consists merely of a vertebral column with ribs, and a head but slightly developed ; but the vertebra:, or little bones composing the spine or vertebral column, are uncommonly numerous, being in some species three hundred, while man has but twenty-four, and the frog only eight. As we rise to higher orders, the offices of the vertebra: become subdivided, and new parts are added, for moving, and for seizing or holding objects (in scientific language, for progression and prehension), suitable to the wants of each species.

13. In the Mammalia, the division of the skeleton into its different regions is complete. Figure 7 is the skeleton of a man,

ANIMAL PHYSIOLOGY.

the highest order of the class. At the top we have the solid skull, to enclose and protect the important urain ; the neck (o')

Fig. 7- Human Skeleton.

a' a, vertebral column, on the top of which is the cranium, r r, ribs, most of which meet in the sternum, or breast-bone x, so as to form the thorax, y y, the clavicles, or collar-bones, 4, the humerus, or upper arm-bone, c, the elbow, d, the radius, e, the ulna, f the wristbones. <f, the phalanges, or finger-bones, s », the bones of the pelvis. w, the sacrum, h t, the thigh-bones. /, the patella, or knee-pan. in, the tibia, n, the fibula, o, the ankle, p, the metatarsal or foot bones.

CLASSIFICATION OF ANIMALS.

invariably composed, in tlxis class, of seven vertebrae,* showing the uniformity of nature’s plan in forming different families; the dorsal vertebrae, or vertebrae of the back, with the attached ribs (r r ) and breast-bone (a?), constituting the chest or thorax, to wnich the anterior extremities are attached ; below these, the lumbar vertebrae (a), forming the posterior boundary of the abdomen or belly; and, lower still, a strong circle of bones called the pelvis (a a), for connecting the inferior extremities with the trunk, aud for supporting the bowels. In most of the Mammalia the vertebral column is still farther prolonged in the form of a tail, but in man it terminates by forming tho posterior boundary of the pelvis.

1 4. These parts tire variously modified in different species ; but the general remark may be made, that the organs of each are connected so closely as to enable an anatomist to tell with certainty, from seeing a single bone, or even a part of a bone, the general form and habits of the animal to which it belonged. This may appear almost incredible, but may easily be illustrated by an example. Let us suppose a properly qualified person to find the broken off lower extremity of the Done called the radius (Fig. 7, d). He could easily tell, from examining its articulations, whether it was intended to be moveable or fixed. If fixed, as we find it in the horse or cow, then he would infer that it served as a solid support to the body, but was not meant to be used in seizing objects, as in man, the cat, monkeys, &c., in all of which it is moveable. But if the animal did not seize objects, it could have no use for claws, and would undoubtedly in their stead have hoofs. Hoofs, again, always imply a vegetable feeder, with grinding teeth, a particular form of alimentary canal, a certain conformation of the spine, &c. ; so that it will thus be seen, that, from this broken piece of bone, a good general idea of the size, form, and habits of the animal, might be formed.t

15. Such reasonings as the above, it will be observed, are all grounded upon the supposition that the animal frame has been put together upon rational principles. This will become abundantly evident as we proceed with our subject. We shall

* The sloth appears, hut only appears, to bo an exception.

+ Dr Bueklana, in his Bridgewater Treatise, relates a fact admirably exemplifying what has been stated. A good many years ago, a few of the bones of an extinct species of animal had been found, before tho general form of the animal was known. From these bones, Mr Connybeare, a celebrated geologist, set himself to construct an animal such as he snpposed that to which the bones belonged would be. Some years afterwards, a complete skeleton of this singular animal, tho Plesiosaurus, was discovered, with which Mr Connybeare’s drawing was found in a surprising degree to correspond.

ANIMAL PHYSIOLOGY.

quickly discover that no organ stands isolated, but that each has intimate relations with the rest, always forming a harmonious whole ; and that, whether we examine the structure of the individual parts, or their relations, we must equally feel that all has proceeded from the hand of an infinitely wise and good Creator.

[A proper acquaintance vritli the classes of the Vertebrata is so important, that we have appended the following table of their chief characters, taken from Edwards's “Elemcns de Zoologie,” an admirable work for the student of natural history. The teacher should have these tables transcribed on a large scale, and as the pupil gains a knowledge of the different organs, exercise him in the details by frequent examinations. In order, also, to render more intelligible our allusions to Cuvier’s classification, we have given a condensed view of the vertebrate classes and orders, with examples.

An excellent plan for impressing on the minds of children Cuvier’s four great divisions of the animal kingdom, is to make a largo diagram, with drawings of each placed under their respective heads. Numerous appropriate figures may be found in Dr Roget’s Bridgewater Treatise, vol. 1., pages 165, 227, 258, 271, 283, 41 1, 441. 447, 530; in Grant’s Comparative Anatomy; and in Edwards’s Zoologie, &c.]

Seal, outline of cuvier’s classes and orders of the vertebrata. Class I. — Mammalia.

Order.

1. Bimana (two-handed). Man.

2. Quadrumana (four-handed). Monkey, Ape, Lemur.

’ Cheiroptera (wing-handed). Bat.

Insectivora (insect-eating). Hedgehog, Mole.

Digitigrada (toe- walking). Dog, Cat, Lion, Weasel.

Amphibia (doubtful, or belonging both to sea and land), cs Walrus.

Marsupialia (pouch-nursing). Kangaroo, Opossum.

7. Pachydermata (thick-skinned). Horse, Elephant, Hog.

9. Cetacea (whale-like). Whale, Dolphin, Narwhal.

Class II— Birds.

2. Passeres (sparrow-like). Sparrow, Thrush, Lark, Crow, Swallow.

Class III.— Reptiles.

1. Cbelonia (tortoise-like). Tortoise, Turtle.

2. Sauna (lizard-like). Crocodile, Lizard, Chameleon.

CLASSIFICATION OF ANIMALS.

4. Batrachia (frog-like). Frog, Newt, Salamander.

Class IV. — Fishes.

2. Malacoptorygii (soft-rayed). Salmon, Cod, Herring, Eel.

4. Plectognathi (jaw-joined). Sun-fish.

5. Chondropterygii (gristle-rayed). Shark, Lamprey, Sturgeon. CHIEF CHARACTERS OF THE CLASSES OF THE VERTEBRATA.

Mammalia.

Birds.

Reptiles.

Fishes.

Viviparous.

Oviparous.

With teats.

' » 1 - ■ Without teats.

Globules of blood circular.

Globe

des of blood elliptical.

Blood warm.

Blood

cold.

Breathe by lung

s.

Breathe by gills.

V simple.

double.

Respiratic

n simple.

Circulation doublo complete.

Circulation doublo incomplete.

Circulation double complete.

Heart with four

V i

compartments.

Heart usually with 3 compartments.

Heart with 2 compartments.

Skin furnished with liairs.

Skin furnished with feathers.

Skin naked or sea

furnished with ies.

Members organised in general for walking.

Anterior members organised for flight.

Members organised in general for walking

Members organised for swimming.

animal physiology.

Section II. Mastication— Deglutition— Digestion

16. The details which follow, refer, for the most part, to the sciences of Anatomy and Physiology, the former of which treats of the structure of animals, the latter of their functions, or of those phenomena which are peculiar to life. It is chiefly of the Physiology of the higher Vertebrate Orders, and particularly of Man, that we design at present to give some account; and in doing this we shall keep in view the important division of Bichat, into the organic or vegetative, and the animal or relative functions, although a slight departure from it will occasionally be necessary, to give connection, and to prevent

17. The first, because the most essential, processes which engage our attention, are those which relate to the introduction of food into the body, its digestion, and its assimilation.

18. It has hitherto been supposed that many of the lower ana more minute creatures possess no cavity for the reception ot nutriment, but are supported by the absorption of aliment through the outer surface. Of late, however, so many animals of this obscure kind, formerly thought stomachless, have been discovered to possess alimentary cavities, that it is now probable that no creature whatever is altogether deficient in a stomach. In the Infusoria, such on opening has been discovered, in some instances surrounded on the outside by cilia, or a series o » the office of which seems to be to draw food towards the mouth. In the Polypes we find the orifice of the alimentary cavi y rounded in like manner by tentacula , or long

with which they seize their prey, and convey it to its p P receptacle. In the star-fish the opening of the stomach is sur rounded with teeth, and the cuttle-fish, of the divismMolW, possesses, in addition to its numerous tentacula, a strong beak, Ea^Vrot's, for crushing the shell-fish 0. which It ™s In Insects we find mandibles and a proboscis or trunk, with a thousand other modifications leading us up to the regular m treating apparatus of the vertebrate division.

19. This masticating apparatus consists of several P^t?-. .

have, 1st, the teeth for seizing or dividing the * the glands which secrete the fluid for moiste g

MASTICATION — DEGLUTITION — DIGESTION.

Fig. 8. Dime* of Lion's Head.

and mouth; and, 3d, the tongue and other muscles which move the food from side to side, or carry it backwards to be swallowed.

20. From what was formerly said, it must be evident that the Teeth are parts of great importance to the zoologist. They at once give him decisive indications of an animal’s habits, conformation, and other qualities. In the adult man they are thirtytwo in number, and consist of four different kinds, namely, 1st, of eight incisors, or cutting teeth, in front; 2d, of four cuspidati (pointed), or canine teeth ; 3d, of eight bicuspidati, or small grinders; and, 4th, of twelve molares, or proper grinders. The molares are flat-crowned in the horse, and in

the other Herbivora, while the canine are either wanting, or, as in the horse, are rudimentary ; that is, imperfectly developed. The canine are largely developed in the lion (Fig. 8), and in the Carnivora, or flesh-eating tribes, generally, which have also the molares pointed instead of flat. In insectivorous animals, such as the mole and hedgehog, the molares are formed as in QZJQ Fig- 9 5 while in the shark (Fig. 10), and other

Fig. 9. fishes, which swallow their prey entire, the teeth are all of the same pointed form, and are numerously set, even on

the lips, sides of the mouth, and throat.

Even the degree in which an animal is carnivorous, or otherwise, is marked very accurately by the Fig. 10. Shark’s Jaw and Teeth. more Or less pointed

form of the teeth. Thus, among the Carnivora, the lion and others of the cat tribe are found to have all the molares pointed except two, which are rounded; while ferrets and polecats have four, and dogs eight, rounded. The teeth of man seem to approximate him, in this respect, to families such as we find among the Quadrumana, or monkey tribes, which, in their natural state, live principally upon fruits, but do not refuse also various kinds of animal food.

21. Nothing can be more plain than nature’s intention in thus varying the forms of the teeth. Without any exception, tnese variations have the most precise relation to the instincts ot each tnbe. It is quite impossible that the sharp teeth of the tiger or shark could ever be used for grinding food, like that ot the cow or the horse, or that the teeth of the latter could be animal physiology.

intended for seizing or tearing flesh ; and this becomes still more striking, when we observe how the jaws in which the teeth are set have been articulated. In the Carnivora, as the teeth merely cut, the jaws are jointed like the blades of a pair of scissors ; "but the jaws of the Herbivora, and of Man, allow of a grinding motion. This motion, in the cow and horse, is from side to side, or nearly circular ; in the Iiodentia, as the rat, &c., the grinding motion is rapidly performed in a longitudinal

direction. . ,

22. The human teeth are composed principally ot two substances, the enamel , and the ivory, or bone. The enamel is placed externally and on the body of the tooth, and forms only a thui layer. It has ninety-eight per cent, ot earthy matter in its composition, is so hard 'as to strike fire with steel, and is viewed by physiologists as void of vitality, so that when once formed, the teeth never increase in size, and when any part of the enamel is destroyed it is never regenerated. The internal part ot the tooth, or ivory, approaches more to the nature of bone, and is shown to possess vitality, from its capability of adhering to other vital structures. It was owing to this property that Duhamel, a physiologist of the last century, and others, were able to transplant the teeth of one person into the jaw ot another, to make them grow upon the combs ot cocks, «c. Transplanting sound for decayed teeth, threatened, indee< , a one time, to become but too common among the better ranks, until, happily for the interests of humanity, it was discovered that this practice produced disorders more serious than the

deficiencies intended to be supplied.

23. Other animals have the enamel differently distributed. The horse, elephant, and other Herbivora, have ayersof it which penetrate interiorly. These, from being harder than the surrounding ivory, are longer of wearing down, and kcnceform protecting ridges, of great importance in triturating their food. II incisors of the beaver, and other Rodentia, want th< .enamel] posteriorly, and, for the same reason, wear soonest in that d ration. As a consequence, they always preserve a sharp edge m front, which is of the greatest importance to this miawmg order

24. The teeth furnish a beautiful example of w hat lias been justly denominated a prospective contrivance, lheir presence above the gums at birth would have been on y an annoyance. Accordingly, they nre then wanting; but nature, u Inch ant cinates our needs, places them deep in the jaw, even befo.e birth to appear in due season. If a section of the jaw of a young animal be made, some of the teeth nniy be seen just beginning to be formed— others cutting through the bonewlnle others are passing through tlieir last covering, the gum.

MASTICATION — DEGLUTITION — DIGESTION.

In the child, the teeth generally begin to appear from the sixth to the twelfth month, and the first, or milk teeth, twenty in number, are usually completed about the third year. These again begin to be shed about the seventh year ; and the second, or permanent set, is not complete until about the sixteenth or eighteenth year.

25. In the process of shedding, the crown, or that part of the tooth which is coming forward, presses upon the fang of the one already occupying the jaw, and causes its absorption. It is the impossibility of growth, in consequence of the nonvital nature of the enamel, which renders the renewal of the teeth necessary. In consequence, also, of the jaw, during youth, increasing its dimensions, mostly posteriorly, it is necessary that the number of the teeth should be increased, in that situation, in order that no part of the jaw may be unfurnished. For this reason, about the time when the human being reaches maturity, a new tooth rises at each extremity of the range, being four new ones in all, which, from the time of their appearance, are called the wisdom teeth. There are some teeth which animals of other species do not shed. The incisors of the Rodentia, and the grinders of the elephant, &c., continue to grow during the whole life of the animals.

26. The second part of the masticating apparatus, is that which moistens the food and mouth. The fluid employed for this purpose is tasteless, and is called saliva. It is produced by six bodies called glands, two of which are placed near the angles of the lower jaw (submaxillary), two under the tongue (sublingual), and one on each side, immediately before the ears (parotid). All these open into the mouth by means of small tubes or ducts — the four first under the tongue, the two latter on the inside of the cheeks, opposite the second or third molar teeth of the upper jaw. The fluid trickling from the latter may be seen at any time by turning out the cheeks, and watching their small openings. Its flow is seen to be increased by pressing with the finger from the ear forwards. The camel is supplied with additional salivary glands in its throat, which are of great service in its long journeys over burning deserts. Fishes, again, from living in water, and masticating but little, do not require, and are therefore destitute of, salivary organs.

27. The muscles of the tongue, cheeks, See., which bring the food under the influence of the teeth, and pass it backwards into the gullet, form the third and most curious part of the masticating apparatus. Indeed, this, which we shall examine in connection with deglutition, or swallowing, is undoubtedly among the most admirable and wonderful of the bodily processes. During the comminution of the food, the mouth forms

ANIMAL PHYSIOLOGY.

a shut cavity, hounded hy the closed lips anteriorly, the cheeks laterally, and the root of the tongue and the curtain of the palate brought together posteriorly. In Fig. 11, which represents the throat or pharynx cut open behind, d is the curtain ( velum palati ), with its central dependent part called the uvula, and c, the tongue. When the food has been properly moistened, find broken down, it is rolled into the form of a ball, and is passed backwards by the tongue and other muscles into the pharynx, which may be conceived of as a kind of bag, having the nostrils (a a) opening into it from above ; the mouth ( b ) opening below these ; the glottis (c), and the gullet, or oesophagus (i), still lower ; besides two other openings, called Eustachian tubes, which open on its sides, and lead to the internal ear.

28. Before the food can get into the pharynx, the curtain (d), which we have said is, during mastication, applied closely to the root of the tongue, must be lifted up. But if nothing more were done, the food or drink might pass from the pharynx into any or all of the openings mentioned, the inconvenience attending which, every one has experienced when a morsel of food or a little fluid gets into the nostrils or the windpipe. This, however, is effectually prevented, for the curtain is not only lifted up, but is also instantly applied closely to the back of the pharynx, so as to cut off the communication with the nostrils, and close the Eustachian openings ; while, at the same instant, the sides of the glottis, or opening into the windpipe (el, are drawn together, and a gristly substance, the epiglottis (g), is folded back over it. The food, being still forced backwards, passes rapidly over the epiglottis into the gullet (i), the only opening it can