CHAPTER XIII. HOW A RABBIT LIVES.

44. THE SKELETON AND MUSCLES

1. =The examination of the bones.=—In a boiled rabbit clear away the flesh from the bones. Before separating a bone, notice carefully how it is attached to neighbouring bones. Notice also, especially in the limbs, the attachment of the bundles of flesh (_muscles_) to the bones, the ends of each muscle being fixed to separate bones. Notice that the places of attachment of the largest muscles are marked by ridges or roughnesses on the bones.

2. =The skull.=—Observe the two rounded knobs at the back of the skull, which fit into hollows on the first bone (vertebra) of the vertebral column. In the skull notice the saw-like boundaries of the various bones, and make out: the great, rounded _brain-case_; the external openings of the _ears_; the _eye-sockets_; the snout, with the two _nasal chambers_; and the upper and lower _jaws_. Before separating the lower jaw observe carefully how it is hinged on the arches which run below the eyes, and notice the great muscle on each side which moves it. Examine the _teeth_ in detail, and remove them one by one from their sockets. Notice the bony shelf or palate separating the nasal chambers from the cavity of the mouth. Draw a side view of the skull.

Shave off the top of the brain-case with a sharp knife, and examine the _brain_.

3. =The vertebral column.=—Make out that each of the bones (vertebrae) which compose the vertebral column is really a ring, and that the whole column is therefore a bony tube. In this tube is enclosed the spinal cord, a backward continuation of the brain. Examine and draw vertebrae from the various parts of the column, and notice how they vary in shape and size. Compare vertebrae from (_a_) the neck, (_b_) the region of the chest (notice the attachment of the ribs), (_c_) the abdominal region, (_d_) the region of the hips (four united vertebrae; leave these for the present in position between the hip-bones), (_e_) the tail.

4. =The ribs and sternum.=—Examine the manner of attachment of the anterior (p. 217) pairs of ribs to the sternum or breast-bone. Some of the more posterior ribs are free at their ventral ends.

5. =The bones of the fore-limb.=—Examine the bones in order and make out: (_a_) the triangular shoulder-blade, (_b_) the single long bone of the upper arm (its upper, rounded end fitting into a socket in the shoulder-blade; its lower end forming, with the bones of the fore-arm, the elbow joint), (_c_) the two bones of the fore-arm (these lie side by side; notice the peg which makes it impossible for the hinged elbow-joint to be bent back beyond a straight line), (_d_) the small bones of the wrist, (_e_) the bones of the hand.

6. =The bones of the hind-limb.=—Notice that the limb as a whole is divided into parts—upper leg, leg, ankle and foot—which correspond to the divisions of the fore-limb. Make out: (_a_) that as the bone of the upper arm fits into a socket in the shoulder-blade, so the bone of the upper leg has a ball-shaped end which works in a socket of the hip-bone. Notice that the two hip-bones are joined together ventrally, and that they are fixed to the welded vertebrae of the hip-region. Does this give increased strength to the hind-limb? Identify: (_b_) the two bones of the leg (between knee and ankle) and notice that they differ from the corresponding bones of the arm in being fused together in the lower part; (_c_) the bones of the ankle, and (_d_) the bones of the foot.

7. =The structure of a long bone.=—Take the long bones of the upper arms and upper legs, and examine them further. Break one across the middle. Is it solid or hollow? What is the advantage of its being hollow? Is the tube empty or does it contain marrow? Are the ends of the bone hollow or solid? Place one of the bones on a bright hot fire. Is there any sign of burning? Does all the bone burn away? Carefully remove what is left, and compare the brittleness of the “burnt” bone with that of an unburnt bone. Place another bone for several days in a cupful of water to which has been added about two teaspoonfuls of strong hydrochloric acid. Then take it out, wash it; and try to bend it. Continue the treatment until the bone is soft enough to tie in a knot. Can you tie an ordinary bone in a knot? Why not? Put a “burnt” bone in similar dilute acid. Does it dissolve? What do you think gives a bone its hardness?

8. =Bones moved by muscle.=—Stretch out one of your arms and grasp the middle of the upper arm firmly with the other hand. Now bend the elbow, and notice that the great muscle (the _biceps_) of the front of the upper arm thickens and becomes shorter. Straighten the arm again, and notice that the muscle becomes thinner and longer again. Examine Fig. 160, which shows how the upper end of the biceps is attached at _a_ to the shoulder-blade and its lower end to one of the bones of the fore-arm at _P_. If the biceps muscle shortens, the fore-arm must be pulled up, because the shoulder remains stationary.

Carefully notice the various movements of which your arm is capable—_e.g._ extension of the arm; bending (flexure) on the elbow-joint; rotation of the fore-arm, so that either the palm or the back of the hand can be turned upwards; and grasping. Observe your power of touching the tip of the little finger with that of the thumb. If a human skeleton is accessible watch how one (which?) of the two long bones of the fore-arm rotates when the hand is turned over. How many of these movements can the rabbit make?

=The uses of the skeleton.=—The bodies of vertebrate animals (p. 220) are supported, and their more delicate parts protected from injury, by an internal framework called the skeleton. In some of the simpler fishes this is composed of gristle or “cartilage”; but in more highly developed animals it consists almost entirely, in the adult state, of bone. Further, almost all the bones are connected with strands or bundles of red flesh called =muscle=, which the animal is able to shorten or “contract” at will. When a muscle shortens, the bones to which its ends are fixed are of necessity pulled nearer to each other. If the bone at one end of the muscle remains stationary, that at the other end is moved into a new position. This is, generally speaking, the manner in which the numberless movements of the limbs, head, etc., are made.

=The movements of the limbs.=—An excellent illustration of this relation between the bones and muscles is seen in the bending of the arm at the elbow. When the arm is bent, a great mass of flesh, the biceps muscle, in front of the upper arm may be observed to become much thicker. The muscle (Fig. 160) thickens because it becomes shorter. Its upper end is attached to the corner of the shoulder-blade (_a_), which remains stationary; while its lower end is connected at _P_ with one of the bones of the fore-arm. The shortening of the muscle therefore draws up the fore-arm. The elbow-joint, about which the motion takes place, is a very perfect hinge. Several other forms of joint are also found in various parts of the body. It is seen, when the skeleton is examined, that in every case the characters of the joints and the attachment of the muscles are most admirably adapted to the movements with which they are associated.

=The study of the skeleton.=—The study of the rabbit’s skeleton is not only highly interesting in itself, but necessary for the intelligent appreciation of the animal’s life. And when it is compared with the skeletons of other familiar animals, a common plan of structure is found which illustrates in the most convincing manner the kinship which often exists between very dissimilar creatures. Such a comparison shows that, almost bone for bone, the skeleton of a rabbit corresponds with that of a man or a horse, and even with that of a bird or a frog. Mounted skeletons are shown in most natural-history museums, and the student should, whenever possible, examine and compare them.[9] He can himself, however, easily separate the bones from a boiled rabbit, and make out their main features and relationships.

=The rabbit’s skull and backbone=—The bones of the head are collectively known as the =skull=. This consists of (1) a large brain case; (2) the cavities for the organs of special sense, viz., (_a_) a pair of nasal chambers (in which the organ of smell is located) in front: their hinder ends open into the top of the throat; (_b_) the eye-sockets at the sides, and (_c_) the flask-shaped chambers for the internal ears at the sides of the hinder end of the brain-case; (3) the jaws: the upper jaw is rigidly attached to the brain-case, but the lower jaw is hinged at each side on the hinder end of the bony arch which runs below the eye. Both upper and lower jaws bear teeth, which are fixed in sockets. On each side the upper jaw contains two incisor teeth (p. 219) and—much further back—six grinding teeth. In the lower jaw are one incisor and five grinding teeth on each side.

The =vertebral column= or backbone is a chain of bony rings or _vertebrae_ which runs dorsally (p. 217) from the hinder end of the skull to the tail, and forms a long tube containing the spinal cord—a backward continuation of the brain. On the anterior face of the first vertebra are two hollows into which a pair of knobs on the skull fit. Although, with the exception of the first and second, all the vertebrae are formed on essentially similar lines, there is considerable variation in shape and size in the different regions of the spine, as may be seen from Fig. 161. There are seven neck vertebrae, and this number is remarkably constant in mammals (p. 220). Following these are the chest vertebrae, which bear pairs of =ribs=. The majority of the ribs curve round and join on to a ventral bar of bone called the =sternum= or breast bone; so that the cavity of the chest, containing those very important organs the heart and lungs, is enclosed in a protective bony cage. The vertebrae of the abdominal region of the body are very large and stout. Between them and the tail-bones are four fused vertebrae, forming a mass which on each side gives attachment to the large hip-bone.

=The bones of the rabbit’s limbs.=—The fore and hind-limbs of the rabbit are obviously comparable: the upper arm corresponding to the upper leg, the elbow to the knee, the fore-arm to the “leg,” the wrist to the ankle, and the fore-foot to the hind-foot. This similarity becomes even more apparent when the limb-skeletons are examined. Commencing in each limb at the end nearest the body we find a single long bone. In the fore-limb the rounded, upper end of this bone works in a socket at the anterior angle of the =shoulder-blade=, a triangular plate on each side which overlies the chest dorsally; in the hind-limb the rounded end of the corresponding bone works in a cup in the =hip-bone=. Again, between the elbow and the wrist are two bones lying side by side; and between the knee and the ankle are two corresponding bones, although here they are only separate in their upper parts. Similarly, the wrist bones correspond to the ankle bones, and the bones of what may be called the fingers to those of the toes. Certain of the ankle bones are, however, much elongated: obviously a great advantage to an animal which progresses by a series of hops—owing to the increased leverage which is thereby given to the hind-foot. The rabbit’s fore-foot bears five toes, the hind-foot four.

=The structure of a long bone.=—The long bones of the limbs are hollow except at the ends. Strength and lightness are thus secured by a device (the hollow cylinder), which has already (p. 72) been seen to be adopted by the supporting structures of plants, and is also copied by human engineers. The cavity of the tube is filled by marrow, which supplies the bone with food. The hard, bony tube itself is partly composed of mineral matter and partly of animal matter. The mineral matter is left as a white, brittle framework when the animal matter is burnt away; while on the other hand the mineral part—which gives the bone its rigidity—may be dissolved out by immersing the bone in dilute hydrochloric acid, leaving the organic tissue as a soft flexible substance having the shape of the original bone.

45. HOW THE RABBIT DIGESTS ITS FOOD.

1. =A solution of starch will not pass through a thin membrane.=—Rub up with water as much starch as will lie on a shilling, so as to form a thin “cream,” and then pour on it about a cupful of boiling water. The starch swells up and largely dissolves in the water. Add a few drops of the starch solution to about half a pint of water, stir, and test it by adding a little iodine solution (p. 2, footnote). A beautiful blue colour is obtained, showing that the test is a very delicate one. Now take a thistle funnel and with a file cut through the stem about six inches below the head. Wet a piece of parchment paper or thin bladder (having previously held it up to the light to be sure there are no holes in it), and tie it tightly across the mouth of the funnel. Fill the head and about an inch of the stem with the starch solution. This can easily be done by means of a “canula,” such as is used for filling fountain pens. Now put the funnel into a beaker of water, in the manner shown in Fig. 162, and put the arrangement aside for a few hours. After that time add iodine solution to the water in the beaker. No blue colour is formed, showing that no starch has passed through the membrane.

2. =A solution of sugar will pass through a thin membrane.=—A delicate test for certain varieties of sugar (not, however, table sugar) is a liquid known as Fehling’s solution.[10] Place a particle of honey in a test tube with a teaspoonful of Fehling’s solution, and put the test tube into a vessel of boiling water. Notice that in a short time the blue colour of the solution disappears and the liquid becomes red and turbid.

Now repeat Experiment =45, 1=, but instead of starch solution use honey dissolved in water. To show that some of the honey-sugar has passed through the membrane, take about a teaspoonful of the water in the beaker, and put it in a test tube with twice as much Fehling’s solution. Heat as before, and notice the red turbidity. If table sugar is used it may be recognised by the sweet taste of the water after the experiment.

3. =The action of saliva on starch.=—(_a_) Chew a piece of india-rubber to induce a free flow of saliva, and collect the liquid. In one test tube put half a teaspoonful of starch solution; in a second tube put a mixture of equal quantities of starch solution and saliva; in a third put saliva alone. Keep the tubes at blood heat for twenty minutes. Then add a little water to each tube and divide its contents into two parts. Test one part of each for starch with iodine solution, and the other part for sugar with Fehling’s solution. The first tube contains only starch. The second now contains no starch, but shows the presence of sugar. The third contains neither starch nor sugar. _Evidently the saliva has changed the starch of the second tube into sugar._

(_b_) Repeat the experiment, but keep the mixture of starch and saliva in a _cold_ place. No sugar is produced.

(_c_) Repeat the experiment as in (_a_), but use saliva which has been heated to boiling in a test tube. No sugar is formed.

=The necessity for food.=—It is common knowledge that a rabbit, like every other animal, must have a regular supply of food if it is to continue healthy, and that it would soon die outright if food were withheld. The reason for this is that the living substance of the animal’s body is incessantly wasting away. The rabbit cannot move a muscle except at the expense of the living substance of the muscle, and the more active is its life, the more rapidly does its body waste. It is to counteract this continual waste by continual formation of new living substance that food is taken.

But the succulent plants which the rabbit eats are not suddenly transformed into animal muscle and bone, and so forth, when they are swallowed. They have first of all to undergo a process which is called =digestion=. This takes place in a tube—the digestive canal—which runs from end to end of the body. The digestive canal of the rabbit is coiled in a somewhat intricate manner. That of the frog is, however, much simpler and more typical of vertebrates (p. 220) generally, and will serve equally well in so elementary a consideration of digestion as the present.[11]

=The frog’s digestive canal.=—The hinder end of the frog’s mouth opens (Fig. 163) into the =gullet= (_gul._), a short wide tube which leads to a capacious bag called the =stomach= (_st._). The termination of this is continuous with a coiled narrow tube called the =small intestine= (_s. in._), which passes suddenly into a much wider =large intestine= or rectum. The rectum opens to the exterior, at the hinder end of the body, by an aperture (_an._) known as the anus. In addition to the digestive tube proper, two large digestive glands, the liver and the pancreas, should be carefully noticed. The =liver= (_lr._) is a large, dark red organ, consisting of two lobes which lie at the sides of the stomach. It makes a digestive fluid called _bile_. The =pancreas= (_pn._) is an elongated body which lies in the loop between the stomach and the first portion—called the _duodenum_ (_du._)—of the small intestine. It makes a digestive fluid called the _pancreatic juice_. In the frog both the bile and the pancreatic juice are discharged into the duodenum by one tube or duct (_b.d._). Small digestive glands also occur in the inner wall of the stomach; these discharge a fluid called _gastric juice_ into the cavity of the stomach.

=The rabbit’s digestive canal=—In its main features the digestive canal of the rabbit resembles that of the frog; here also the mouth opens into a long tube consisting of gullet, a bag-like stomach, a small intestine, and a large intestine. There are also a liver and a pancreas which discharge their fluids—by separate ducts, however,—into the first part of the small intestine; and the stomach is supplied with gastric juice by small glands in its inner wall. There are, however, certain differences besides those of size in the digestive tubes of the two animals. In the first place, a fluid called _saliva_ is poured into the rabbit’s mouth by the ducts of salivary glands which occur near the mouth. Secondly, the coils of the small intestine are very much more complicated than in the frog. And lastly, at the junction of small and large intestines there is given off, in the rabbit and in herbivorous mammals generally (when these have simple stomachs,—p. 261), a great, spirally-constricted tube which ends blindly in a finger-like process.

=How the rabbit digests its food.=—A rabbit needs food to repair the constant waste of substance which the activities of its life entail; and the same is true of every other living thing, be it plant or animal. Now, every part of a rabbit’s body is irrigated and drained by the finest branches of a system of pipes through which blood is always flowing; and it is in this blood-stream that the food is conveyed to the muscles and other organs which are to be repaired. The food finds its way into the blood when that fluid is flowing through the small vessels which lie in the thickness of the wall of the digestive tube. Before food can gain access to the blood it must be in a condition in which it is capable of diffusing through the thin membrane which separates them. =Digestion is the process which renders food soluble and diffusible=, and hence capable of passing into the blood. The food of animals is of several different kinds. A few of these are soluble and diffusible at the time they are eaten, but most of them are neither, and therefore require treating according to their nature. This is why so many different fluids are poured into the rabbit’s digestive canal. Saliva, gastric juice, bile, and pancreatic juice are each capable of acting upon certain constituents of the food and rendering them soluble and diffusible.

It would be beyond the scope of this book to consider the work of these fluids in detail, but the =action of saliva= is not only fairly typical, but it can easily be imitated outside the body. =Starch= is a very common constituent of vegetable foods, and its presence or absence is readily determined by the blue colour which it gives with a solution of iodine. Starch is quite insoluble in cold water, but when treated with hot water it swells up and, to a great extent, dissolves. But a mere solution of starch cannot get into the blood, for it is incapable (Expt. =45, 1=) of passing through a thin membrane. On the other hand, if starch is mixed with saliva, and the mixture is kept at the temperature of the body, it is found in a short time that the starch has been changed into sugar, which is not only soluble, but readily diffuses through a thin membrane. In other words, starch _as such_ is useless to the rabbit as food; only after it has been digested by conversion into sugar can it be used by the body.

Something very similar often takes place in plants. It was seen (p. 33) that the cotyledons of a pea become sweet during germination. Starch is a convenient form of food for storing in the cotyledons of a pea, the endosperm of the maize seed, the short stem of the crocus-corm, and so on; but before the plant can use it as food the starch must be made soluble and diffusible by being changed into sugar. In plants the change is brought about, not by saliva, but by a substance known as _diastase_.

The pancreatic juice continues the change of starch into sugar which is commenced by the saliva, but it also digests other food-stuffs as well. Similarly, gastric juice and bile are each concerned with the digestion of certain foods. The result of the action—separate and combined—of the digestive fluids is that, when a rabbit eats no more food than it requires, all the useful parts of the food are absorbed into the blood, and then distributed to the tissues by the blood as it flows through them.

46. THE CIRCULATION OF THE BLOOD.

1. =Evidence of the circulation.=—(_a_) _The arterial pulse._—Feel your pulse at the wrist and count the number of beats per minute.

(_b_) _The beats of the heart._—Similarly count how many times your heart beats per minute. Put your ear over the heart of a friend, and listen to the sounds of the heart. How many different sounds can you distinguish?

(_c_) _The valves of the veins._—Press your thumb on your arm inside the elbow, and then move it down the arm towards the wrist. Notice the small knots which rise under the skin between the point of pressure and the wrist.

2. =The structure of a sheep’s heart.=[12]—Procure from the butcher a sheep’s heart with, if possible, the lungs still attached. If the thin transparent bag which naturally surrounds the heart has been left on, carefully cut it away. Make out the main external features of the heart before cutting into it. The shape is conical, the apex of the cone being posterior, the base (where the blood-vessels are attached) anterior. The ventral face is rounded; the dorsal face is flatter. Compare Fig. 164. The line of fat (_3_) marks the line of division between two chambers, the right (_R.V._) and left (_L.V._) _ventricles_. Feel that the right ventricle yields to pressure more readily than the left. Two more chambers, the right and left _auricles_, are situated at the basal (thick) end. _R.A._ and _L.A._ are flaps of the right and left auricles respectively. Identify the great vessels _SVC_ and _IVC_ which discharge into the right auricle. Cut them open to see the entrance. Then lay open the right auricle. Pass your finger down and notice that the right auricle communicates freely with the right ventricle. Observe that you can from the right ventricle pass your finger into the tube _P.A._; make out that this goes to the lungs. Lay open the left auricle, and see that it receives vessels _from_ the lungs. Pass your finger from the left auricle into the left ventricle, and notice that this latter chamber leads into the _Aorta_, _Ao._ (_A´o´._ a branch of _Ao._). Notice that the walls of the blood-vessels in connection with the two auricles are collapsed, while those (_P.A._ and _Ao._) leading from the two ventricles are more elastic and remain open. Which of the two auricles has the thicker wall? Which of the two ventricles has the thicker wall? Can you pass your finger from one auricle to the other? From one ventricle to the other? Cut away the auricles and pour water into the ventricles. Then squeeze the heart gently, and notice the flaps which rise to close the openings into the ventricles. Could blood pass from the ventricles to the auricles? Why not? Lay open the bases of the vessels _P.A._ and _Ao._ leading from the right and left ventricles respectively, and notice the pockets of thin membrane which are attached there. Open them out gently with the point of a pencil. Put the heart under the tap and let water trickle down the vessels towards the ventricles; notice how the pockets stand out as they fill with water. What would be the effect of blood trying to pass from _P.A._ or _Ao._ back into their respective ventricles?

=The circulation of the blood.=—The blood of such an animal as the rabbit is contained in a system of closed tubes called blood-vessels, through which the fluid is continually flowing. The regular flow of the blood in one direction is maintained by the action of the =heart=, a four-chambered organ which is situated in the chest, between the lungs. The heart is muscular, and, like other muscles (p. 225), has the power of “contracting” in definite directions. The contraction of the walls of the chambers of the heart lessens their capacity, and therefore drives the blood out of them, the direction of flow being determined by valves. The vessels which carry blood _to_ the heart are called =veins=; those which transmit the blood which is pumped _out of_ the heart are called =arteries=. The arteries branch into smaller and smaller tubes which supply the various organs of the body, and the small arterial branches divide again and again in the organs until their finest ramifications form a close-meshed network of vessels with excessively thin walls, through which diffusion can readily take place between the tissues and the blood. These finest blood-vessels are called =capillaries=. They can be studied in the transparent web of the frog’s foot with the aid of a low power of the microscope. The blood can then be seen flowing, at a speed which varies with the size of the vessel, its course being rendered obvious by the tiny oval particles (red corpuscles) which are suspended in it. The smallest capillaries are so thin-walled that they appear to be merely channels in the substance of the web, and the corpuscles creep along in single file. But these channels unite to form larger vessels with obvious walls, and these unite again and again until a main vein is formed in which the blood, with its suspended corpuscles, rushes along in a swift torrent towards the heart.

=The heart.=—The beginner will find the sheep’s heart more convenient for examination than the rabbit’s, on account of its larger size; but apart from some difference in the arrangement of the great blood-vessels opening into them, the two hearts are broadly similar.

The heart (Fig. 164) consists of four chambers. Two of these, the =auricles=, are receiving-chambers, and are placed at the thick, anterior end of the heart. Into the right auricle open the great veins (_SVC_, _IVC_,) which bring blood from all parts of the body except the lungs; the left auricle receives only blood from the lungs. Each auricle opens into a more posterior chamber called a =ventricle=, the right auricle opening into the right ventricle and the left auricle into the left ventricle. The ventricles pump blood into the arteries. The blood from the right ventricle is sent into the artery (_P.A._) which supplies the capillaries of the lungs; while the blood of the left ventricle is forced into the aorta (_Ao._), a great artery which branches and supplies with blood all parts of the body except the lungs. The two auricles contract simultaneously, forcing their contents into the flaccid ventricles. Then both the filled ventricles contract at once, and pump blood into the great arteries, flaps of membrane between the auricles and ventricles preventing a backward flow into the auricles. Similarly the bases of the great arteries are provided with membranous pockets which readily admit the blood from the ventricles when these contract, but entirely prevent a return of blood to the ventricles. The appearance of these four sets of valves, as seen from above, is shown in Fig. 165. After the contraction of the two ventricles there is a short rest, then the auricles contract again and the whole process is repeated.

The =sounds= which are heard when the ear is placed over another person’s heart are often compared to the syllables _lub-dup_. The “lub” is partly caused by the contraction of the ventricles; the “dup,” which immediately follows, is caused by the sudden closure of the valves at the bases of the great arteries. The throb of the heart, which can be felt from the outside, is really the thrust of the apex of the heart against the chest-wall at each beat. The sudden forcing of blood into the already-full but elastic arteries causes a wave to travel along these vessels, which can readily be felt, or even seen, at places where a fairly large artery lies just beneath the skin. This arterial wave is called the =pulse=. Many of the veins are provided with pouch-shaped valves which permit the blood to flow freely towards the heart, but which bar the passage of blood in the opposite direction. Their action will readily be understood from Figs. 166 and 167.

=The importance of the capillaries.=—The capillaries are the least conspicuous part of the circulatory system, but they are by far the most important. The heart, arteries, and veins exist merely to renew constantly the blood which flows through these minute channels. The excessive thinness of the walls of the capillaries makes it possible for a ready exchange to take place between the living tissue and the blood, and the vessels themselves form a network of such extremely close texture that it is practically impossible to prick any living part of the body with a fine needle without puncturing some of them and “drawing blood.” The work of the blood in supplying the various organs of the body with food has already been referred to. We have next to see how this all-important fluid is of service in supplying the organs with oxygen.

47. RESPIRATION.

1. =Carbon dioxide is formed when flesh burns.=—Dry a piece of meat and attach it to the end of a wire. Then light it, and when it is burning vigorously lower it into a clean glass jar. When the flame goes out remove the charred flesh, and at once pour a little clear lime-water into the jar and shake up. The lime-water turns milky, showing the presence of carbon dioxide gas in the jar. Examine what is left of the meat. It is charred, showing that meat contains carbon. How was the carbon dioxide formed during the burning?

2. =Carbon dioxide is formed by the living body.=—Breathe through a glass tube into clear lime-water, so that the air you expel from your lungs bubbles through the liquid. Does the lime-water remain clear, or turn milky? Does the air you breathe out contain a considerable quantity of carbon dioxide gas?

=Burning and life.=—When a piece of the flesh of any animal has been dried it may easily be set on fire. The burning is caused by the union of the constituents of the flesh with some of the oxygen of the air to form various gases. One of these gases is carbon dioxide, formed by the combination of the carbon of the flesh with oxygen. Carbon is present in all the parts of animals and plants, as is evident from the separation of charcoal (an impure form of carbon) in the first stages of burning; the carbon dioxide gas which is formed may easily be recognised by the milkiness which it produces in clear lime-water. The liberation of heat, and the formation of carbon dioxide, which always accompany the burning of animal and plant tissues, are worthy of very careful attention.

It is well known that the body of a living animal such as a rabbit or a man is always _warm_; and the experiment (Fig. 168) of passing through clear lime-water the air breathed out from the lungs shows, by the milkiness produced, that the animal is also constantly producing carbon dioxide during its life. Is life, then, always accompanied by a peculiar form of burning, in which the living substance of the body is the combustible material? It seems so, and the experiments of physiologists tend to confirm this view.

=The necessity for breathing.=—The energy which enables a muscle to contract is derived from the oxidation—the slow burning—of part of its substance, just as the energy which enables a steam-engine to move is derived from the burning of fuel in the boiler fires. The fires soon go out, and the engine stops, unless fresh fuel is added from time to time and a plentiful supply of air is available. Similarly, a muscle loses its power of contracting, a gland that of secreting, the brain that of thinking, unless the waste matters resulting from previous activities are cleared away and replaced by fresh food and fresh oxygen. Wherever vital action is taking place, whether in a contracting muscle, a secreting gland, or a thinking brain, there is continual consumption of oxygen and continual production of waste material, chiefly carbon dioxide. In the higher animals the renewal of oxygen and the removal of waste material are performed by the blood. Blood-vessels are to the body what rivers and canals are to a country: they act as highways for the transport of material. We may perhaps carry the analogy a little farther and find in the red corpuscles of the blood the rough equivalents of boats or canal-barges, for they carry with them tiny loads of oxygen. As the blood creeps along the narrow channels in an active tissue the red corpuscles relinquish their oxygen, and the fluid portion of the blood takes up carbon dioxide. The blood continues its course, and sooner or later arrives at a place where it can obtain a fresh supply of oxygen and get rid of its surplus carbon dioxide. In the rabbit this exchange takes place as the blood is passing through the capillaries of the =lungs=. There the blood is separated from the air by a membrane so delicate that gases can readily pass through it; and, hence, on leaving the lungs the blood has got rid of the waste carbon dioxide, and its red corpuscles are laden with fresh oxygen. _This exchange of useless carbon dioxide for oxygen constitutes respiration or breathing._

=The mechanism of respiration.=—In active animals the air inside the lungs soon becomes vitiated, unless there is some means of changing it. Under ordinary conditions a man changes the air in his lungs from thirteen to fifteen times a minute. He does this quite automatically, and without thinking about it. Every four seconds or so a set of muscles contracts and pulls his ribs upwards and outwards; another muscular contraction pulls down the floor of his chest at the same time. As a consequence the cavity is much enlarged. The lungs follow the movements of the walls of the chest, and some thirty cubic inches of air are sucked in. Immediately the ribs fall back to their former position, the chest-floor rises, and air is driven out. Then after a short pause the process is repeated. It should be noticed that only about thirty cubic inches of air are changed at each respiration, although the capacity of the human lungs averages about 230 cubic inches. All mammals breathe in much the same way.

=Plants and animals.=—There are considerably more points of similarity than of difference between plants and animals. In every case the vital activities are accompanied by an oxidation of living substance, and from this fact arises the necessity for food and oxygen. The breathing of plants is essentially like that of animals, and consists in taking in oxygen and giving out carbon dioxide; though the _mechanism_ of respiration is—except in the lowest plants and animals—entirely different in the two cases. It is in the sources from which they obtain their =food= that plants and animals are most unlike. An animal must be supplied with food which has already been prepared by some other living thing; and it is obvious that the food even of carnivorous animals can ultimately be traced back to plants, for the flesh-eater preys upon the vegetarian. Animals are therefore entirely dependent upon plants for food. In this sense the saying “all flesh is grass” is full of significance.

Green plants (Chapter II.) are quite independent of all other forms of life, and can build up their substance from water, mineral matter, and the carbon dioxide of the air. The taking in of carbon dioxide and giving out of oxygen by green plants has nothing whatever to do with respiration; it is part of their process of feeding. Green plants breathe in the usual manner—by taking in oxygen and giving out carbon dioxide. It should, however, be noticed that the peculiar method by which a green plant obtains its carbonaceous food is of the highest importance to animal life; for by this process the amount of injurious carbon dioxide in the air is considerably lessened, while the proportion of life-supporting oxygen in it is greatly increased.

Fungi (Chapter XI.) seem to be intermediate, as regards their method of feeding, between green plants and animals. They require their carbonaceous food in an organic form, that is, already prepared by other living things; but they can obtain the other elements of their food from mineral salts and water.

The thoughtful student will be increasingly impressed by the extent to which the plant and animal kingdoms are dependent upon each other, and by the manner in which each utilises the waste products of the other for carrying on its own life processes.

EXERCISES ON CHAPTER XIII.

1. Of what parts does the skull of a rabbit consist? What is the use of each part?

2. Describe one of the long bones of a rabbit’s leg. To what features does it owe its strength?

3. The bones of the skeleton are useful (1) as affording points of attachment for the muscles; (2) as affording protection for delicate tissues and organs. Give examples of each of these uses. Do not give the technical names for the various muscles. (King’s Schol., 1902)

4. Draw and describe one of the middle joints of the backbone of a quadruped, and explain the uses of the various parts. (1898)

5. What is meant by “digestion”? Why must food be digested? Where does digestion take place?

6. Give full practical instructions for demonstrating the chief properties of saliva, and its action upon various kinds of food. (1897)

7. Prove that the action of human saliva upon starch is not due to living particles contained in it. (1898)

8. What are the chief uses of the blood? Why is it necessary that it should be kept in motion? (1901)

9. Where would you look for the Aorta in a sheep’s heart? What valves are found in it? How does it differ in appearance and feel from a large vein? (1901)

10. What kinds of valves are found in a sheep’s heart, and where are they placed? Describe a valve of each kind. (1898)

11. How does air breathed out from the lungs differ from common air? How can the differences be demonstrated? (1898)

12. Describe the process by which a mammal renews the air in its lungs.

13. What is meant by “respiration”? Why is respiration necessary?

14. Point out the most remarkable differences between the nutrition of a green plant and that of an animal. (1897)

15. Name organisms which can derive nourishment from carbon dioxide, from sugar, and from the dead bodies of animals. (1898)

16. What are the simplest functions which distinguish living animal matter from inanimate matter? (King’s Schol. 1903)

17. Explain as fully as you can how food taken into the stomach acts upon organs, such as the brain, which are not closely connected with the stomach. (1904)

18. The uses of bone are, generally speaking, to protect delicate structures, to support weight and to gain leverage. Illustrate this statement by a simple description of one example of each type. (Certificate, 1904)

19. The flow of liquids through the body is regulated in certain localities by valves. Explain the action of a valve, and indicate where they are to be found in the body. (Certificate, 1904)

20. In which kind of blood-vessel can the pulse be felt? In which kinds can it not be felt? Explain the reason of the difference. (Certificate, 1905)

21. How is oxygen conveyed from the lungs to the various parts of the body? Describe what could be observed if a drop of blood were spread out on a piece of glass and examined under a microscope. (King’s Schol. 1905)

FOOTNOTES:

[9] See footnote, p. 231.

[10] Prepared by adding to a solution of copper sulphate first tartaric acid, and then caustic soda until the blue mixture is clear. It may be obtained from a chemist if the materials are not available.

[11] NOTE TO TEACHERS.—A general dissection of a frog should be made and exhibited to the class. Detailed instructions for such a dissection will be found in Marshall’s _The Frog_ (Smith, Elder & Co.) or in Huxley and Martin’s _Elementary Practical Biology_ (Macmillan). The frog’s heart continues to beat for some time after the death of the animal, and may be shown as an illustration of the next section of this chapter. Teachers who are unskilled in dissection may obtain prepared dissections, mounted skeletons, etc., from Newmann & Co., 84 Newman Street, London, W.

[12] More detailed instructions will be found in Foster and Shore’s _Physiology for Beginners_ (Macmillan).