Fig. 1.-The Viscera of a Rabbit as seen upon simply opening the Cavities of the Thorax and Abdomen without any further Dissection.
A, cavity of the thorax, pleural cavity of either side; B, diaphragm; C, ventricles of the heart; D, auricles; E, pulmonary artery; F, aorta; G, lungs, collapsed, and occupying only the back part of the chest; H, lateral portions of pleural membranes; I, cartilage at the end of sternum; K, portion of the wall of body left between thorax and abdomen; a, cut ends of the ribs; L, the liver, in this case lying more to the left than the right of the body; M, the stomach; N, duodenum; O, small intestine; P, the cæcum, so largely developed in this and other herbivorous animals; Q, the large intestine.
the leg, a great cavity. This is something quite new-there is nothing like it in the leg-a great cavity, quite filled with something, but still a great cavity; and if you slit the rabbit right up the front of its trunk and turn down or cut away the sides as has been done in Fig. 1, you will see that the whole trunk is hollow from top to bottom, from the neck to the legs.
If you look carefully you will see that the cavity is divided into two by a cross partition (Fig. 1, B) called the diaphragm. The part below the diaphragm is the larger of the two, and is called the abdomen or belly; in it you will see a large dark red mass, which is the liver (L). Near the liver is the smooth pale stomach (M), and filling up the rest of the abdomen you will see the coils of the intestine or bowel, very narrow in some parts (O), very broad (P Q), broader even than the stomach, in others. If you pull the bowels on one side as you easily can do, you will find lying underneath them two small brownish red lumps, one on each side. These are the kidneys.
In the smaller cavity above the diaphragm, called the thorax or chest, you will see in the middle the heart (C), and on each side of the heart two pink bodies, which when you squeeze them feel spongy. These are the two lungs (G). You will notice that the heart and lungs do not fill up the cavity of the chest nearly so much as the liver, stomach, bowels, &c. fill up the cavity of the belly. In fact, in the chest there seems to be a large empty space. But as we shall see further on, the lungs did quite fill the chest before you opened it, but shrank up very much directly you cut into it, and so left the great space you see.
9. The trunk then is really a great chamber containing what are called the viscera, and divided into an upper and lower half, the upper half being filled with the heart and lungs, the lower with the liver, stomach, bowels, and some other organs. In front the abdomen is covered by skin and muscle only. But if all the sides of the trunk were made of such soft material it would be then a mere bag which could never keep its shape unless it were stuffed quite full. Some part of it must be strengthened and stiffened. And indeed the trunk is not a bag with soft yielding sides, but a box with walls which are in part firm and hard. You noticed that when you were cutting through the front of the chest you had to cut through several hard places. These were the ribs (Fig. 1, a), made either of hard bone or of a softer gristly substance called cartilage. And if you take away all the viscera from the cavity of the trunk and pass your finger along the back of the cavity, you will feel all the way down from the neck to the legs a hard part. This is the backbone or vertebral column. When you want to make a straw man stand upright you run a pole right through him to give him support. Such a support is the backbone to your own body, keeping the trunk from falling together.
In the abdomen nothing more is wanted than this backbone, the sides and front of the cavity being covered in with skin and muscle only. In the chest the sides are strengthened by the ribs, long thin hoops of bone which are fastened to the backbone behind and meet in front in a firm hard part, partly bone, partly cartilage, called the sternum.
But this backbone is not made of one long straight piece of bone. If it were you would never be able to bend your body. To enable you to do this it is made up of ever so many little flat round pieces of bone, laid one a-top of the other, with their flat sides carefully joined together, like so many bungs stuck together. Each of these little round flat pieces of the backbone is called a vertebra, and is of a very peculiar shape. Suppose you took a bung of bone, and fastened on to one side of its edge a ring of bone. That would represent a vertebra. The solid bung is what is called the body, and the hollow ring is what is called the arch of the vertebra. Now if you put a number of these bodies together one upon the top of the other, so that the bodies all came together and the rings all came together, you would have something very like the vertebral column (see Frontispiece, also Fig. 2). The bungs or bodies would make a solid jointed pillar, and the rings or arches would make together a tunnel or canal. And that is really what you have in the backbone. Only each vertebra is not exactly shaped like a bung and a ring; the body is very like a bung, but the arch is rough and jagged, and the bodies are joined together in a particular way. Still we have all the bodies of the vertebræ forming together a solid pillar which gives support to the trunk; and the arches forming together a tunnel or canal which is called the spinal canal, (Fig. 2, C.S.) the use of which we shall see
Fig. 2. A, a diagrammatic view of the human body cut in half lengthways. C.S., the cavity of the brain and spinal cord; N, that of the nose; M, that of the mouth; Al. Al., the alimentary canal represented as a simple straight tube; H, the heart; D, the diaphragm.
B, a transverse vertical section of the head taken along the line a b; letters as before.
C, a transverse section taken along the line c d; letters as before.
directly. The round flat body of each vertebra is turned to the front towards the cavity of the trunk, and it is the row of vertebral bodies which you feel as a hard ridge when you pass your fingers down the back of the abdomen. The arches are at the back of the bodies, so you cannot feel them in the abdomen; but if you turn the rabbit on its belly and pass your finger down its back, you will feel through the skin (and you can feel the same on your own body) a sharp edge, formed by what are called the spines, i.e. the uneven tips of the arches of the vertebræ (Fig. 2) all the way down the back.
So that what we really have in the trunk is this. In front a large cavity, containing the viscera, and surrounded in the upper part or thorax by hoops of bone, but not (or only slightly) in the lower part or abdomen; behind, a much smaller long narrow cavity or canal formed by the arches of the vertebræ, and therefore surrounded by bone all the way along, and containing we shall presently see what; and between these two cavities, separating the one from the other, a solid pillar formed by the bodies of the vertebræ. So that if you were to take a cross slice, or transverse section as it is called, of the rabbit across the chest, you would get something like what is represented in Fig. 2, C, where C.S. is the narrow canal of the arches and where the broad cavity of the chest containing the heart H is enclosed in the ribs reaching from the vertebra behind to the sternum in front. Both cavities are covered up on the outside with muscles, blood-vessels, nerves, connective, and skin, just as in the leg.
10. We have now to consider the head and neck. If you cut through the skin of the neck of the rabbit, you will see, first of all, muscles and nerves, and several large blood-vessels; but you will find no large cavity like that in the trunk. So far the neck is just like the leg. But if you look carefully you will see two tubes which are not blood-vessels, and the like of which you saw nowhere in the leg. One of these tubes is firm, with hardish rings in it; it is the windpipe or trachea; the other is soft, and its sides fall flat together; this is the gullet or osophagus, leading from the mouth to the stomach. Behind these and the muscles in which they run you will find, just as in the trunk, a vertebral column, without ribs, but composed of bodies, and behind the bodies there is a vertebral canal. This vertebral column and vertebral canal in the neck are simply continuations of the vertebral column and canal of the trunk.
The neck, then, differs from the leg in having a vertebral column and canal with a trachea and osophagus, and differs from the trunk in having no cavity and no ribs.
The head, again, is unlike all these. Indeed, you will not understand how the head is made unless you take a rabbit's skull and place it side by side with the rabbit's head. If you do this, you will at once see how the mouth and throat are formed. You will notice that the skull is all in one piece, except a bone which you will at once recognize as the jawbone, or, to speak more correctly, the lower jawbone; for there are two jawbones. Both these carry teeth, but the upper one is simply part of the skull, and does not move; the lower one does move; it can be made to shut close on the upper jaw, or can be separated a good way from it. The opening between the two jaws is the gap or gape of the...
