[8.] If now you turn to the trunk and cut through the skin of the belly, you will first of all see muscles again, with nerves and blood-vessels as before. But when you carefully cut through the muscles (for you cannot easily separate them from each other here), you come upon something which you did not find in
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.