Section 2. At the fore end, or as English zoologists prefer to say, anterior end, of the vertebral column of the rabbit, is of course the skull, containing the anterior portion of the nerve axis, the brain (Figure 1 br.). Between the head and what is called "the body," in the more restricted sense of the word, is the neck. The neck gives freedom of movement to the head, enables the animal to look this way and that, to turn its ears about to determine the direction of a sound, and to perform endless motions in connexion with biting and so forth easily. We may note that in types which swim through the water, the neck dose not appear-- in the fish and frog, for instance-- and the head simply widens out as one passes back to the body. The high resistance offered by water necessitates this tendency to a cigar or ship outline, just as it has determined the cigar shape of the ordinary fish torpedo. Section 3. In the body of the rabbit, as examined from the outside, we can make out by feeling two distinct regions, just as we might in the body of a man; anteriorily a bony cage, having the ribs at the sides, a rod-like bone in the front, the sternum (Figure 1 -st.-, [stm.]), and the backbone behind, and called the chest or thorax; and posteriorily a part called the abdomen, which has no bony protection over its belly, or ventral surface. These parts together with the neck constitute the trunk. As a consequence of these things, in the backbone of the rabbit there are four regions: the neck, or cervical part, consisting of seven vertebrae, the thoracic part of twelve joined to ribs, the abdominal (also called the lumbar) region of seven without ribs, and the tail or caudal of about fifteen. Between the lumbar and caudal come four vertebrae, the sacral, which tend to run together into a bony mass as the animal grows old, and which form a firm attachment for the base of the hind limb. Section 4. The thorax and abdomen are separated by a partition, the diaphragm (Figure 1 dia.). This structure is distinctive of that class of the vertebrata called mammals, and which includes man, most of the larger and commoner land animals, and whales and manatee. We shall find later that it is essentially connected with the perfection of the air breathing to which this group has attained. Another characteristic shared by all mammals, and by no other creature, is the presence of hair. In birds we have an equally characteristic cover in the feathers, the frog is naked, and the fishes we find either naked skins or scales.