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.

Section 5. The short strong fore limbs are adapted to the burrowing
habit, and have five digits; the hind limbs are very much longer and
muscular, enable the animal to progress rapidly by short leaps, and
they have four toes. If the student thinks it worth while to attempt to
remember the number of digits-- it is the fault of examiners if any
value dose attach to such intrinsically valueless facts-- he should
associate the number 54 (5 in front, 4 behind) with the rabbit, and
observe that with the frog the revers