CHAPTER IV
THE NERVOUS SYSTEM
Spinal Cord
The spinal cord extends the whole length of the vertebral canal and ends near the end of the tail as a thin, round thread. It varies in thickness and shape in cross section, being nearly always elliptical, but at places approaching a circle. Large, spindle-formed thickenings of about equal diameter are present in the cervical and lumbar regions.
A cauda equina is absent in the alligator, the nerves of the large tail leaving the cord like the intercostals.
On its ventral surface the cord has a deep perpendicular fissure, the fissura ventralis, that extends almost to the center; it extends even along the reduced region in the tail. A vascular membrane extends into this fissure.
A shallow but distinct furrow extends along the dorsal side of the cord, parallel to which, on either side, is a fine, linear furrow.
The first two spinal nerves have no dorsal roots.
Brain
The cervical cord passes insensibly into the medulla, the dorsal furrow becoming wider and more shallow as it merges into the fourth ventricle.
A dorsal view of the brain is shown in [Figure 30], A. The most prominent structures here seen are the cerebral hemispheres, VH, whose combined transverse diameter is greater than their longitudinal. The tapering, cephalic end of each hemisphere forms an olfactory tract, I, which extends cephalad to form the olfactory bulb, B. ol. Lying between the caudal ends of the hemispheres is a small conical body, G.p., called by Bronn and others the pineal body. The writer has found ([62]), however, that this body is the paraphysis rather than the epiphysis. Caudad to the cerebra-hemispheres and in contact with them are the optic lobes, MH; they have about the same shape and position as in the frog, but are much smaller in proportion to the size of the hemispheres. Immediately caudad to the optic lobes is the cerebellum, HH, somewhat elliptical in outline as seen from above.