The alligator and its allies

by A. M. Reese

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.

Extending caudad from beneath the cerebellum is the medulla, NH, with its triangular fourth ventricle. The outlines of the medulla are somewhat obscured by the numerous roots of the eighth to eleventh cranial nerves, VIII-XI, which arise along its dorsal border. The medulla, as was said above, passes, as is usually the case, without any line of demarcation into the spinal cord, the obex filling in the apex of the fourth ventricle at the anterior end of the median dorsal fissure.

Fig. 30. Brain of Alligator. (A, dorsal; B, ventral; and C, lateral view.) (From Wiedersheim, slightly altered.)

VH, cerebral hemispheres, each of which gives rise postero-laterally to a hippocampal lobe partially overlying the corresponding optic tract, Tr.opt; ZH, thalamencephalon; MH, optic lobes; HH, cerebellum; NH, medulla oblongata; I-XII, cranial nerves; 1, 2, first and second spinal nerves; B.ol, olfactory bulb; Tro, olfactory tract; G.p, paraphysis; Jnf, infundibulum; Hyp, hypophysis; Med, spinal cord.

A lateral view of the brain is shown in [Figure 30], C. The hemisphere, VH, is conical in outline, with a small projection from the posteroventral region; its continuation forwards as the olfactory tract, Tro., and bulb, B. ol., is plain. Beneath it and extending forwards are the prominent optic nerve, II, and tract. Caudad to the latter and projecting ventrad and caudad are the infundibulum, Inf., and hypophysis, Hyp.

Caudad to the cerebrum are seen the end of the paraphysis, G.p., the optic lobes, MH, and the cerebellum, HH. From the cerebral peduncles (ventrad to the optic lobes) arises the oculomotor nerve, III, and dorsocaudad to this, between the optic lobe and the cerebellum, arises the trochlear nerve, IV. From the middle zone (in a dorsoventral direction) of the medulla, ventrad to the cerebellum, arises the very large trigeminal nerve, V; while from its usual place, on the ventral surface of the medulla, the abducens nerve, VI, takes its origin by several roots. At some distance caudad from the trigeminal, from the dorsal surface of the medulla, as noted above, the very large acoustic nerve, VIII, arises; and immediately ventrad to this, on the side of the medulla, the facial nerve, VII, may be seen. Commencing just caudad to the acoustic and extending along the upper border of the medulla and beginning of the spinal cord, are seen a dozen or more small nerve roots, which unite to form the glossopharyngeal, IX, vagus, X, and spinal accessory, XI, nerves. Ventral to the roots of the last, on the ventral surface of the medulla, arise the roots of the hypoglossal nerve, XII. A short distance caudad to this nerve are seen the first two spinal nerves, 1 and 2, which have, as noted above, no dorsal roots.

A ventral view of the brain is shown in [Figure 30], B. The cerebral hemispheres, VH, have the same outline, of course, as in the dorsal view, but the rounded projection from the caudal end of each is here seen on each side of the infundibulum, Inf. The infundibulum is in close contact with the chiasma anteriorly, and lies close between the converging optic tracts, Tr. opt. From the chiasma the optic nerves, II, extend, in an antero-lateral direction, almost at right angles to each other. The appearance of the olfactory tracts, I, is the same as in the dorsal view. Caudad to the infundibulum, from the cerebral peduncles, ZH, arise the rather small oculomotor nerves, III. Caudad to these, from near the ventral fissure, on the middle region of the medulla, arise the abducens nerves, VI, and from the ventral side of the posterior part of the medulla and of the anterior end of the cord arise the hypoglossal, XII, and the first two spinal nerves, 1 and 2. The origins of the other cranial nerves were described in connection with the lateral view of the brain, where they show more clearly. On each side of the cerebral peduncles is seen the ventrolateral edge of the corresponding optic lobe. The pyramidal tracts are seen, extending caudad from the region of the peduncles, as a swelling on each side of the median ventral fissure.