. The elasmobranch fishes. Chondrichthyes. 222 THE ELASMOBRAXCH FISHES. Fig. 201. Cross-section of cord, Hep- tanchus cinereus. (From Sterzi.) (Grey matter stippled.) , dorsal horn; , ventral horn. the mesencephalon and near the cerebellnm the fourth cranial or trochlear nerve arises {IV). Ventrally the mesensephalon is composed of large fiber tracts through which the third cranial or oculomotor nerve (7/7) emerges and passes forward to muscles of the eye. The metencephalon consists, in large part, of the cerebellum (c6.), a large shield-shaped mass, separated dorsally into right and l
. The elasmobranch fishes. Chondrichthyes. 222 THE ELASMOBRAXCH FISHES. Fig. 201. Cross-section of cord, Hep- tanchus cinereus. (From Sterzi.) (Grey matter stippled.) , dorsal horn; , ventral horn. the mesencephalon and near the cerebellnm the fourth cranial or trochlear nerve arises {IV). Ventrally the mesensephalon is composed of large fiber tracts through which the third cranial or oculomotor nerve (7/7) emerges and passes forward to muscles of the eye. The metencephalon consists, in large part, of the cerebellum (c6.), a large shield-shaped mass, separated dorsally into right and left halves by a median groove. The cerebellum extends anteriorly over the posterior half of the optic lobes, and posteriorly it overlies the fol- lowing division of the brain. Ventrally and under the cerebellum are heavy fiber tracts which also belong to the segment of the metencephalon. The myelencephalon is the last seg- ment of the brain. It comprises the me- dulla oblongata (»ief?.) which from above extends upward on each side almost to the tip of the cerebellum as the resti- form bodies (corpora restiforme, ). Back of the restif orm bodies the medulla grows smaller and smaller in diameter until it joins the cord. Within the myelencephalon and the metencephalon is the fourth ventricle which is covered over by a thin roof. The myelencephalon gives rise to all the cranial nerves back of the fourth. SPINAL COED The spinal cord extends from the medulla practically to the tip of the tail. Externally, as is seen in figure 201, it presents no evidence of a dorsal or a ventral groove which in some Elasmobranchs (see fig. 217b) may mark the boundaries of the cord into right and left halves. Superficially the cord con- sists of numerous fiber tracts surrounding masses of nerve cells. In the section (fig. 201) the central, more solid part of the cord constitutes the grey matter (cells) roughly in the form of an X, the upper arms of which are the dorsal horns {) and the lower
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