222 



THE ELASMOBRAXCH FISHES 



Fig. 201. Cross-section of cord, Hep- 

 tanchus cinereus. (From Sterzi.) (Grey 

 matter stippled.) 



d.h., dorsal horn; v.h., 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, c.r.). 

 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 {d.h.) and the lower arms, the ventral horns {v.h.) of the 

 cord. The less conspicuous dorsal horns lie near the middorsal line, while the 

 ventral horns extend outward and downward as an expanded mass on each 

 side. Furthermore there is a median ventral mass extending toward the mid- 

 ventral line from the union of the ventral horns. Wliere the crossing of the X 

 takes place there is a considerable thickening, in which is located the neuro- 

 coele or cavity of the spinal cord. It will be observed that the neurocoele is 

 practically at the middle of the section and is circular in outline. 



Peripheral Nervous System 



The cranial and spinal nerves arising from the central nervous system per- 

 forate the cranium and spinal column respectively. Also perforating the cra- 

 nium but back of the last cranial nerve are certain accessory nerves. 



