THE ELASMOBRANCII FISHES 



235 



It is so in LainiKi, Scoliodon (fig. 211a), Galeus, Tvijcjon, and Myliobaiis (fig. 

 212). In all these the surface is thrown into numerous irregular folds or 

 convolntions. 



The myelenc'ei)halon (medulla) {nied., figs. 211-21:]) when seen in dorsal 

 view is shaped like the letter Y, the anterior limits of the Y being made by the 

 restiform bodies, corpora restiforme {cr.). These in many of the simpler types 

 of sharks, such for example as Scymnus, appear as prominent structures. In 

 others the corpora restiforme are entirely hidden by the enlarged cerebellum 

 (Myliohafis, fig. 212) . Both in dorsal and in ventral view the medulla is conical 



Fig. 214. Stages A and B in the development of the pineal region of Acanthios. (From 

 Minot.) 



ep., epiphysis; pa., paraphysis; p.c, posterior commissure; p.v., postvelar arch; s.c, 

 superior commissure; v., velum. 



in shape and tapers gradually l)ack to the spinal cord. It is from this segment 

 that most of the cranial nerves arise; all in fact except the first four take 

 their origin here. 



INTEENAL VIEW OF BRAIN 



A sagittal section through the brain ol Scymnus (fig. 213b) shows within the 

 medulla the large cavity of the fourth ventricle. The floor and sides compose 

 the fossa rhomboidealis. Along each side of the middle line of this fossa are 

 two median longitudinal bundles, fasiculi longitudinales mediales (flm.). 

 Running parallel to these in the posterior part of the fossa are the ventro- 

 lateral bundles, fasiculi lateroventrales. Above these bundles and in the side 

 wall are the lobes of the vagus (l.v.) which vary in numl)er of segments from 

 types in which only a few are present to those in which there are several 

 nodules {Heptanchns and Hexanchus). Above the lobes is a dorsolateral 

 bundle which continues forward as the bundle to the tuberculum acusticum. 

 Above this are other bundles which continue into the ridge of the restiform 

 bodies or corpora restiforme. 



In the region of the mesencephalon the cavity is so large that it is an aque- 

 duct of Sylvius only in name. It connects the cavity of the metencephalon and 



