NERVOUS SYSTEM 



373 



cerebellum is entirely absent. The ventricles are subject to 

 some individual variation. Third and fourth ventricles are 

 generally recognisable, either as isolated cavities or connected 

 by a remnant of the mesocoele. In the feeble development of 

 the prosencephalon, in the striking preponderance of the mid-brain 



A 



FIG. 213. The brain of a Dog-Fish (Scyllium canicula). A, dorsal view ; B, ventral 

 view. The choroid plexuses covering the roof of the third and fourth ventricles 

 have been removed, b.o, Olfactory lobe ; ep, origin of the stalk of the pineal body ; 

 f.b (in A), prosencephalon ; f.b (in B), cerebral hemispheres ; fr, fourth ventricle ; 

 h. b, cerebellum; h.p, pituitary body; i.f, lobi iuferiores ; m. b, optic lobes; m./l, 

 medulla oblongata ; sc, saccus vasculosus ; th, thalameucephalon ; t.o (i) olfactory 

 peduncle; i.-x. cranial nerves. (From Wiedersheim. ) 



over the rest of the brain, and in the absence of a cerebellum, 

 Myxine is unique amongst Craniates. 



In Elasmobranchs among Fishes the brain attains a much 

 higher grade of structure. In Scyllium (Fig. 213) there is 

 a large prosencephalon, and directly in front of it a pair of 

 imperfectly differentiated cerebral hemispheres, while from its 

 antero-lateral regions the large olfactory lobes arise. The proso- 

 coele divides in front into four diverticula, of which the two 



