NERVOUS SYSTEM. VERTEBRATA. 73 



third ventricle and extends slightly into the lateral ven- 

 tricles. 



The cerebellum has been divided in the mid-line, and the 

 two halves have been separated to show the large cerebellar 

 cavity and the course of the longitudinal zones along the 

 roof on either side of the mid-line. The latter are specially 

 well seen coursing backwards upon the ventral wall of the 

 anterior lobe. 



The floor of the fourth ventricle has been exposed by 

 turning the vascular roof to one side. Owing to the divi- 

 sion of the cerebellum and to the somewhat unnatural 

 separation of the side walls of the medulla, the fore parts 

 of the fasciculi longitudinales and of the vagal lobes are 

 brought into view. The fine transverse strands visible at 

 intervals on either side of the fasciculi posteriores are 

 tracts connecting them with the motor nuclei of the cranial 

 nerves. The motor nucleus of the vagus and the relation 

 of the root of the superficial ophthalmic lateral-line nerve 

 to the lobus linese lateralis are particularly clear. 



O.C. 1311 BC. 

 Presented by W. Coivan, Esq. 



D. 70. Parts of the cranium of a Spinous Shark (Ecliinorhinus 

 spinoms) with the brain shown from the dorsal aspect. 

 This brain, like those of the two other representatives of 

 the Spinacidse (Acantliias and Lcemargus), is of an essen- 

 tially simple type. This is particularly shown by the 

 relatively large size of the medulla (it occupies about half 

 the brain-length), its extensive rhomboid fossa and gradual 

 passage into the cord. The cerebellum is rounded and, for 

 that of an Elasmobranch, very poorly developed ; it shows 

 a feeble transverse groove. The parts of the brain in front 

 of the cerebellum are somewhat indefinite in outline owing 

 to imperfect preservation, but it can be v seen that the optic 

 lobes and nerves are small, the thalamencephalon fairly 

 long, and the cerebrum of medium size, without median 

 division and apparently with a considerable cavity. 



From either side of the cerebrum is given off an 

 immense solid olfactory nerve, without however any cor- 



