THE NERVOUS SYSTEM 129 



missure^ followed by a clearer area — the pulvinar. It is from the 

 posterior end of this region that the epiphysis (epi.) and its terminal 

 pineal body evaginate. They are only weakly developed in the Sala- 

 mander and never perforate the cranial roof, but flatten out over the 

 surface of the brain between it and the parietals. In the adult the 

 epiphysis becomes very closely applied to the surface of the brain, 

 but is distinguishable in sections (see Fig. 50). Eycleshymer (i 892) 

 states that the paraphysis and epiphysis in Urodela never come into 

 close relation as in Reptilia, but remain widely separated. The condi- 

 tion in the Salamander supports this statement. 



(iv) The corpus opticus (lob.op.) or mesencephalon follows. It is not 

 divided into two distinct lateral lobes as in the Frog, but expands 

 gradually from the roof of the diencephalon into a more or less hemi- 

 spherical dome. There is a faint groove in the middle line and the 

 nervous tissue is thinner there than elsewhere, this feature being more 

 obvious in transverse sections. The corpus opticum is far removed 

 from the cerebrum. At the posterior border of the mesencephalon 

 there is a deep groove between this and the subsequent portion of 

 the brain, and from this groove the N. trochlearis (fourth cranial 

 nerve) emerges. 



(v) The cerebellum (cerb.) or epencephalon is a very narrow trans- 

 verse band of nervous tissue forming the anterior boundary of 

 the fourth ventricle, and separated from the mesencephalon by the 

 groove above mentioned. This region is surprisingly slender, being 

 much smaller comparatively than the corresponding region in the 

 Frog. 



(vi) The medulla oblongata (med.ob.) or myelencephalon is a fairly 

 long (about 6| mm.) shallow trough of nervous tissue, open dorsally. 

 It forms about 42 per cent, of the total brain length. Its anterior end 

 is laterally expanded, and tapers gradually into a more or less circular 

 cross-section posteriorly, merging imperceptibly into the spinal cord. 

 Its cavity — the fourth ventricle — is roofed over dorsally by a rich 

 vascular plexus. The trigeminus (Vth) root arises ventro-laterally 

 from its anterior angles, and the acustico-facialis (Vllth and Vlllth) 

 root appears about | mm. posterior to this. The glossopharyngeus- 

 vagus complex (IX and X) arises laterally from the sides of the 

 medulla by three roots, of which the middle root is the largest, the 

 other two being very fine. 



The following additional features are evident on the ventral sur- 

 face of the brain (Fig. 52): 



(i) The emergence of the olfactory nerves (n.i) from the brain. 



4038 K; 



