THE NERVOUS SYSTEM 131 



In addition to the commissures and the thickness of the walls of 

 the brain in its various parts, the chief features of interest in a 

 sagittal section are: 



(i) The. foramen of Monro (fo.M.) connecting the lateral ventricles 

 — the cavities of the cerebral hemispheres — with the third ventricle 

 — the cavity of the thalamencephalon. This is a long oval slit 

 of relatively large dimensions, and through it the blood-vessels 

 pass from the nodus chorioideus to the lateral plexus of the first 

 ventricle. 



(ii) The remainder of the pituitary body may now be dis- 

 tinguished: 



(a) The pars intermedia (Atwell) (pit/). This is very small, 

 and lies dorsal to the pars anterior and behind the infundi- 

 bulum. 



(Jy) The pars anterior (pit.") is bi-convex in section. It forms by 

 far the largest portion of the gland. 



[c] The infundibular portion, or pars posterior^ shows considerable 

 retrogression and is almost non-existent. 



(iii) The purely epithelial portions of the brain surface are as 

 follows : 



{a) The lamina terminalis (lam.ter.) and antero-dorsal roof of the 

 thalamencephalon. 



{h) The dorsal wall of the infundibulum. 



{c) The posterior median portion of the mesencephalon. 



{d) The roof of the fourth ventricle. 



With regard to the commissures on the floor of the foramen of 

 Monro, the upper bundle was first homologized with the corpus 

 callosum by Osborn (1886). It had previously been thought to be 

 homologous with the pars olfactoria of the anterior commissure, but 

 Osborn showed that this part was present in the lower bundle of 

 fibres as well as the pars temporalis^ and thus the lower bundle is the 

 entire anterior commissure. 



Summary. 



The brain of the Salamander is straight, or very slightly flexed, 

 and small compared with that of the Frog, but long in comparison 

 with its breadth and depth. The pineal organ is much reduced, 

 while the paraphysis shows considerable development. The sepa- 

 ration of the optic lobes is very incomplete and the cerebellum 

 poorly developed. The olfactory lobes are paired and not fused 

 to form a median body as in the Frog. The foramen of Monro 

 is large. 



