THE BRAIN 



81 



The first of the primary divisions, the prosencephalon, or 

 primary forebrain, gives rise during development to a pair of 

 hollow outgrowths and thus becomes divisible into an anterior 

 portion, the endbrain or telencephalon, which is largely a 

 paired structure, and a second portion, unpaired, the diencephalon 

 or interbrain. The larger, paired portion of the telencephalon is 



Fig. 44. Plan of the divisions of the vertebrate brain: A, embryonic; 

 B, adult, projection from dorsal surface; C, adult, sagittal section. The con- 

 tour of the mammalian brain is indicated by broken lines. 



Primary divisions — PR, prosencephalon; T, telencephalon; DI, dience- 

 phalon; MS, mesencephalon; RH, rhoihbencephalon ; MT, metencephalon; 

 MY, myelencephalon; S, spinal cord. 



a.c, cerebral aqueduct; b.o, olfactory bulb; c, corpora quadrigemina; 

 c.a., anterior commissure; cb., cerebellum; cm., mamillary body; c.o., optic 

 chiasma; c.p., pineal body; f.i., interventricular foramen; h., hypophysis; 

 h.c, cerebral hemisphere; in., infundibulum; l.t., lamina terminalis; p., pons; 

 pi., chorioid plexus of third ventricle; p.c, cerebral peduncle; t., thalamus, 

 also indicates position of massa intermedia; v.l., lateral ventricle; v.m.p., 

 posterior medullary velum; v.q., fourth ventricle. 



