THE BRAIN 



157 



indicate that they had well-developed parietal or pineal organs. The 

 roof of the brain in this region, behind the lamina terminalis, also 

 gives rise to a chorioid plexus like that of the fourth ventricle, a part 

 of which invades the third ventricle and another portion, the inferior 

 plexus, sends branches through the foramina interventricuiares into 

 the ventricles of the hemisphere, thus providing for a blood supply on 

 the interior of these structures (fig. 160). 



The floor of the diencephalon remains thinner behind the optic 

 recess, a portion of it becoming funnel-shaped and pushing out from 

 the ventral surface toward the roof of the mouth. This is the jn- 

 fundihulUBi, which meets an ectodermal diverticulum, the hypophy- 

 sis- This arises, in the cyclostomes from 

 the ectoderm between the nostril and the 

 mouth (fig. 212, //); in other vertebrates 

 from the roof of the oral cavity (figs. 159, 

 161). It retains its connexion with the 

 parent epithelium for a time, the point of 

 ingrowth being known as Rathke's pock et. 

 Later the stalk disappears, and the infun- 

 dibulum and hypophysis, closely associated. 

 He just beneath the brain in the sella tur- 

 cica on the floor of the skull (p. 68). In 



the hypophysis (pituitary body) two parts /. interventricular foramen; 



,. ,. • 1 J /7 T-^ 1 I \ • 1. • *' region of hippocampus; /, 



are distmguished (fig. 160, hg, hn) rich m lateral ventricle; n, neopallial 



blood 



gland of internal secretion whose action is 



connected with the fat-storing powers of the animal. The infundib- 



ulum may be a simple pit, as in most vertebrates, or its lateral walls 



may become enlarged and folded, blood-vessel lying in the folds, 



and the whole forming the so-called saccus vas culosus. The paired 



eyes are also connected with the 'twixt-bram, their nerves crossing 



the ventral surface (opticxhiasma) just in front of the infundibulum, 



on their way to th5ir termination in the optic lobes. Associated 



with the optic chiasma may be a postoptic commissure, connecting 



the ventral parts of this region of the brain. 



Cerebrum. — The cerebrum or telencephalon consists of two hemi- 

 spheres separated in front by the longitudinal or intercerebral fissure, 

 which is slight in fishes, but well marked in the other groups. It is 

 the extreme anterior part of the brain and is bounded posteriorly by 

 the velum transversum and the optic chiasma. Hence (see fig. 163) 



Fig. 163. — Section through 

 brain of embryo Sphenodon, 

 just behind the lamina ter- 

 minalis, after G. Elliot Smith. 



and lymph-vessels and forming a ^^g^^'^J />, paraterminal body; 

 J t' & 5, corpus striatum. 



