590 BRAIN CHAP, xm 



from its dorsal surface. The brain now consists of five 

 divisions : the prosencephalon (prs. en) and diencephalon 

 (dieri) derived from the fore-brain, with the pineal 

 structures (pn. b, pn. e) and the infundibulum, to which 

 the pituitary body becomes attached (inf, pty) ; the 

 mid-brain or mesencephalon (m. b), which gives rise to 

 the optic lobes and crura cerebri ; and the epencephalon or 

 cerebellum (cblm) and metencephalon or medulla oblongata 

 (med. obi) derived from the hind-brain. The original 

 cavity of the brain becomes correspondingly divided 

 into a series of chambers or ventricles (compare Figs. 156 

 and 50), all communicating with one another, and called 

 respectively the fore-ventricle or prosoccele, third ventricle 

 or diaccele, mid-ventricle or mesoccele (iter, and optic ven- 

 tricles or optocceles), cerebellar ventricle or epiccele, and 

 fourth ventricle or metaccele. 



In some fishes (e.g. dogfish, Fig. 124) the brain con- 

 sists throughout life of these five divisions, but in most 

 cases (Figs. 49 and 141) the prosencephalon grows out 

 into paired lobes, the right and left cerebral hemispheres 

 or parencephala (Figs. 156, I-L, c. h), each containing a 

 cavity, the lateral ventricle or paraccele (pa. cce) which 

 communicates with the diacoele (di. cce) by a narrow 

 passage, the foramen of Monro (/. m) . From the pros- 

 encephalon or the hemispheres are given off a pair of 

 anterior prolongations, the olfactory lobes or rhinencephala 1 

 (olf. I), each containing an olfactory ventricle or rhinoccele 

 (rh. cce). 



In the preceding description the brain has been described 

 as if its parts were in one horizontal plane ; but, as a matter 

 of fact, at a very early period of development the anterior 

 part becomes bent down over the end of the notochord so 

 that the whole organ assumes a retort-shape, the axis of the 



1 The prosencephalon together with the olfactory lobes is often 

 described as the telencephalon. 



