568 NERVOUS SYSTEM chap. 



metencephaloTi or medulla oblongata {nied. obi) derived from 

 the hind-brain. The original cavity of the brain becomes 

 correspondingly divided into a series of chambers or 

 ventricles (compare Figs. 148 and 50), all communicating 

 with one another, and called respectively the fore-ventricle 

 or prosocxle, third ventricle or diaccek, mid-ventricle or 

 ?nesoccele {iter and optic ventricles or optocceles), cerebellar 

 ventricle or epicmle, and fourth ventricle or me.'accele. 



In some fishes (e.g. dogfish, Fig. 115) the brain consists 

 throughout life of these five divisions, but in most cases 

 (Figs. 49 and 131), the prosencephalon grows out into 

 paired lobes, the right and left cerebral hemispheres or 

 parencephala (Figs. 148, I-L, c. h), each containing a cavity, 

 the lateral ventricle or paraccele {pa. coe) which communi- 

 cates with the diacoele {di. coe) by a narrow passage, the 

 foramen of Monro {f. m). From the prosencephalon or 

 the hemispheres are given off a pair of anterior prolonga- 

 tions, the olfactory lobes or rhinencephala {olf. I), each 

 containing an olfactory ventricle or rhinoccele {rh. cos). 



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 fore-brain being strongly inclined to that of the 

 hind-brain. The bend is known as the cerebral flexure (Fig. 155) : it is 

 really permanent, but as the hemispheres grow forwards parallel to the 

 hind-brain and the floor of the mid-brain and hind-brain thickens, it 

 becomes obscure and is not noticeable in the adult. 



The ganglia of the dorsal roots of the spinal nerves are developed 

 from a paired neural ridge formed close to the junction of the medullary 

 plate and outer ectoderm, and the dorsal roots themselves appear as 

 outgrowths from their ganglia (see Fig. 147, above sf. c) : the 

 ventralj^oots arise as direct outgrowths from the medullary cord. Certain 

 of the cerebral nerves are developed in an essentially similar manner to 



