94 ZOOLOGY SECT. 



As already mentioned, the anterior end of the nervous system 

 undergoes, at a very early period, a marked dilatation, and is 

 distinguished as the brain (Fig. 733). Constrictions appear in the 

 dilated part and divide it into three bulb-like swellings or vesi- 

 cles, the fore-brain (A,/. &.), mid-brain (m. b.) and hind-brain (h. b.). 

 Soon a hollow outpushing grows forwards from the first vesicle 

 (B, prsen), and the third gives off a similar hollow outgrowth 

 (eblm.) from its dorsal surface. The brain now consists of five 

 divisions : the prosencephalon (prs. en.) and the diencephalon (dien.), 

 derived from the fore-brain : the mid-brain or mesencephalon (m. b.) 

 which remains unaltered : and the epencephalon, or cerebellum 

 (cblm.\ and the metencephalon, or medulla oblongata (med.obl.) 

 derived from the hind-brain. Additional constrictions appear in 

 the medulla oblongata giving it a segmented appearance, but they 

 disappear as development proceeds, and, whatever may be their 

 significance, have nothing to do with the main divisions of the 

 adult organ. The original cavity of the brain becomes corre- 

 spondingly divided into a series of chambers or ventricles, all 

 communicating with one another and called respectively the fore- 

 ventricle or prosoccele, third ventricle or diaccele, mid-ventricle or 

 mesoccele, cerebellar ventricle or epiccele, and fourth ventricle or 

 metaccele. 



In some Fishes the brain consists throughout life of these five 

 divisions only, but in most cases the prosencephalon grows out 

 into paired lobes, the right and left cerebral hemispheres or 

 parencephala (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 (f. m.). 

 Moreover, each hemisphere gives off a forward prolongation, the 

 olfactory lobe or rhinencephalon (olf. I.), containing an olfactory 

 ventricle or rhinocosle (rh. cce.) : when there is an undivided pro- 

 sencephalon the olfactory lobes (C, D, olf. I.) spring from it. In 

 the embryo of some forms there is a median unpaired olfactory 

 lobe, like that of Amphioxus. 



The brain undergoes further complications by the unequal 

 thickening of its walls. In the medulla oblongata the floor becomes 

 greatly thickened (D, H, K.), while the roof remains thin, con- 

 sisting of a single layer of epithelial cells, assuming the character 

 therefore of a purely non-nervous epithelial layer (ependyme). In 

 the cerebellum the thickening takes place to such an extent that 

 the epicoele is usually obliterated altogether. In the mid-brain 

 the ventral wall is thickened in the form of two longitudinal 

 bands, the crura cerebri (cr. crb.), the dorsal wall in the form of 

 paired oval swellings, the optic lobes (opt. I.): extensions of the 

 mesocoele into the latter form the optic ventricles or optocosles 

 (G. opt. cce.) : the median portion of the mesoccele is then called 

 the iter (I) or aqueduct of Sylvius. In the diencephalon the sides 



