xra PHYLUM CHORDATA 97 



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. 787). Constrictions appear in the 

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

 the fore-brain (A, f. b.), mid-brain (m. b.) and hind-brain (h. b.). 

 Soon a hollow outpushing grows forwards from the first vesicle 

 (B, prs. en), and the third gives off a similar hollow outgrowth 

 (cblm.) 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 : the epencephalon or cerebellum 

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

 derived from the hind-brain. 1 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 

 mesoco3le, cerebellar ventricle or epicosle, 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 diaccele 

 (di. cce.) by a narrow passage, the foramen of Monro (f. m.). 

 Moreover, each hemisphere gives off a forward prolongation, the 

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

 ventricle or rhinoccele (rh. cos.) : when there is an undivided prosen- 

 cephalon, the olfactory bulbs (C, D, olf. I.) spring from it. In the 

 embryo of some forms there is a median unpaired olfactory bulb, 

 like that of Amphioxus. The part of the cerebral hemisphere 

 with which the olfactory bulb is immediately related is the olfactory 

 lobe. 



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 epiccele 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 



1 The prosencephalon is sometimes called the telencephalon, the epen- 

 cephalon the. metencephalon, and the metencephalon the myelencephalon. 



