xni PHYLUM CHORDATA 103 



the roof or pallium of the undivided fore-brain is reduced to a 

 layer of epithelium (D and E. pal.), its -floor is thickened so as to 

 form large paired masses, the corpora striata (c. s.). When hemi- 

 spheres are developed the corpora striata form the floors of the 

 two lateral ventricles (L. c. s.), and the roof (pallium) of each is 

 formed of nervous tissue. In such cases the front wall of the 

 diencephalon remains very thin, and is distinguished as the 

 lamina terminals (I. t.): this is the actual anterior extremity of 

 the central nervous system, the cerebral hemispheres being lateral 

 outgrowths. 



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: it is really permanent, but, as the hemispheres 

 grow forward parallel to the hind-brain and the floor of the mid- 

 and hind- brain thickens, it becomes obscure, and is not noticeable 

 in the adult. 



The brain, like the spinal cord, is composed of grey and white 

 matter, but the grey matter either forms a thin superficial layer 

 or cortex, as in the hemispheres and cerebellum, or occurs as 

 ganglionic masses surrounded by white matter. 



The whole cerebro-spinal cavity is lined with a tough membrane, 

 the dura mater, and both brain and spinal cord are covered by a 

 more delicate investment, the pia mater: the space between the 

 two contains a serous fluid. In the higher forms there is a delicate 

 arachnoid membrane outside the pia, and in many cases the regions 

 of the pia in immediate contact with the thin epithelial roofs of 

 the diencephalon and medulla become greatly thickened and 

 very vascular, forming in each case what is known as a choroid 

 plexus. 



From the brain are given off cerebral or cranial nerves : these, 

 like the spinal nerves, are paired, but, unlike them, are strictly 

 limited in number, the number being constant, at least within 

 very narrow limits : there typically are ten pairs in Fishes and 

 Amphibians, twelve in Reptiles, Birds, and Mammals. 1 



The first or olfactory nerve (Fig. 779, I.) is rather a bundle of 

 fibres than a single nerve ; it arises from the olfactory bulb, and 

 supplies the organ of smell i.e., the epithelium of the olfactory sac 

 (see below). It is therefore a purely sensory nerve. 



The second or optic nerve (II.) arises from the ventral region or 



1 In many Fishes a pair of very small nerves the nervi terminates are given 

 off from the cerebral hemispheres and run forward to the olfactory sacs : they 

 seem to be the nerves of ordinary sensation for these organs. As they have not 

 been found in higher forms, they are not here counted as the first pair. 



