100 ZOOLOGY SECT. 



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 gang- 

 lionic 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 are typically ten pairs in Fishes and 

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



The first or olfactory nerve (Fig. 788, I.) is rather a large number 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 of 

 the diencephalon, just in front of the infundibulum. It differs 

 from all the other nerves in being originally a hollow outpushing 

 of the brain, containing a prolongation of the diaccele (see Fig. 795). 

 It supplies the retina or actual organ of light, and is therefore a 

 purely sensory nerve. 



The third or oculomotor nerve (III.) arises from the crus cerebri 

 or ventral region of the mid-brain. In its course is a ganglion, the 

 oculomotor or ciliary ganglion (c. gn.}. It supplies four out of the 

 six muscles -of the eye-ball (see below, Fig. 796), viz., the superior, 

 inferior, and internal recti, and the inferior oblique, as well as the 

 ciliary muscles and muscles of the iris in the interior of the eye. 

 It is therefore a purely motor nerve. 



1 In ma:iy Fishes a pair of very small nerves the nervi terminales are 

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

 they seem to be the nerves of ordinary sensatinn for these organs. Repre- 

 sentatives of these nerves have also been described in higher forms. 



