an PHYLUM CHORD AT A 97 



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 are ten pairs in Fishes and Amphibians, 

 twelve in Reptiles, Birds, and Mammals. 



The first or olfactory nerve (Fig. 734, I.) is rather a bundle of 

 fibres than a single nerve : it arises from the olfactory lobe, 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 out-pushing 

 of the brair containing a prolongation of tne diaccele (see Fig. 741). 

 It supplies\the retina or actual organ of sight, 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. 742), viz., the superior, 

 inferior, and internal recti, and the inferior oblique (Fig. 742, III.), 

 as well as the ciliary muscles and muscles of the iris in the 

 interior of the eye. It is therefore a purely motor nerve. 



The fourth or trochlear nerve (Figs. 734 and 742, IV.) arises from 

 the dorsal surface of the brain at the junction of the mid-brain 

 with the medulla oblongata. It is a very small and purely motor 

 nerve, supplying only the superior oblique muscle of the eye. 



The fifth or trigeminal nerve (Fig. 734;V.) is of great size and 

 wide distribution. It arises from the side of the medulla, fre- 

 quently by two roots, a dorsal and a ventral, thus resembling 

 in its origin a spinal nerve. In some instances each root, or the 

 dorsal root only, has a ganglion near its origin, in others the two 

 roots enter a single Gasserian ganglion (g. gn.) The trunk of the 

 nerve early divides into two principal branches, the ophthalmic and 

 the mandibular (V. md.) : the latter sends off a maxillary nerve 

 (V. mx.), and we thus get the three divisions to which the name 

 trigeminal is due. The ophthalmic nerve frequently divides into 



VOL. II H 



