348 INTERNAL ANATOMY OF HOUSE. 



runs through the Ijase of the cerebellum and enters the cerebral 

 hemispheres close behind two thick white cords, the optic nerves 

 (Dji.G). Further portions of the isthmus and other parts of 

 the brain are named in the figure (fig. 197). 



Both from the brain and spinal cord nerves are given off to 

 the various parts of the body. Those coming from the brain 

 are called cranial nerves, those from the spinal cord ajiinal 

 nerves ; both groups are alv?ays paired. There are twelve pairs 

 of " cranials," as follows : — 



Olfactory nerves (fig. 164, 1). 



Optic nerves (2). 



Oculo-motor nerves (3). 



Trochlear nerves (4). 



Trigeminal nerves (5). 



I'jxternal oculo-motor nerves (6). 



Facial nerves (7). 



jVuditory nerves (8). 



(ilosso-pharyngeal nerves (9). 



A';igus or pneumogastric nerves (10). 



Accessory nerves (10). 



Hypoglossal nerves. 

 The olfactory constitute the olfactory lobes in the brain; 

 they pass to the nose and form the sense of smell (fig. 197, OJ). 

 They arise from two roots at the brain. Impressions of odours 

 are reoei^'ed by them and are transmitted to the brain. The 

 optics are, of course, the nerves of sight : they cross one another 

 and form the optic chiasma (fig. 197, Op.C). The third and 

 fourth pairs are for the movements of the eyes. The trigeminal 

 are a \-ery large pair, have many branches, and are connected 

 with the sympathetic system. They arise b}"" two roots, one 

 sensory, while the other contains motor fibres, there being a 

 ganglion on the sensory root. The sixtli arises by five to eight 

 roots from the medrdla, and also supplies tlie eye-muscles. The 

 facial nerves are motor in origin, but receive sensory fibres; 

 they excite the contraction of the muscles of the face in general 



