FUNCTIONS OF THE CRANIAL NERVES. 263 



ultimate distribution, but also by experiments on animals, and obser- 

 vations made in cases of injury or disease in man. In this way, the 

 properties of the several cranial nerves have been determined, and the 

 nerves themselves have been classified accordingly. Thus, the olfac- 

 tory, the optic, and the portio mollis of the seventh pair, or auditory 

 nerve, are purely and specially sensory, and contain afferent fibres 

 only ; the third pair, oculo-motor, or motor nerves of the eye, the 

 fourth pair, trochlear, or pathetic nerves, the sixth pair, or abducent 

 nerves, the portio dura of the seventh pair, or facial nerve of each side, 

 and the ninth pair, or hypoglossal nerves, are purely motor nerves, and 

 contain efferent fibres only ; whilst, lastly, the fifth pair and the eighth 

 pair, its three divisions being considered as one nerve, are, like the 

 spinal nerves, mixed sensory and motor. The fifth nerve arises, in- 

 deed, as we have seen, like the spinal nerves, by two roots, of which 

 the larger one is sensory, partly serving for common sensation, and 

 partly, it is believed, for the gustatory sense, whilst the smaller one 

 is motor. The glosso-pharyngeal division of the eighth pair is sensory, 

 partly tactile, and partly gustatory ; the spinal accessory division is 

 chiefly motor, containing a few sensory fibres derived from the pneu- 

 mogastric, whilst the great pneumogastric itself is partly motor, and 

 partly sensory, some of its terminal branches being sensory, and others 

 motor. 



The following table shows briefly these relations : 



CRANIAL NERVES. 



( First, or olfactory. 

 First Group. Sensory, . . . -j Second, or optic. 



( Auditory (portio mollis of seventh). 



Third nerve (motor of eye). 



Fourth nerve (pathetic). 



Sixth nerve (abducent). 



Facial (portio dura of seventh). 



Ninth nerve (hypoglossal). 



Third Group.-Mixed Nerves, ^ three divisions). 



The special functions of these nerves are as follows : The olfactory, 

 optic, and auditory nerves appear to have special endowments, or to 

 react only under the effects of peculiar stimuli, producing odorous, 

 luminous, or sonorous impressions; for no other sensations can be pro- 

 duced by their irritation. Thus, pinching, or the electric stimulus, 

 does not cause pain, but the sensation of light or noise, if applied to 

 the optic or auditory nerves. Pain may, however, be produced by an 

 excess of their proper stimuli, as by intense light and very loud noise. 

 Their division destroys the function of the sensory organs to which 

 they are distributed. It has not been proved that an ordinary stimulus 

 applied to the olfactory nerves, produces smell. Further details on 

 these subjects will be given in the Chapter on the Senses. 



The third cranial or oculo-motor nerve, governs all the muscles of 

 the eyeball, except the external rectus and the superior oblique mus- 

 cle; through its connection with the lenticular ganglion, it effects the 

 contraction of the pupil, exciting the circular fibres of the iris. This 



