450 THE CRANIAL NERVES. 



large root of the fifth pair, the glosso-pharyngeai, and the pneumo- 

 gastric. It will be observed that, in all their essential properties, 

 they correspond with the posterior roots of the spinal nerves. Like 

 them they are inexcitable, but extremely sensitive. Irritated at 

 their point of origin, they give rise to acutely painful sensations, 

 but to no convulsive movements. Secondly, if divided at the same 

 situation, the operation is followed by loss of sensibility in the 

 parts to which they are distributed, without any disturbance of the 

 motive power. Each of these nerves, furthermore, like the poste- 

 rior root of a spinal nerve, is provided with a ganglion through 

 which its fibres pass : the fifth pair, with the Casserian ganglion, 

 situated near the inner extremity of the petrous portion of the tem- 

 poral bone ; the glosso-pharyngeai, with the ganglion of Andersch, 

 situated in the jugular fossa; while the pneumogastric presents, 

 just before its passage through the jugular foramen, a ganglion 

 known as the ganglion of the pneumogastric nerve. Finally, the 

 sensitive fibres of all these nerves, beyond the situation of their gan- 

 glia, are intermingled with others of a motor origin. The large root 

 of the fifth pair, which is exclusively sensitive, is accompanied by 

 the fibres of the small root, which are exclusively motor. The 

 glosso-pharyngeai receives motor filaments from the facial and spi- 

 nal accessory, becoming consequently a mixed nerve outside the 

 cranial cavity ; while the pneumogastric is joined by fibres from the 

 spinal accessory and various other nerves of a motor character. 

 These nerves, accordingly, are exclusively sensitive only at their 

 point of origin. Though they afterward retain the predominating 

 character of sensitive nerves, they are yet found, if irritated in the 

 middle of their course, to be intermingled with motor fibres, and 

 to have consequently acquired, to a certain extent, the character of 

 mixed nerves. 



The resemblance, therefore, between the cranial and spinal nerves 

 is complete. 



MOTOR OCULI COMMUNIS. This nerve, which is sometimes known 

 by the more convenient name of the oculo-motorius, originates from 

 the inner edge of the crus cerebri, passes into the cavity of the 

 orbit by the sphenoidal fissure, and is distributed to the lexator 

 palpebras superioris, and to all the muscles moving the eyeball, 

 except the external rectus and the superior oblique. Its irritation 

 accordingly produces convulsive movements in these parts, and 



