CHAPTER VI. 

 THE CRANIAL NERVES. 



THE cranial nerves, which take their origin from the base of the 

 brain, are in great measure analogous in anatomical and physio- 

 logical character, with the spinal nerves. An exception to this rule 

 exists only in the three nerves of special sense, the olfactory, optic, and 

 auditory, which are endowed neither with tactile sensibility nor motor 

 power, and which are connected in a special way with the gray sub- 

 stance of the hemispheres. 



The remaining cranial nerves are distributed either to the integu- 

 ment, mucous membranes, or muscular tissues, and are either sensitive 

 or motor, or have both properties combined. Some of them, like the 

 oculomotori us, the patheticus, and the facial, are distinctively motor 

 in character, are distributed to muscles, produce convulsive motion on 

 being irritated, and, when injured or divided, leave the corresponding 

 parts in a state of paralysis. Others, such as the trigeminus, the 

 glosso-pharyngeal, and the pneumogastric, are sensitive nerves, pos- 

 sessing either an acute tactile sensibility, like the trigeminus, or one 

 of more special nature, like the glosso-pharyngeal and pneumogastric. 

 Like the posterior roots of the spinal nerves, they are provided with a 

 ganglion near their points of emergence from the base of the brain ; 

 and they are distributed either to the integument or mucous mem- 

 branes or to both. 



The anatomical similarity between the cranial and spinal nerves is 

 in some instances very marked. The fifth pair, or trigeminus, emerges 

 from the tuber annulare by two roots, of which one is sensitive, the 

 other motor ; the sensitive root presenting a well developed ganglion, 

 with which the fibres of the motor root do not mingle. Beyond the 

 ganglion, accordingly, the nerve contains both motor and sensitive 

 fibres, and is distributed both to muscles and to the integument. The 

 glosso-pharyngeal nerve is joined, beyond its ganglion, by motor fibres 

 from the facial ; and the pneumogastric receives communications from 

 the spinal accessory and other motor nerves. Both sensibility and 

 motion are therefore provided for, in a manner not essentially dif- 

 ferent, by the cranial and spinal nerves. 



The other points, both of difference and analogy, in the cranial 

 nerves, relate to their origin and distribution. Their apparent origin, 

 or the point at which they become detached from the surface of the 

 brain, is not their real origin ; but in every case their fibres can be 

 traced inward, often for a considerable distance, between the tracts of 

 white substance, until they reach a central mass of gray matter from 



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