CHAPTEE YI. 



THE CRANIAL NERYES. 



OP the twelve pairs of nerves which take their origin from the brain, 

 the greater number present distinct analogies, both anatomical and 

 physiological, with the spinal nerves. All those which are distributed 

 to the integument and mucous membranes, or to the superficial and deep 

 muscles of the head and face, correspond in all important characters 

 with the sensitive and motor nerves formed from the anterior and pos- 

 terior spinal nerve roots. Three of them, however, show no resem- 

 blance, either in their anatomical distribution or their physiological 

 properties, with the rest. They are the so-called olfactory, optic, and 

 auditory nerves. After leaving their points of origin in the brain, they 

 are distributed neither to muscles nor to the integument or mucous 

 membranes ; but terminate in nervous expansions of special form and 

 structure, in which the gray substance or collections of nerve cells 

 reappear as prominent elements of the tissue. During their passage 

 through the cavity of the cranium, these nerves are neither sensitive 

 nor excitable in the ordinary sense of the word. Their irritation causes 

 no tactile or painful sensation, nor any direct muscular contraction ; 

 and their section produces no paralysis of the voluntary muscles, nor 

 any loss of general sensibility in the neighboring parts. They are to 

 be considered rather in the light of tracts or commissures than of ordi- 

 nary nerves, and their physiological properties are those connected with 

 the operation of the special senses alone. 



The remaining cranial nerves, on the other hand, are similar, both 

 in structure, arrangement and function, to those in other parts of the 

 cerebro-spinal system. Some of them, like the oculo-motorius, the 

 patheticus, and the facial, are plainly 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 pneumo- 

 gastric, are sensitive nerves, possessing either an acute tactile sensibility, 

 like the trigeminus, or one of a more obscure and special nature adapted 

 for the production of involuntary reflex actions, like the glosso-pharyn- 

 geal and pneumogastric. Like the posterior roots of the spinal nerves, 

 these are also provided with a ganglion situated at a short distance 

 from their points of emergence at the base of the brain ; and they are 

 distributed either to the integument or mucous membranes or to both. 



The analogy in anatomical arrangement between the spinal and cranial 



(511) 



