454 THE CRANIAL NERVES. 



upon the paralysis of its motor fibres, which are distributed, as we 

 have seen, to the muscles of mastication. In many of the lower 

 animals, consequently, the movements of mastication become ex- 

 ceedingly enfeebled upon the affected side. In the cat, for example, 

 an animal in which mastication is usually very thoroughly per- 

 formed, this process becomes excessively laborious, so that the 

 animal after this operation cannot masticate solid meat, but requires 

 to be fed with that which has already been cut in pieces. 



The fifth pair, beside supplying the sensibility of the integument 

 of the face, has a peculiar and important influence on the organs of 

 special sense. This influence appears to consist in some connection 

 between the action of the fifth pair and the processes of nutrition ; 

 so that when the former is injured, the latter very soon become 

 deranged. For the perfect action of any one of the organs of 

 special sense, two conditions are necessary : first, the sensibility of 

 the special nerve belonging to it, and secondly, the integrity of the 

 component parts of the organ itself. Now as the nutrition of the 

 organ is, to a certain extent, under the control of the fifth pair, any 

 serious injury to this nerve produces a derangement in the tissues 

 of the organ, and consequently interferes with the due performance 

 of its function. 



The mucous membrane of the nasal passages, for example, is 

 supplied* by two different nerves; first, the olfactory, distributed 

 throughout its upper portion, by which it is endowed with the 

 special sense of smell ; and, secondly, the nasal branch of the fifth 

 pair, distributed throughout its middle and lower portions, by 

 which it is supplied with ordinary sensibility. 



Since the fifth pair, accordingly, supplies general sensibility to 

 the nasal passages, this property will remain after the special sense 

 of smell has been destroyed. If, however, the fifth pair itself be 

 divided, not only is general sensibility destroyed in the Schneiderian 

 mucous membrane, but a disturbance begins to take place in the 

 nutrition of its tissue, by which it is gradually rendered unfit for 

 the performance of its special function, and the power of smell is 

 finally lost. The mucous membrane, under these circumstances, 

 becomes injected and swollen, and the nasal passage is obstructed 

 by an accumulation of puriform mucus. According to Longet, the 

 mucous membrane also assumes a fungous consistency, and is liable 

 to bleed at the slightest touch. The effect of this alteration is to 

 blunt or altogether destroy the sense of smell. It is owing to a 

 similar unnatural condition of the mucous membrane that the power 



