CHAP. XX.] THE FIFTH NERVE. 113 



Function of the Fifth Nerve. The determination of the functions 

 of the roots of spinal nerves has afforded the clue to that of the func- 

 tions of the roots of the fifth nerve. The analogy of the smaller 

 root of the fifth with the anterior spinal root, and of the larger one 

 with the posterior spinal root has long been admitted by anatomists. 

 Hence, an analogy of function must be admitted, and the former 

 must be viewed as consisting of motor fibres, the latter of sensitive 

 ones ; and by tracing each of the three great divisions of the nerve, 

 we may determine its function by its constitution, according as it 

 derives its fibres from either root or from both. The ophthalmic 

 and superior maxillary are composed of fibres derived exclusively 

 from the larger root; they are, therefore, sensitive nerves. The 

 inferior maxillary consists of fibres derived from both roots, and 

 consequently is both motor and sensitive. Sir C. Bell, in his original 

 exposition of the functions of this nerve, fell into error from having 

 neglected to avail himself of this method of analysing the constitu- 

 tion of each of its three divisions, from which he would have seen that 

 it is the inferior maxillary alone which derives its fibres from both 

 roots, and which perfectly resembles a spinal nerve in constitution. 



The distribution of the three divisions of the fifth nerve confirms 

 most amply the view of its physiology suggested by the anatomy of 

 its origin. The ophthalmic and superior maxillary are distributed 

 entirely to sentient surfaces, or anastomose with motor nerves (the 

 facial) . They supply the skin of the forehead, of the eyelids, the 

 conjunctiva, the eyeball, the mucous membrane of the nostrils, the 

 integuments of the face, the upper lip, the nose, the beard on the 

 upper lip, the integument of the ear, the temple, and the whiskers; 

 they are the sensitive nerves to these regions. The inferior max- 

 illary has two distinct sets of branches, the one by which the mus- 

 cles of mastication are supplied the other, which go to the integu- 

 ments of the lower lip and chin, and the beard, and the mucous 

 membrane of the mouth and tongue. This nerve is, therefore, 

 the nerve of mastication, and of sensation to the surfaces above- 

 named. 



Repeated experiments in the hands of various physiologists, none 

 of which, however, were more conclusive than those of Mayo, indi- 

 cate the same views of function. Division of the ophthalmic or of 

 the superior maxillary induced loss of sensibility without muscular 

 paralysis, leaving only such an impairment of the motor power as 

 destruction of the sensitive nerves invariably produces, by impairing 

 the power of exact adjustment, for which a high degree of ssnsibility 

 is necessary. But when the inferior maxillary nerve was cut, then 



VOL. II. I 



