Ill 



CHAPTER XX. 



OF THE COMPOUND ENCEPHALIC NERVES. THE FIFTH PAIR. THE 



EIGHTH PAIR. 



IT is proposed to devote this chapter to the examination of the 

 physiological history of those encephalic nerves which, from their 

 compound nature, combine the functions of sensitive and motor 

 nerves. These are the fifth pair and the eighth pair. 



The fifth pair of nerves is one of the most interesting and ex- 

 tensively connected nerves in the body. It presents a remarkable 

 resemblance to spinal nerves in its mode of origin, a fact which bears 

 strongly on the determination of its functions. The first point of 

 resemblance is that its origin is by two roots, one large, and the 

 other small; and secondly, its larger root is involved in a ganglion, 

 the two roots being quite distinct until after the formation of the 

 ganglion, when the lesser one coalesces with one of the nerves which 

 springs from the ganglion, to form the inferior maxillary nerve. 



The two roots are implanted in the same column of the medulla 

 oblongata. They remain, however, quite distinct in the 'substance 

 of the medulla. Penetrating the latter at the crus-cerebelli, between 

 the transverse fibres of the pons, each root may be traced through 

 a separate but nearly parallel course downwards to the olivary 

 column, where each forms its separate connection with the vesicular 

 matter. 



The ganglion (ganglion Gasserii) which is formed upon the larger 

 root of the fifth nerve is situate in the middle fossa of the cranium 

 upon the upper surface of the petrous bone, and the middle lacerated 

 foramen, and behind the great ala of the sphenoid. It is of a trian- 

 gular form, its base curvilinear and directed forwards and outwards. 

 From this base there proceed three nerves, the ophthalmic on the 

 inside, the superior maxillary in the middle, and the inferior maxil- 

 lary on the outside. Of these the first two consist exclusively of 

 fibres derived from the larger root and ganglion; the third the 

 inferior maxillary, is composed of fibres derived from both roots. 

 This, therefore, is the only portion of the fifth nerve which is 

 strictly compound, and it constitutes the largest portion of the nerve. 

 The distribution of the nerve may be understood by reference 



