CHAP. XI.] ENCEPHALIC NERVES. 303 



two in his arrangement ; three pairs being allotted to his eighth 

 pair of nerves, and two to his seventh. Willis' arrangement, there- 

 fore, comprises the following nine pairs of nerves, which he enume- 

 rates in passing from the anterior to the posterior part of the base ; 

 the first pair, or olfactory nerves; the second pair, or optic; the 

 third pair, motor es oculorum; the fourth pair, pathetici; the fifth 

 pair ; the sixth pair, abducentes oculum ; the seventh pair, including 

 the portio mollis or auditory nerve, and the portio dura or facial nerve; 

 the eighth pair, including the glosso-pharyngeal, the pneumo-gastric, 

 and the spinal accessory; the ninth pair, or hypoglossal. Willis 

 included among his encephalic nerves the first cervical nerve or 

 sub-occipital, which he therefore numbered as the tenth pair. 



As the cranium may be shewn to be composed of the elements 

 of three vertebra, it has been attempted to prove that among these 

 nerves some may be classed with the vertebral or spinal nerves. 

 The fifth is obviously of this kind, from its anatomical characters, 

 namely, two roots : one small, ganglionless ; the other large, gan- 

 glionic : and with the former, the analogue of the anterior spinal 

 root, the third, fourth, and sixth nerves maybe conjoined from 

 their similarity in structure and distribution. Thus one cranio- 

 vertebral nerve is formed, the anterior or motor root of which con- 

 sists of the smaller portion of the fifth, the third, fourth, and sixth 

 nerves, and the posterior or sensitive root of the larger portion of 

 the fifth. A second cranio-vertebral nerve consists of the eighth 

 pair, to which might be added the facial, contributing to its motor 

 portion. A third is formed by the hypoglossal,^ but the analogy, 

 in the latter case, is certainly far from obvious. 



How to determine the Function of a Nerve. It has been stated, in a 

 former chapter, that nerves evince special properties, depending on 

 the connexions which they form at the periphery, or at the centre; 

 that they may be divided into motor, sensitive, and, according to 

 one view, excito-motor, according to the manner in which they re- 

 spond to particular stimuli ; and that fibres possessing each of these 

 endowments may be bound together in a common sheath as one 

 nerve. To determine with precision the office which each nerve per- 

 forms, is a problem of great importance, not only from its bearing upon 

 the physiology of the nervous centres, but from its great practical 

 valr.o in the diagnosis and treatment of disease. (Introd. p. 28.) 



The following are the means on which we should rely, in order 

 to determine the function of a nerve. 



* Miiller's Physiology, by Baly, vol.i. p. 841. 



