FUNCTIONS OF THE CEPHALIC NERVES. 



167 



A view of the origin and distribution of the Portio 

 Mollis of the Seventh pair or Auditory Nerve; 1, the 

 medulla oblongata ; 2, the pons varolii ; 3, 4, the crura 

 cerebelli of the right side ; 5. the eighth pair of nerves ; 

 6, the ninth pair; 7, the auditory nerve distributed to 

 the cochlea and labyrinth; 8, the sixth pair of nerves; 

 9, the portio dura of the seventh pair; 10, the fourth 

 pair; 11, the fifth pair.] 



whose function is to minister, either [Fig. 35. 



to common sensation, or to that of 

 taste which approaches nearly to it, 

 namely, the Fifth pair and the 

 Glosso-pharyngeal, which issue 

 from the posterior part of the Ence- 

 phalon, and are more nearly analo- 

 gous to the Spinal nerves. The 

 primitive fibres are not so soft as 

 those of the Olfactive, nor so slender 

 as those of the Optic ; and they are 

 softer than those of the Glosso-pha- 

 ryngeal. Moreover, the Auditory 

 never forms a plexus with the Fa- 

 cial, to which there is no analogy 

 in the Optic and Olfactive nerves, 

 but to which a similar one exists in 

 the Glosso-pharyngeal. This inter- 

 mediate structural character is inte- 

 resting, when we compare it with 

 the intermediate character of the 

 function ; for the impressions made 

 upon the sense of Hearing are pro- 

 duced through vibrations of a mate- 

 rial fluid, instead of being, as in the case of Sight, the result of changes so 

 subtle as to be almost inscrutable to our means of research, or, as in the case 

 of Taste and Touch, being produced by the direct contact of the substance 

 which gives rise to the sensation. 



224. Passing by for the present the Motor nerves of the Orbit, as consti- 

 tuting a distinct subject for future inquiry, we may advantageously proceed 

 with the other Sensory nerves connected with the Encephalon. It should be 

 noticed, however, that the Third pair, or Motor Oculi, certainly possesses some 

 degree of sensibility, as is evidenced by the signs of pain given by the animal 

 when it is cut or compressed ; but this sensibility is not nearly so great as that 

 of the Fifth pair ; and it may be doubted whether it is possessed by it, in 

 virtue of its direct connection with the nervous centres, or whether it does 

 not derive it by its anastomosis with the Fifth pair, some filaments of which 

 may pass backwards as well as forwards, so as to confer sensibility on the 

 Third pair, both before and after their junction with it. No sensory fibres 

 can be proved to exist in the Fourth and Sixth nerves. 



225. We next come to the Fifth pair, or Trifacial, the true nature of the 

 functions of which was ascertained in part by Sir C. Bell; his views receiving 

 modification, however, from the experimental researches of others. As for- 

 merly stated, it possesses two distinct sets of roots, of which one is much larger 

 than the other ; on the larger root, as on the posterior root of the spinal nerves, 

 is a distinct ganglion ; and the fibres arising from the smaller root do not blend 

 with the others, until after the latter have passed through this ganglion. The 

 trunk of the nerve separates, as is well known, into three divisions, the 

 Ophthalmic, the Superior Maxillary, and the Inferior Maxillary ; and it can 

 easily be shown by careful dissection, that the fibres of the smaller root pass 

 into the third of these divisions alone. When the distribution of this nerve 

 is carefully examined, it is found that the first and second divisions of it pro- 

 ceed almost entirely to the skin and mucous surfaces, a very small proportion 

 only of their fibres being lost in the muscles ; but of the branches of the third 

 division, a large part are distinctly muscular. Hence analogy, and the facts 



