CRANIAL NERVES. THIRD TO SEVENTH. 



313 



being confined to the superior ob- [Fig- 152 - 



lique, and the Sixth to the abdu- 

 cens. Of these nerves, the Third 

 pair is the only one which exhibits 

 any app.earance of sensibility, when 

 its trunk is irritated ; but this sen- 

 sibility is not nearly so great as 

 that of the Fifth pair ; and it may 

 be doubted whether it is really pos- 

 sessed by the Third, in virtue of 

 its direct connection with the nerv- 

 ous centres, or whether it is not 

 imparted by the anastomosis of that 

 nerve with the Fifth, some fila- 

 ments of which may pass back- 

 wards as well as forwards, so as to 

 confer sensibility on tbe trunk of 

 the Third, above as well as beyond 

 their point of entrance. The pe- 

 culiar mode in which these motor 

 nerves ordinarily excite the mus- 

 cles to action, will be considered 

 in the next Section. Although 

 commonly ranked as cephalic 

 nerves, they have no direct con- 

 nection with the Cerebrum ; their 

 real origin being from the upper 

 part of the Medulla Oblongata, and 

 those prolongations of it which are 

 known as the Crura Cerebri. The 



roots of the Third pair may be traced into direct connection with the Cor- 

 pora Quadrigemina ; a fact of considerable physiological importance, as will 

 hereafter appear. The chief actions of a purely reflex nature, to which this 

 group of nerves ordinarily ministers, are the government of the diameter of 

 the pupil, which is accomplished through the Third pair ; and the rolling of 

 the eyeball beneath the upper lid during sleep, as well as in the efforts of 

 sneezing, coughing, &c. But irregular movements of the eyeballs^ which 

 must be referred to the same group, are continually seen to accompany various 

 other forms of convulsive action. 



406. The Portia Dura of the Seventh pair, or facial nerve, has been 

 supposed, since the first researches of Sir C. Bell, to be a nerve of motion 

 only ; but some recent physiologists have maintained, that it both possesses 

 sensory endowments, and arises by a double root. According to Valentin, 

 however, who experimented on the roots exposed within the cranium, it pos- 

 sesses no sensory endowments at its origin ; since, when these roots were 

 touched, the animals gave no signs of pain, though violent muscular move- 

 ments were excited in the face. Subsequently to its first entrance into the 

 canal by which it emerges, however, it anastomoses with other nerves ; and 

 thus sensory fibres are introduced into it from many different sources, ante- 

 riorly, from the Fifth pair, and posteriorly, from the Cervical nerves, which 

 cause irritation of several of its branches to produce pain. The number 

 and situation of the anastomoses vary much in different animals ; so that it 

 is impossible to make any very comprehensive statement in regard to them. 

 Experimental researches leave no doubt that the Portio Dura is the general 

 motor nerve of the face ; ministering to the influence of volition and Emo- 

 27 



A view ofthe Third, Fourth and Sixth pairs of 

 Nerves; 1, ball ofthe eye and rectus externus muscle; 

 2, the superior maxilla ; 3, the third pair, or motores 

 oculi, distributed to all the muscles of the eye except 

 the superior oblique and external reclus; 4, the fourth 

 pair, or palhetici, going to the superior oblique muscle; 

 5, one of the branches of the seventh pair ; 6, the sixth 

 pair, or motor externus, distributed to the external 

 rectus muscle ; 7, spheno-palatine ganglion and 

 branches; 8, ciliary nerves from the lenticular gan- 

 glion, the short root of which is seen to connect it with 

 the third pair.] 



