THE FACIAL. 539 



of the pons Yarolii. From this point, the nerve, which is about two mil- 

 limetres in thickness, runs nearly straight forward, beneath the under 

 surface of the pons, passes, in company with the oculomotorius and 

 patheticus, along the wall of the cavernous sinus and through the 

 sphenoidal fissure, into the cavity of the orbit, where it terminates in 

 the external straight muscle of the eyeball. 



Physiological Properties of the Abducens. The physiological pro- 

 perties of this nerve have been examined, in the experiments of Longet 

 on rabbits, and in those of Chauveau on rabbits and horses, by irritating 

 its trunk within the cranium and at its point of emergence from the 

 pons Yarolii. The abducens is thus shown to be, at its origin and for 

 some distance beyond, exclusively a motor nerve ; since its galvanization 

 produces at once continued contraction in the external straight muscle 

 of the eyeball, and mechanical or other irritation applied to its fibres 

 causes no indication of suffering. In the experiments of Longet, which 

 were performed upon the living animal, the difference in this respect 

 between the abducens nerve and the trigeminus was very marked ; irrita- 

 tion of the trigeminus always giving rise to signs of acute sensibility, 

 while that of the abducens had no other effect than local muscular con- 

 traction. 



Division of this nerve causes internal strabismus from paralysis of 

 the external straight muscle, and loss of the lateral motion of the eye- 

 ball in a horizontal plane ; although its vertical movements are still 

 preserved, owing to the continued activity of the oculomotorius nerve. 

 Cases of internal strabismus, in man, are recorded, with the accompa- 

 nying symptoms mentioned above, which were apparently due to com- 

 pression of the abducens nerve by morbid growths within the cranial 

 cavity. 



Seventh Pair. The Facial. 



With regard to the innervation of the external parts of the face, this 

 nerve holds an equal rank with the fifth pair, and may be regarded as 

 complementary to it in physiological endowments. As the fifth pair is 

 the nerve of sensation for the integument of this region, the facial is the 

 motor nerve for its superficial muscles. It is the nerve of facial expres- 

 sion, by which the features are animated in their varying movements, 

 corresponding with the different phases of mental or emotional activity. 

 Although at its origin an exclusively motor nerve, it receives, soon after 

 its emergence from the cranium, a communicating branch from the fifth 

 pair, which gives to it, and to the muscles in which it terminates, a cer- 

 tain share of sensibility. 



The facial nerve has its principal source in a collection of gray matter, 

 which has already been described as also giving origin to the fibres of 

 the abducens (Fig. 175). This nucleus extends for a short distance 

 longitudinally along the floor of the fourth ventricle near the median line, 

 as a layer about 1.5 millimetre in thickness, and containing, according 



