THE CRANIAL NERVES. 467 



its auriculo-temporal branch, which supply the anterior border of the 

 concha and the anterior wall of the external auditory meatus. Its 

 relation with the deeper parts of the organ is established through the 

 otic ganglion of the sympathetic, which receives a few fibres from its 

 inferior maxillary division, and sends a filament backward to the plexus 

 on the inner surface of the membrane of the tympanum. This plexus is 

 also supplied with filaments from the ganglion of the glosso-pharyngeal 

 nerve ; and is consequently made up of fibres from both these sources. 

 Its sensitive fibres terminate in the lining membrane of the middle ear. 

 The secretions, both of this cavity and of the external auditory meatus, 

 are important for the preservation of the integrity of the parts and for 

 the mechanism of audition ; and a considerable portion of their nervous 

 supply is derived from the fifth pair. 



Sixth Pair. The Abdncens. 



The abducens nerve, so called because it is distributed to the single 

 muscle causing abduction of the eyeball, originates mainly from a 

 deposit of gray substance on the floor of the fourth ventricle near its 

 widest part, at a point corresponding with the posterior border of the 

 pons Varolii. It is situated next the median line, and is indicated 

 on each side by a longitudinal prominence, known as the "fasciculus 

 teres." This nucleus is designated as the common nucleus of the 

 abducens and facial nerves ; since the root fibres of both these nerves 

 are traced, through somewhat different routes, to its gray substance. 

 The fibres of the abducens, as shown by Dean, Meynert, and Henle, 

 originate from the inner border of the nucleus without apparent decus- 

 sation with those of the opposite side. They pass almost directly down- 

 ward and forward, through the tuber annulare, to their emergence at 

 the base of the brain, at the posterior edge of the pons Yarolii. From 

 this point, the nerve, which is about two millimetres in thickness, runs 

 forward, beneath the pons, passing, in company with the oculomotorius 

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

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

 external straight muscle of the eyeball. 



Physiological Properties of the Abducens. By the experiments of 

 Longet on rabbits, and those of Chauveau on rabbits and horses, the 

 abducens is shown to be, at its origin, exclusively a motor nerve ; since 

 its irritation in this region produces contraction of the external straight 

 muscle of the eyeball, without any indication of sensibility. According 

 to Longet, the difference in this respect between the abducens and the 

 trigeminus is very marked ; irritation of the trigeminus always giving 

 rise to signs of acute sensibility, while that of the abducens has no 

 other effect than local muscular contraction. 



Division of this nerve causes internal strabismus from paralysis of 

 the external straight muscle, and loss of the power of horizontal rotation 

 of the eyeball ; while its vertical movements are still preserved, owing 

 to the continued activity of the oculomotorius nerve. There are cases 



