538 



THE CRANIAL NERVES. 



the ganglion situated upon the trunk of the glosso-pharyngeal nerve; 

 and is consequently made up of interlacing fibres derived from both 

 these sources. Its peripheral sensitive fibres terminate in the mucous 

 membrane lining the cavity 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 they are undoubtedly in great measure under the control 

 of the nervous supply, of which a considerable portion is derived from 

 the fifth pair. 



Sixth Pair. The Abducens. 



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

 single muscle which causes the movement of abduction of the eyeball, 

 originates mainly from a collection of gray matter on the floor of the 

 fourth ventricle, near its widest part and at a point corresponding with 

 the posterior section of the pons Varolii. It is situated next the median 



TRANSVERSE SECTION OF THE FLOOR OF THE FOURTH VENTRICLE of the 

 Human Brain, showing the nucleus and roots of the abducens and facial nerves. Nf, Nu- 

 cleus of gray matter. VI', Fibres of the abducens nerve (6th pair). VII', Fibres of the facial 

 nerve (7th pair). VII", Bundle of longitudinal fibres, connected with the root of the facial 

 nerve. R, Raphe, at the median line, showing transverse or decussating fibres from the 

 facial nerve roots. Magnified 35 diameters. (Henle.) 



line, and is indicated on each side by a longitudinal prominence, known 

 as the " fasciculus teres." This collection of gray matter is the common 

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

 nerves are traced to a connection with it, although running in some- 

 what different directions. The fibres of the abducens, as shown by Dean, 

 Meynert, and Henle, originate from the inner border of the nucleus 

 without showing any apparent decussation with those of the opposite 

 side. They then pass almost directly downward and forward, in a verti- 

 cal longitudinal plane, through the substance of the tuber annulare, to 

 their point of emergence at the base of the brain, at the posterior edge 



