250 THE CRANIAL NERVES. 



SUMMARY. Cutaneous filaments of the inferior maxillary nerve ramify on the 

 side of the head and the external ear, in the external auditory canal, the lower lip, 

 and the lower part of the face ; sensory branches are supplied by it to the greater 

 part of the tongue ; and branches are furnished to the mucous membrane of the 

 mouth, the lower teeth and gums, the salivary glands, the articulation of the 

 lower jaw, the dura mater' and skull, and the mucous lining of the mastoid 

 cells. 



This nerve supplies the muscles of mastication, viz., the masseter, temporal, and 

 two pterygoids ; also the mylo-hyoid, the anterior belly of the digastric, the tensor 

 palati and tensor tympani muscles. 



VI. ABDUCENT NERVE. 



The sixth nerve (abducent nerve of the eyeball, external oculomotor) is distri- 

 buted solely to the external rectus muscle of the eye. Its fibres arise from a nucleus 

 placed close to the floor of the fourth ventricle immediately above the striae acusbicae, 

 beneath the eminentia teres. Leaving the inner side of the nucleus, they pass for- 

 wards and somewhat downwards through the lower part of the pons, and form a 



Fig. 166. PLAN OF THE ORIGIN AND COURSE WITHIX THE 



PONS OF THE SIXTH AND SEVENTH NERVES. 



The nerves and their nuclei are projected on to a sagittal 

 section of the pons near the median plane : VI, sixth nerve ; 

 N.VI, its nucleus ; VII, facial nerve ; Gr, its inner genu ; 

 N.VII, its nucleus ; S.O. , superior olive. 



flattened band, which emerges in the depression 

 at the lower edge of that body immediately 

 external to the pyramid. One or two of the 

 innermost bundles frequently issue between the 

 fibres of the pyramid, or from the lower edge of 

 the pons. 



The nerve speedily becomes rounded, and is directed upwards and forwards, 

 lying for a distance of about 15 mm. between the anterior surface of the pons and 

 the occipital bone. It then enters an aperture in the dura mater to the inner side 

 of and slightly below that of the fifth nerve (see Vol. II, fig. 182), and passing on 

 the outer side of the inferior petrosal sinus, it crosses over the apex of the petrous 

 bone, and beneath the petro-sphenoidal ligament (see Osteology, p. 43), to enter the 

 cavernous sinus. In that cavity the nerve is directed forwards along the outer side 

 of the internal carotid artery, and reaching the outer wall of the space anteriorly, it 

 then passes into the orbit through the sphenoidal fissure, and between the heads 

 of the external rectus, to which muscle it is distributed on its ocular surface 

 (fig. 149, 6 ; 156, 5). As the nerve enters the orbit, it is placed below the 

 other nerves passing through the sphenoidal fissure, but above the ophthalmic 

 veins. 



While contained in the cavernous sinus, the sixth nerve is joined by filaments 

 from the carotid plexus of the sympathetic, and as it enters the orbit it receives a 

 small filament from the ophthalmic nerve. 



The number of fibres in the sixth nerve has been estimated at 2,000 to 2.500 by Rosenthal, 

 3.600 by Tergast. They are mostly large, but there are some of medium size ; fine fibres are 

 scanty. 



Variety. Absence of the sixth nerve upon one side is recorded, its place being supplied 

 by a branch of the third nerve (Generali). 



