810 THE NERVOUS SYSTEM. 



down to the bone, the nerve will be divided. Hilton divided it opposite the second molar tooth, 

 where it is covered only by the mucous membrane, and Lucas pulls the tongue forward and over 

 to the opposite side, when the nerve can be seen standing out as a firm cord under the mucous 

 membrane by the side of the tongue, and can be easily seized with a sharp hook and divided or 

 a portion excised. This is a simple enough operation on the cadaver, but when the disease is 

 extensive and has extended to the floor of the mouth, as is generally the case when division of 

 the nerve is required, the operation is not practicable. 



Sixth Nerve (Figs. 384, 481). 



The Sixth or Abducent Nerve supplies the External rectus muscle. Its super- 

 ficial origin is by several filaments from the constricted part of the pyramid close 

 to the pons, or from the lower border of the pons itself in the groove between this 

 body and the medulla. Its deep origin is a little lower than the motor root of the 

 fifth, and close to the median line, beneath the superior portion (above the audi- 

 tory striae) of the fasciculus teres on the floor of the fourth ventricle. 



The nerve pierces the dura mate.r on the basilar surface of the sphenoid bone, 

 runs through a notch immediately below the posterior clinoid process, and enters 

 the cavernous sinus. It passes forward through the sinus, lying on the outer side 

 of the internal carotid artery. It enters the orbit through the sphenoidal fissure, 

 and lies above the ophthalmic vein, from which it is separated by a lamina of dura 

 mater. It then passes between the two heads of the External rectus, and is dis- 

 tributed to that muscle on its ocular surface. 



Branches of Communication. It is joined by several filaments from the carotid 

 and cavernous plexus, by one from Meek el's ganglion (Bock), and another from 

 the ophthalmic nerve. 



The above-mentioned nerve, as well as the third, fourth, and the ophthalmic 

 division of the fifth, as they pass to the orbit, bear a certain relation to each other 

 in the cavernous sinus, at the sphenoidal fissure, and in the cavity of the orbit, 

 which will now be described. 



In the cavernous sinus (Fig. 384) the third, fourth, and ophthalmic division of 

 the fifth are placed on the outer wall of the sinus, in their numerical order both 

 from above downward and from within outward. The sixth nerve lies at the 

 outer side of the internal carotid artery. As these nerves pass forward to the 

 sphenoidal fissure, the third and fifth nerves become divided into branches, and 

 the sixth approaches the rest, so that their relative position becomes considerably 

 changed. 



In the sphenoidal fissure (Fig. 486) the fourth and the frontal and lachrymal 

 divisions of the ophthalmic lie upon the same plane, the former being most 



Lachrymal. 

 Frontal. 



Superior division of 3d. 

 Nasal. 



'Inferior division of 3d. 

 "6th. 

 ~0phthalnir vein. 



FIG. 486. Relations of structures passing through the sphenoidal fissure. 



internal, the latter external, and they enter the cavity of the orbit above the mus- 

 cles. The remaining nerves enter the orbit between the two heads of the 

 External rectus. The superior division of the third is the highest of these; 

 beneath this lies the nasal branch of the ophthalmic ; then the inferior division of 

 the third ; and the sixth lowest of all. 



