THE BRANCHES OF THE SIXTH NERVE. 739 



tudinal bundle to the oculo-motor nucleus of the opposite side and into the third 

 nerve, along which they are carried to the Internal rectus muscle. The External 

 rectus of one eye and the Internal rectus of the other may therefore be said to 

 receive their nerves from the same nucleus -a factor of great importance in con- 

 nection with the conjugate movements of the eyeball, and one that may explain 

 certain paralytic phenomena of the Recti muscles, which are often associated with 

 lesions in the pons. 



The nerve pierces the dura mater 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 

 distributed to that muscle on its ocular surface. 



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

 and cavernous plexus, and by one 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. 327) 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. 398) 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. 



"Ophthalmic vein. 



FIG. 398. 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 ot 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 oJ 

 the third ; and the sixth lowest of all. ^a^v,-! 



In the orbit the fourth and the frontal and lachrymal divisions of the ophth 

 mic lie on the same plane immediately beneath the periosteum, the fourth v 

 being internal and resting on the Superior oblique, the frontal resting 01 tthe 

 Levator palpebne, and the lachrymal on the External rectus Next a older 

 comes the superior division of the third nerve, lying immediately beneath t 

 Superior rectus, and then the nasal branch of the ophthalmic crossing the optic 

 ner P ve from the outer to the inner side of the orbit. Beneath these is found he 

 optic 'nerve, surrounded in front by the ciliary nerves, and having the lenticular 



