648 



DISSECTION OF THE ORBIT. 



and an- 

 terior. 



Branches 

 to eyelids. 



Frontal 

 branch. 



Nasal 

 branch. 



Ophthalmic 

 veins : 



superior 



and inferior, 



Optic nerve 



ends in 

 retina. 



Dissection. 



Lower 

 division of 

 third nerve 



supplies 

 muscles, 



and joins 

 ganglion, 



with the supraorbital artery. It ends in offsets to the mucous 

 membrane of the upper part of the nose and the ethmoidal cells. 



The anterior branch (internal nasal) accompanies the nasal nerve to 

 the cavity of the nose, and gives anterior meningeal offsets to the 

 fore part of the falx cerebri and the dura mater of the anterior fossa 

 of the skull. 



The palpebral branches, one for each eyelid, generally rise togethei 

 opposite the pulley of the superior oblique muscle, and then separate 

 from one another. The arches they form have been dissected with 

 the eyelids (p. 569). 



The frontal branch turns round the margin of the orbit, and is 

 distributed on the forehead (p. 503). 



The nasal branch (external) supplies the skin and muscles of the 

 upper part of the nose, and anastomoses with the angular and lateral 

 nasal branches of the facial artery. 



The OPHTHALMIC VEINS are two in number, superior and inferior, 

 and leave the orbit by the sphenoidal fissure, between the heads of 

 the external rectus, to end in the cavernous sinus. The superior vein 

 is the larger and accompanies the artery : it begins in front by 

 a wide communication with the angular vein, and on its way back- 

 wards it receives tributaries corresponding to most of the offsets of 

 the artery. The inferior vein lies below the optic nerve, and is 

 formed by the lower ciliary and muscular veins ; it communicates 

 through the spheno-maxillary fissure with the pterygoid plexus. 

 The supraorbital, frontal and palpebral veins do not join the oph- 

 thalmic, but pass to the veins of the face. 



The OPTIC NERVE in the orbit extends from the optic foramen to 

 the back of the eyeball. As the nerve leaves the foramen it is sur- 

 rounded by the recti muscles; and beyond that spot the ciliary 

 arteries and nerves entwine around it. It terminates in the retinal 

 expansion of the eye. 



Dissection (fig. 230). Take away the ophthalmic vessels, and 

 divide the optic nerve about its middle, together with the small 

 ciliary vessels and nerves. Turn forwards the eyeball, and fasten 

 it in that position with hooks. On removing some fat the three 

 recti muscles inner, inferior, and outer, will appear ; and lying on 

 the first two are the offsets of the lower division of the third nerve. 



The LOWER DIVISION OF THE THIRD NERVE (fig. 230) supplies 



three muscles in the orbit. As it enters this space, between the 

 heads of the external rectus, it lies below the nasal, and rather above 

 the sixth nerve. Almost immediately the nerve divides into three 

 branches. One ( 5 ) passes to the internal, another ( 4 ) to the inferior 

 rectus, both entering the muscles on their ocular surfaces, and the 

 third ( 3 ), the longest and most external, is continued forwards to the 

 inferior oblique muscle, which it pierces at its hinder border. 



Soon after its origin the last branch communicates with the 

 lenticular ganglion, forming the short root (fig. 228, 4 ) of that body ; 

 and it furnishes two or more filaments to the inferior rectus 

 muscle. 



