54 DIHSECTION OF THE OKBIT. 



Dissection. The superior rectus muscle is next to be divided about the 

 middle, and turned backwards (fig. 11), when a branch of the third nerve 

 to its under surface will be found. At the same time the nasal nerve and 

 the ophthalmic artery and vein will come into view as they cross inwards 

 above the optic nerve : these should be traced forwards to the inner angle, 

 and backwards to the posterior part of the orbit. 



By taking away the fat between the optic nerve and the external rectus, 

 at the back of the orbit, the student will find easily fine nerve-threads 

 (ciliary) with small arteries lying along the side of the optic nerve ; and 

 by tracing the ciliary nerves backwards, they will guide to the small len- 

 ticular ganglion (the size of a pin's head) and its branches. The dissector 

 should find then two branches from the nasal and third nerves to the 

 ganglion : the nasal branch is slender, and enters the ganglion behind, and 

 that of the third nerve, short and thick, joins the lower part. 



Lastly, the student should separate from one another the nasal, third, 

 and sixth nerves, as they pass between the heads of the external rectus 

 muscle into the orbit. 



The THIRD NERVE is placed highest in the wall of the cavernous sinus 

 (fig. 4, 3 ) ; but at the sphenoidal fissure it descends below the fourth, and 

 the two superficial branches (frontal and lachrymal) of the ophthalmic 

 nerve. It comes into the orbit between the heads of the outer rectus, 

 having previously divided into two parts. 



The upper piece (fig. 11, 8 ), the smallest in size', ends in the under sur- 

 face of the levator palpebrae and superior rectus muscles. 



The lower piece supplies some of the other muscles, and will be dissected 

 afterwards (p. 57). 



The nasal branch of the ophthalmic nerve (fig. II, 1 ) enters the orbit 

 between the heads of the rectus, lying between the two parts of the third 

 nerve. In the orbit the nerve is directed obliquely inwards to reach the 

 anterior of the two foramina in the inner wall. Passing through this 

 aperture with the anterior ethmoidal (nasal) artery, the nerve appears in 

 the cranium at the outer margin of the cribriform plate of the ethmoid 

 bone. Finally, it enters the nasal cavity by an aperture at the front of 

 the cribriform plate : and after passing behind the nasal bone, it is directed 

 outwards between that bone and the cartilage, to end on the outer side of 

 the nose. 



In the orbit the nasal crosses over the optic nerve, but beneath the 

 superior rectus and levator palpebrae muscles, and lies afterwards below 

 the superior oblique ; in this part of its course it furnishes the following 

 branches : 



The branch to the lenticular ganglion ( 5 ) is about half an inch long and 

 very slender, and arises as soon as the nerve comes into the orbit : this is 

 the long root of the lenticular ganglion. 



Long ciliary branches. As the nasal crosses the optic nerve, it supplies 

 two or more ciliary branches (fig. 11) to the eyeball. These lie on the 

 inner side of the optic, and join the ciliary brandies of the lenticular 

 ganglion. 



The infra-trochlear branch ( 2 ) arises as the nasal nerve is about to leave 

 the cavity, and is directed forwards below the pulley of the superior oblique 

 muscle, to end in the upper eyelid, the conjunctiva, and the side of the 

 nose. Before this branch leaves the orbit it receives an offset of commu- 

 nication from the supra-trochlear nerve. 



In the nose (fig. 34). Whilst in the nasal cavity the nerve furnishes 



