644 



DISSECTION OF THE ORBIT. 



Dissection. 



For the use of the muscle, see the description of the inferior 

 oblique (p. 650). 



Dissection (fig- 228). The superior rectus muscle is next to be 

 divided about the middle and turned backwards when a branch of 

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

 the nasal nerve and the ophthalmic vessels 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 

 nerves (ciliary] w r ith small arteries lying along the side of the optic 

 nerve ; and by tracing these ciliary nerves backwards, he will be 

 guided to the small lenticular ganglion (the size of a pin's head) 

 The dissector should find then two branches from the nasal am 

 third nerves to the ganglion : the nasal branch is slender, and entei 

 the ganglion behind ; while that of the third nerve, short and thicl 

 joins the lower part. 



The eyeball is to be fully exposed by dissecting off its investing 

 fascia (capsule of Tenon), which will be seen to send processes 

 around the several muscles inserted into the sclerotic. 



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; 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 parts. 



The upper division (fig. 228, 8 ) is the smaller, and ends in the under 

 surface of the levator palpebrae and superior rectus muscles. 



The lower division supplies the internal and inferior recti and the 

 inferior oblique muscles, and will be dissected afterwards (p. 648). 



The NASAL BRANCH OF THE OPHTHALMIC NERVE (fig. 228, A ) enters 



the orbit between the heads of the external rectus, Ivin^ between 



? J O 



the two parts of the third nerve, and is then directed obliquely 

 inwards to reach the anterior of the two internal orbital canals. 

 Passing through this aperture with the anterior ethmoidal 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 issues between that bone and the 

 cartilage, to end on the outer surface 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 

 ! L enSar f and Very slender > and ari ses as soon as the nerve comes into the 

 ganglion. orbit : this is the long root of the lenticular ganglion. 



Find len- 

 ticular 

 ganglion, 



and roots. 



Clean 

 eyeball. 



Separate 

 nerves. 



Third nerve 



as it enters 

 orbit ; 



its upper 

 branch, 



lower 

 branch. 



Nasal nerve. 



General 

 course to 

 the face. 



In the orbit. 



Branches. 



