246 Mammalia, 



eventually lodged in grooves on the inner surface of the 

 sclerotic, pierce the ciliary muscle, and ramify in the Iris. 



The Third nerve should now be traced forwards ; first 

 along the posterior edge of the inferior rectus, then across the 

 outer surface of the upper part of this muscle, to terminate 

 in the Inferior Oblique muscle. This muscle is situated near 

 the anterior margin of the orbit, and differs from the other 

 muscles in being directed across, instead of parallel to, the 

 axis of the orbit. It arises from the lachrymal bone, immedi- 

 ately beneath the lachrymal duct. From this spot the mus- 

 cle passes outwards above the internal rectus, and between 

 the suspensorius and external rectus, to be inserted into the 

 upper margin 'of the sclerotic coat at this point, close to the 

 insertion of the superior oblique muscle. 



The next stage of the dissection requires very great care, 

 viz., in tracing back the several nerve branches above enume- 

 rated through the dura mater and upper part of the cavernous 

 sinus to the Brain. 



The Cavernous sinus is so named from the reticulate 

 structure in its interior ; the space results from the separation 

 of the layers of the dura mater. The piece of dura mater 

 bounding the sinus externally is of some thickness, and con- 

 tains in its substance the following nerves. The most super- 

 ficial is the Fourth Nerve : it lies almost on the surface of the 

 internal aspect of that ridge of the dura mater which passes 

 along the tips of the clinoid processes : next below it, is the 

 Third Nerve ; and below this, again, the two quite distinct 

 though apposed branches of the Ophthalmic division of the 

 Fifth Nerve. Through the cavity of the Cavernous sinus the 

 Sixth Nerve passes. 



It will require considerable patience to trace these nerves, 

 and the dura mater in which they are imbedded should be 

 kept moist during the dissection by applying a damp sponge. 



To complete the dissection of the Orbit there only remains 



