534 BULBUS OCULI 



the external tunic a further dissection is required. Select an 

 eyeball for that purpose, and make an incision, with a sharp 

 knife, through the sclera at the equator. The incision must be 

 made carefully, and the moment that the subjacent black 

 chorioid coat appears the knife should be laid aside. The cut 

 edge of the sclera should now be seized with the forceps, and the 

 incision carried completely round the eyeball, with the scissors, 

 along the line of the equator. The outer fibrous tunic is thus 

 divided into an anterior and a posterior portion. Both parts 

 must now be raised from the subjacent structures. As the 

 anterior portion is turned forwards, some resistance will be met 

 with, close to the margin of the cornea ; it is due to the attach- 

 ment of the ciliary muscle to the deep surface of the sclera. 

 The attachment can easily be broken through with the blunt 

 point of the closed forceps ; as soon as that is done the aqueous 

 humour escapes. In the case of the posterior part of the sclera, 

 the complete separation of the sclera can be effected by dividing 

 the fibres of the optic nerve close to the point where they enter 

 the sclera from the inner side. 



When the above dissection is successfully carried out, the 

 outer fibrous tunic is isolated in two portions, whilst a con- 

 tinuous view of the intermediate vascular coat is obtained. 

 The eyeball, denuded of its external tunic, should now be placed 

 in a shallow vessel filled with water. 



Sclera. The sclera is what is commonly known as the 

 white of the eye. It is a dense, resistant tunic, opaque- 

 white in colour, which envelops the posterior five-sixths of 

 the globe of the eye. It is thickest posteriorly, and becomes 

 thinner as it is traced forwards. Near the cornea, however, 

 it again becomes thicker, owing to the accession of fibres 

 which it receives from the tendons of the ocular muscles. 

 Except at the entrance of the optic nerve and close to the 

 margin of the cornea, where it adheres to the surface of the 

 subjacent ciliary muscle, the deep surface of the sclera is very 

 loosely attached to the chorioid coat. Some pigmented floc- 

 culent connective tissue, called the lamina fusca, connects the 

 two coats and traverses what is, in reality, an extensive lymph 

 space, termed the perichorioidal space. 



The point at which the optic nerve pierces the sclera 

 does not correspond with the posterior pole of the eyeball. 

 The optic entrance, as it is termed, is situated about 3 mm. to 

 the medial or nasal side of the posterior pole and i mm. 

 below it. There the outer fibrous sheath of the optic 

 nerve, which is derived from the dura mater, blends with 

 the sclera, and the bundles of nerve fibres pass through a 

 number of small apertures. The perforated portion of the 

 sclera through which the fibres of the optic nerve pass is 

 called the lamina cribrosa. 



