572 BULBUS OCULI 



chamber; and (3) the vitreous body, occupying the posterior 

 part of the interior of the eyeball. 



Dissection. The superficial surface of the sclera and the cornea 

 should now be examined ; but to complete the study of the external tunic 

 a further dissection is required. Selecting an eyeball for this purpose, an 

 incision should be made, with a sharp knife, through the sclera at the 

 equator. This must be done carefully, and the moment that the sub- 

 jacent 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. These must now be raised from the subjacent parts. 

 As the anterior portion is turned anteriorly, some resistance will be met 

 close to the margin of the cornea from the attachment of the ciliary muscle 

 to the deep surface of the sclera. This can easily be broken through with 

 the blunt point of the closed forceps ; as soon as this is done the aqueous 

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

 complete separation can be effected by dividing the fibres of the optic 

 nerve close to the point where they appear through the sclera. 



When the above dissection is successfully carried out, the outer fibrous 

 tunic is isolated in two portions, whilst a continuous view of the inter- 

 mediate 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 anteriorly. 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 optic entrance 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 flocculent 

 connective tissue, the lamina fusca, passes between 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 

 series of small apertures. This perforated portion of the 

 sclera is called the lamina cribrosa. 



