DISSECTION OP THE DOG 189 



the shortest distance between the two poles, and by meridians (meridiani) 

 which follow the surface of the eyeball. The equator (sequator) is a line drawn 

 equidistant from the two poles cutting the meridians at right angles. 



The eyeball is composed of three concentric tunics enclosing three refracting 

 media. The tunics are : (1) An outer, dense, fibrous coat (tunica fibrosa 

 oculi) consisting of the posterior opaque sclera and the anterior transparent 

 cornea ; (2) a middle vascular and pigmented envelope (tunica vasculosa oculi) 

 comprising a posterior and most extensive portion, the choroid (chorioid), an 

 intermediate ciliary body (corpus ciliare), and an anterior diaphragm, the iris ; 

 and (3) an internal and thin nervous tunic, the retina. 



The refracting media are : (1) The most posterior and most voluminous, the 

 vitreous body (corpus vitreum), of jelly-like consistence; (2) the crystalline 

 lens (lens crystallina) , the most dense of the media; and (3) an aqueous 

 humour which occupies the most anterior part of the eyeball. 



Dissection. — In order that a complete examination may be made of the 

 sclera and cornea it is necessary to separate them from the structures 

 within. With great care make an incision with a knife through the 

 sclera at the equator. The underlying choroid must not be injured ; 

 therefore the incision must stop as soon as pigment becomes visible. 

 Now complete the incision round the equator by means of scissors. The 

 separation of the sclera can be very readily effected, except at the junction 

 of the sclera and cornea and at the entrance of the optic nerve. In the 

 former position resistance is offered by the attachment of the ciliary 

 muscle, but can be easily overcome by using a pair of closed forceps or 

 similar blunt instrument. The optic nerve must be severed immediately 

 it has pierced the sclera. The fibrous tunic of the eyeball is now 

 divided into two parts. The rest of the specimen must be preserved 

 for future examination. 



Sclera. — The sclera is a dense, resisting, white tunic, of considerable 

 thickness in the neighbourhood of the optic nerve, and fairly thick close to 

 the margin of the cornea. The intermediate part, however, is relatively 

 thin, so much so, indeed, that generally the shimmer of the dark underlying 

 choroid imparts a bluish colour to this part of the sclera. 



The free anterior margin of the sclera (rima cornealis) , with which the cornea 

 is connected, is almost perfectly circular. The point of entrance of the optic 

 nerve is not quite at the posterior pole of the eyeball, but slightly ventral and 

 lateral to it. Here the fibrous sheath of the nerve — derived from the dura 

 mater of the brain — is continuous with the sclera, and bundles of nerve-fibres 

 pass through small orifices in the sclera. The area so pierced is known as the 

 cribrose lamina of the sclera (lamina cribrosa sclerse). 



Between the sclera and the choroid is the perichoroideal lymph space 



