HEAD AND NECK OF THE HORSE 117 



Dissection. — The dissection should begin witli a separation of the 

 sclera and cornea from the structures that lie within them. With great 

 care, make a short incision with ihe knife through the sclera at the 

 equator. The underlying choroid must not be injured: therefore the 

 incision must stop immediately pigment becomes visible. Now complete 

 the incision round the equator by means of scissois. The separation of 

 the sclera can be effected readily, except on a level witli the junction of 

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

 first-named position resistance is offered by the attachment of the ciliary 

 muscle, and can be overcome by using a pair of closed forceps or a 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 later 

 examination. 



The sc^errt^ is a dense, resisting, white tiuiic, composed of 

 interlacing meridional and circular bundles of fibres, and forming some 

 four-fifths of the whole tunica fibrosa. It is of considerable thickness 

 in the region of the posterior pole, and fairly thick close to the margin 

 of the cornea. The intermediate part, hovvever, is relatively thin and 

 may have a bluish tinge from the underlying pigment. The relative 

 thickness of the different parts may be expressed in figures as follows: 

 In the region of the corneo-scleral junction the sclera is about 1'3 mm. 

 in thickness; at the equator the thickness is reduced to ()'4 or O'o mm. ; 

 while at the posterior pole it attains a thickness of 1'5 to 2 mm. 



The external surface of the sclera gives attachment to the various 

 ocular muscles, and is partly covered (anteriorly) by conjunctiva. The 

 anterior margin circumscribes an oval opening, with a transverse long 

 axis, into which the cornea fits in much the same manner as does a 

 watchglass into its metal rim. The point of entrance of the optic 

 nerve, as has been previously noted, is below and lateral to the posterior 

 pole. Here the fibrous sheath of the nerve — derived from the dura 

 mater of the brain — is continuous with the sclera. The bundles of 

 fibres of the nerve pass through small orifices, the area containing them 

 being the lamina crlbrosa'' sclera'. 



Between the sclera and the choroid there is a perichoroidal lymph 

 space (spatium perichorioidale) crossed by fine ciliary vessels and 

 nerves and a loose network of pigmented connective tissue that con- 

 stitutes the lannina fusca.^ 



The sclera is poorly supplied with blood by the ciliary arteries. 

 Relatively large vems— vence vorticosce — pierce it about the equator, 

 and into these the small veins of the tunic open. A circular venous 

 sinus (si7ius venosus sclerw), which communicates with the scleral 



^ Sclera [L.], (TKXrjpoi (sklC-ros) [Gr.], hard. 

 - Grihriim [L.], a sieve, 

 ^ F'uscus [L.], brown. 



