656 



DISSECTION OF THE EYE. 



to the sclerotic, and to remove the cornea from the front of the eyeball ; 

 on piercing the cornea the aqueous fluid escapes from the containing 

 chamber. The outer structures may be then abstracted from the interior 

 of the sclerotic covering, and may be set aside with the cornea for subse- 

 quent use. 



Fig. 227. 



a. Outer or sclerotic coat, and d, the 



cornea. 



b. Middle or choroidal coat. 

 m. Ciliary ligament. 



s. Ciliary process. 



e. Ciliary muscle, and/, iris. 



c. Inuer c at of retina, continuous 



with the optic nerve behind, 

 with a dark layer outside it. 



g. Lens. 



t. Suspensory ligament of the lens. 



ft. Vitreous body. 



n. Hyaloid membrane. 



i. Posterior chamber. 



0. Canal of Petit 



r. Sinus circularis iridis. 



1. Optic nerve. 



The dotted line through the centre 

 is the longitudinal axis of the 

 ball. 



DIAGRAM OF A HORIZONTAL SECTION OP THE EYEBALL. 



The sclerotic tunic of the eye (fig. 227, a) is bell-shaped, and extends 

 from the entrance of the optic nerve to the margin of the cornea, forming 

 about five-sixths of the ball. 



At its back, and a little to the inner side of the centre (one-tenth of an 

 inch), the optic nerve (1) is transmitted through an aperture in it : this 

 opening decreases in size from without inwards, and is cribriform when 

 the nerve is drawn out the lattice-like condition being due to the bundles 

 of fibrous tissue between the funiculi of the nerve. Small apertures for 

 the passage of vessels and nerves are situate around the optic nerve ; and 

 there are others for vessels at the front and centre of the ball. Anteriorly 

 the sclerotic is continuous with the transparent cornea. 



On the outer surface this coat is smooth, except where the muscles are 

 attached ; on the inner aspect it is of a dark color with flocculi of fine 

 areolar tissue (membrana fusca) uniting it to the next coat, and with the 

 ends of ruptured vessels and nerves. 



The sclerotic covering is thickest at the back of the eyeball, but it be- 

 comes thinner and whiter about a quarter of an inch from the cornea, 

 where it is visible as the " white of the eye." Where it joins the cornea 

 it becomes again somewhat thickened. In its substance near the union 

 with the cornea is a small flattened venous space, the canal of Schlemm 

 (sinus circularis iridis) (fig. 23o, r ). 



Structure. The sclerotic coat is formed of layers of white fibrous tissue, 

 collected into bundles, with a fine network of yellow or elastic fibres. In 



