TUNICA SCLEROTICA. 411 



the recti muscles, about half a line. The latter is the weakest 

 part of the parietes of the eye-ball, and a blow applied upon 

 the cornea will cause a rupture at that part, rather than at any 

 other. — 



It has been considered, by many anatomists, as continued 

 into the cornea ; but it can be separated from it by putrefac- 

 tion ; and it is essentially different from it in structure. 



The aperture in it, which is occupied by the cornea, is not 

 perfectly circular, but inclines somewhat to the oval form, the 

 transverse diameter being rather longer than the vertical. 

 • Posteriorly it is intimately connected with the optic nerve, 

 which enters it, not directly at the extremity of the axis that 

 passes through the centre of the cornea and pupil, but on the 

 inside of this spot. 



The optic nerve has a firm coat (neurilema) which invests it 

 rather loosely ; this coat seems to be continued into, or expanded 

 upon the sclerotica. Within it the delicate nerve diminishes 

 considerably before it perforates the sclerotica, and appears to 

 be composed of fibres. At the small place of its penetration, 

 the sclerotica is very thin ; and it seems that the nerve does not 

 penetrate through one aperture, but these fibres pass separately 

 through very small foramina, in this thin part. The foramina 

 are necessarily very small, as the diameter of the whole perfo- 

 rated portion in some eyes, does not exceed two lines, or 

 one-sixth of an inch.* 



The sclerotica, in its natural state, has few, if any, vessels, 

 that carry red blood. The great vascularity, which is so evi- 

 dent in ophthalmia, is in the tunica conjunctiva. 



— It is perforated posteriorly immediately around the optic 

 nerve, with a great number of oblique foramina, through which 



* We find one or two of these larger than the rest, for the transmission of 

 the central artery and vein of the retina. The space occupied by these fora- 

 mina, is called the lamina cribrosm, or sieve-like plate of the sclerotica. The 

 optic nerve is made up of bundles of medullary filaments, each of which is 

 enclosed in a minute envelop of cellular tissue, formed by a process reflected 

 inwards from the general neurilema of the nerve. By opening the eye, and 

 removing all the coats but the sclerotic, and then squeezing slightly and press- 

 ing forward the nerve, in the direction of the ball, the medullary matter will be 

 seen to exude through the foramina of the cribriform plate. — f . 



