STRUCTURE OF THE EYE. 149 



the rectus anticus^ internus, or adducetis, f, directs it forwards. 

 Acting simultaneously, these muscles would by compressing 

 the sclerotic coat, produce an effect upon the internal parts 

 which will be afterwards explained. The two oblique muscles 

 arise from the inner or fore part of the walls of the orbit, and 

 are inserted in the same manner as the recti : the ohliquus 

 superior, g, and ohliquus inferior^ /^, of which the action is to 

 direct the eye forwards. 



Viewing the eyeball laterally, as in Fig. 7, we observe that 

 its posterior surface forms the segment of a sphere, of which 

 the radius is seven and a half twelfths of an inch, while the 

 anterior or transparent part forms the half of a sphere, of which 

 the radius is three twelfths and a half. These two segments 

 are united by an intervening portion, convex in the greater 

 part of its extent, but concave toward the anterior part. We 

 have now to examine the structure of the ball of the eye. 



The dense membrane or coat which inverts the posterior part 

 of the eye, as far as the cornea, b b, is named the Tunica scle- 

 rotica, on account of its comparative hardness. It is rather 

 thin, firm, somewhat elastic, and of a glistening bluish white 

 colour. Although, according to authors, divisible into three 

 layers, it seems to me to be separable into two only, of which 

 the inner is transparent, and of an almost horny hardness. But 

 at its anterior part, c c, it is strengthened by a broad belt, com- 

 posed of a series of distinct flat bones, overlapping each other 

 by their edges, and interposed between the inner and outer 

 membranes. At the anterior edge of this bony circle, the 

 membranous structure is resumed ; and lastly, there is a thick- 

 ened margin of the same nature, but of a dark colour, h b, to 

 which the remaining convex part of the outer coat of the eye 

 is attached. 



Before describing the sclerotic bones, we may examine this 

 anterior membrane, b b b, which, on account of its bearing some 

 resemblance to clear horn, is named the Tunica cornea. It is 

 of considerable thickness, and has its outer and inner layers of 

 denser structure than the intervening portion. Being per- 

 fectly transparent, it allows the free transmission of the rays 

 of light to the interior of the eye. It is so firmly attached to 



