SENSITIVE SYSTEM. 381 



out the tunic, that the opinion of its being divisible into layers 

 seems to be resisted by anatomical evidence. The jirincipal use 

 of the sclerotica is, to give configuration, support, and protection 

 to the abstractedly formless delicate parts encased within it. 

 It also afibrds attachment to the muscles moving the globe. 



Cornea. 

 The CORNEA (also called the transparent cornea) is the part 

 completing the sphere of the eyeball in front, filling up the el- 

 liptical vacuity left by the sclerotica, thereby forming rather more 

 than one-fifth of the whole superficies. In quadrupeds (not over- 

 looking that notable exception, the hog), this is the only exter- 

 nally visible part of the eye; they have not therefore, according 

 to the common meaning of the term, any xvhite to their eyes ; at 

 least none is apparent but on such occasions as when the animal, 

 tvithout turning his head, looks behind him, a glance that gene- 

 rally betokens slyness, if not viciousness : and this, I presume, 

 is the origin of the vulgar assignation of vice to a horse who is in 

 the habit of shewing the white of his eye. The convexity of the 

 cornea, which varies somewhat in different individuals, is greater 

 than that of the sclerotica ; it being, as was said before, the seg- 

 ment of a smaller sphere let into the segment of a larger one. Its 

 figure (taking the circumferent outline of the cornea) is hori- 

 zontically elliptical ; it is not a perfect ellipsis, however, for the 

 part turned inwards is bounded by an arc of larger sweep than 

 that turned outv/ards : a remark that will serve us, so long as we 

 remember that the optic nerve pierces the inner and inferior part 

 of the globe, to distinguish a right eyeball from a left, in the de- 

 tached state. The vertical diameter of the cornea bears the same 

 relative ratio to its longitudmal that five does to eight. Its mar- 

 gin is cut aslant to fit that of the sclerotica, by which it is over- 

 lapped ; in this manner, the surfaces of apposition are consider- 

 ably broadened, and their union so much the more strengthened : 

 indeed, their union is so firm, that as much resistance is opposed 

 to their mechanical separation as to the laceration of either of 

 their textures. Its convex surface is covered by conjunctiva, con- 

 tinued over it from the sclerotica, upon which its limits are marked 

 by a black elliptical line relieved by a dusky shade: this part of 

 the membrane, however, is so condensed and attenuated, that it 

 becomes perfectly diaphanous, .which circumstance has given rise 

 to doubt, and even denial, of its presence here ; the fact, however, 

 is nowadays admitted to receive indubitable evidence from long 

 protracted maceration. At first view we might suspect that the 

 cornea itself, being so pellucid a part, was thin and delicate in its 

 texture; but, in point of fact, it is allied in density, fiinuicss, and 



