THE STRUCTURE AND FUNCTIONS OF THE EYE. 257 



portion of the choroid coat which accomplishes this effect is 

 termed the tapetum lucidum, or kiminous carpet. — Ed.] 



The last and most considerable coat of the eye is the sclerotic 

 coat, which is a very strong, thick, white, and opaque membrane, 

 including all the other coats and humours, except the cornea. 

 The sclerotic coat forms the greater portion of the globe of tlie 

 eye, and is intimately united, towards the anterior part, with 

 the cornea, which may be viewed as bearing the same relation 

 to the sclerotic coat as the glass of a watch does to the case. 

 By maceration in water the coi'nea separates from the sclerotic 

 coat completely. 



The optic nerve penetrates the posterior part of the eye, not 

 in the centre, but rather inwardly ; still, however, it is central 

 with respect to the axis of vision, for both eyes are always 

 exerted in looking at objects, and. In doing this, incline a little 

 inwardly, so as to make the optic nerve central with respect to 

 the object, or rather to the axis of vision. 



Anatomists have supposed that, were this the case, the insertion 

 of the optic nerve would cause a dark spot to appear before the 

 eye, and that the nerve, therefore, enters a little inwardly. But 

 this is an error, as has been demonstrated by the experiment of 

 the late Dr. Darwin, in which, after looking at a coloured object 

 stedfastly for some time, upon shutting the eyes the impression 

 will remain a considerable time, and in an equal degree upon 

 the insertion of the optic nerve, and every part of the retina, 

 without the appearance of any dark spot whatever. This not 

 only proves that tliat part of the optic nerve which enters the 

 sclerotic coat, and from which the retina proceeds, is susceptible 

 of the impressions of light in the same degree as the retina, but 

 that the vitreous humour, or rather its arachnoid membrane, 

 performs an important office. In addition to that which it per- 

 forms as a part of a most beautiful and wonderful optical appa- 

 ratus : that is, it serves to collect and retain the impressions of 

 objects, to afford time for the intellect to judge of their distance, 

 as well as of their different properties. The optic nerve passes 

 through an opening in the bottom of the orbit, mxmQ({ foramen 

 lacerum, and crossing the opposite nerve, terminates in that part 

 of the brain named thalamus nei'vi optici. This decussation of 

 the optic nerves has been a subject of controvei'sy with anato- 

 mists. But the fact is completely proved by an examination of 

 the brain and optic nerves of a horse affected with one cataract 

 only ; and by an examination of the brain and optic nerves of a 

 sheep affected with that kind of hydrocephalus which depends 

 upon the existence of an hydatid in the right ventricle of the 

 brain, for it is the right ventricle that is generally affected. 



In both cases, the nerve which proceeds from the affected 

 thalamus will be found smaller than the other, and it will be 



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