SENSE OF SIGHT. 



191 



the eye on every side except the front, forming about four- 

 fifths of the external covering, and extending from the 

 entrance of the optic nerve, at its base, to the cornea. This 

 coat has almost the firmness of leather, and possessing but 

 li** 1 '} sensibility, it is rarely exposed to inflammation or other 



Fig. 1. ' 



Plan of the eye, seen in section. 



A, The Sclerotic Coat. F, The Aqueous Humour. 



B, The Choroid Coat. G, The Iris. 



C, The Retina. H, The Ciliary Processes. 



D, The Optic Nerve. I, The Crystalline Lens. 



E, The Cornea. K, The Vitreous Humour. 



diseases* The sclerotic coat is much denser in the eyes of 

 fishes than in the eyes of creatures which live on the surface 

 of the earth. In the whale, it is more than an inch thick, 

 in order, that when he dives some hundreds of fathoms deep, 

 the pressure of the water on the eye may not be greater than 

 its structure can withstand. That this pressure is very great, 

 is shown by the common experiment of corking an empty 

 bottle tight, and then letting it down into the sea by a cord ; 

 the cork will always be found forced in, and the bottle full 

 of water ; the pressure being equal to the weight of the 

 column of water above it, of which it is the base. Dr. Buck- 



