228 ANIMAL PHYSIOLOGY. 



power which enables us to perceive, as in touch, the spot 

 where the stimulus is applied, but one by which the relative 

 positions of all the rays cantering the eye at one time may be 

 recognised. 



It is further necessary that every nerve-termination or 

 sensitive point in the eye shall receive only one ray at a time, 

 and that it shall be the proper ray. In the case of insects, 

 this is managed by every nerve-termination being placed at 

 the bottom of a long dark- walled tube, so that it is affected 

 by none but the ray which falls vertically on it. But in all 

 vertebrata, as well as in the cephalopodous molluscs, of which 

 the cuttle fishes are a familiar example, the object is achieved 

 in much greater perfection by an optical apparatus which 

 throws the inverted image of the landscape on a sensitive 

 surface at the bottom of a dark chamber. 



The whole optical apparatus, as well as the sensitive sur- 

 face, is contained within the eyeball ; the range of vision is 

 increased, and the two eyes are enabled to act in concert, by 

 means of muscles which turn the eyeballs; and inasmuch 

 as the fore part of the eye must be preserved from opacity, 

 whether from dryness, scratching, or the nutritive changes 

 consequent on irritation, it is protected by the eyebrows, 

 eyelids, eyelashes, and a lachrymal apparatus. 



169. The eyeball is a nearly spherical structure, about an 

 inch in diameter, pierced at the back, at a point about a tenth 

 of an inch internal to the centre, by the optic nerve, which, 

 being in its sheath a stout cylinder a sixth of an inch thick, 

 looks like the stalk of a berry. The outer investment of the 

 eyeball is protective, and, in the greater part of its extent, is 

 an exceedingly tough felted fibrous coat, called the sclerotic, 

 thickest behind; but in front it becomes abruptly trans- 

 parent, so as to form a clear window, the cornea, through 

 which one can look into the interior. At the edge of junc- 

 tion, the fibres of the sclerotic are continuous with those of 

 the cornea, the same bundle being opaque in the outer part 

 of its extent, and transparent in the inner; but in the cornea 

 the fibres are arranged in numerous parallel laminae, with 

 intercommunicating branched spaces between them. The 

 cornea is altogether destitute of blood-vessels, though sup- 

 plied with a network of nerves near its surface; and it is 



