ORGANS OF THE SPECIAL SENSES. 289 



This faculty draws upon the brain only for the- 

 stimulus derived from sensation and will, both of 

 which are, as we have seen, of insignificant 

 strength. The apparatus of motion includes the 

 spinal marrow, and the greater part of the large 

 and numerous nerves which proceed therefrom; 

 and, in addition to this commanding power resi- 

 dent in the nervous system, a host of inferior 

 agents, the muscles, by which the immediate 

 phenomena of motion are accomplished. 



The organs of special sense, namely, of sight, 

 hearing, smell, and taste, are each provided with a 

 vital and a mechanical apparatus. The mechanical 

 apparatus of sight, the eye, or, as it is popularly 

 termed, the eye-ball, or apple of the eye, is an 

 optical instrument of great perfection. In man 

 and in the higher animals, the eye-ball is very 

 nearly globular, the deviation from the perfectly 

 spherical shape being in favour of a greater con- 

 vexity of the transparent front of the organ. In 

 fishes, on the contrary, the breadth of the eye 

 greatly exceeds its depth, and it is flattened in 

 front to a remarkable degree, presenting, in fact, 

 the form of a segment of a sphere in place of a 

 perfect globe. It is obvious that this peculiarity 

 of shape is less easily maintained than that of an 

 entire sphere ; hence, it is no uncommon thing to 

 find the circumference of the eye-ball strength- 

 ened by a thin cup of gristle, and, in some instances, 

 U 



