220 ORGANS OF THE SPECIAL SENSES. 



motion is effected the largest in their whole 

 economy. This faculty draws upon the brain 

 only for the stimulus derived from sensation and 

 will, both of which are, as we have seen, of in- 

 significant 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 resident in the nervous system, a host of 

 inferior agents, the muscles, by which the imme- 

 diate 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 me- 

 chanical 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 convexity 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, pre- 

 senting, 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 main- 

 tained than that of an entire sphere; hence, it 

 is no uncommon thing to find the circumference 



