ANATOMICAL CONSIDERATIONS. 33: 



warmer climate; while the likeness between the equine race and "the 

 ship of the desert" demonstrates that that locality was the hottest portion 

 of the earth. 



The eye of the horse is also provided with a power which could sel- 

 dom be needed in these Northern climes, where the fleetness of the 

 equine tribe might readily set at defiance the comparative feebleness of 

 all the predatory beasts of prey. Besides, the wooded state of this 

 country must have rendered the presence of telescopic vision unneces- 

 sary. Upon the far-stretching level of the desert, however, where 

 larger and more ferocious animals prowl by night, the possession of 

 such a faculty would be a needed protection. Accordingly, we find the 

 interior of the globe to consist chiefly of water, the outward covering 

 being formed of a tough substance, which is easily compressible ; while 

 all the hidden portion of the exterior is enveloped by muscular fiber. 



Situated directly upon the forward portion of the ball are the two 

 oblique muscles. These are inserted at opposite places, and each pulls 

 in a contrary direction to the other. The two, simultaneously acting, 

 could not move the organ, but would, obviously, tend to fix it or to 

 render the globe stationary. The outer substance of the horse's eyes 

 is composed of a thick and pliable covering, purely tendinous in char- 

 acter. The interior consists of fluid perfectly pure and transparent. At 

 the back of all is placed the optic nerve ; while the exterior is enveloped 

 by several thick and straight muscles. 



diagram, displatino the coatino op muscular fiber which covers the boft globe op the 

 , horse's eye. 



The motor agents are endowed with an ability to contract or to 

 shorten in their reach. When parts of this nature operate upon a 

 plastic substance, which is filled only with a fluid, they must of neces- 

 sity tend to alter the shape of that body on which they repose. The 

 oblique muscles act to prevent rotation; the pressure, therefore, can 

 only compress, elongate, and force backward the ball of the eye. By 

 such a capacity that telescopic property is produced which man feebly 

 imitates by a complex and costly machine. 



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