LESSON 96.] THE EYES IN MAMMALIA AND MAN. 



307 



when the head is bowed down to the ground, and the creatures are 

 occupied in cropping the herbage, microscopic vision is required ; but 

 when seeking the best pastures, or keeping a watchful look-out for 

 fear of surprises from their natural enemies, the feline species, tele- 

 scopic vision is essential. For this purpose they possess a seventh 

 muscle to the eye-ball one more than belongs to other animals 

 this is called the retractor muscle, and its office is to draw the eye- 

 ball back into the orbit, thereby effecting great alteration in its focal 

 capabilities. The eyes of a dead Cow, or Sheep, are generally re- 

 tracted deep into the orbit, by the contraction of these muscles. 

 This muscle may be fitly compared to the coarse adjusting screw of 

 a microscope; the fine adjustment has yet to be explained. 



1398. While the muscles of the eye in all animals contribute, by 

 their action, in altering the focal length of the eye, the last act, by 

 which thorough sharpness of definition is acquired, is reserved for 

 the ciliary processes. It has been stated that these bodies impinge 

 upon the crystalline lens, at its margins ; in 



addition to being highly vascular, they are 

 also provided with erectile tissues, and when 

 the capillaries are distended with blood, the 

 processes become erect. In this condition 

 their combined action slightly moves the lens, 

 by which means the last process of perfect ad- 

 justment prevails, and is made complete. 



1399. The ciliary processes of the rumi- 

 nant are without parallel in the animal king- 

 dom ; they consist, even in the injected state, 

 of a vast number of folds (Fig. 435), and 

 when straightened out by the action of the 

 erectile tissue, they must exercise an unusual 

 influence on the position of the lens. 



FIG. 435. 



Ciliary processes, Ox. 



LESSON XCYI. 



THE EYES IN MAMMALIA AND MAN, CONTINUED. 



1400. In the Feline animals, the ciliary processes are beautifully 

 developed, and more nearly resemble those of Man. 



1401. In the domestic Cat (Fig. 436), they consist of a number 



