VISION. 551 



to all the ocular muscles. When the globe of the eye is with- 

 drawn by its muscles, the third eyelid is pushed out in a me- 

 chanical way with little or no direct assistance from muscles. 



In this connection may also be mentioned the gland of 

 Harder, a yellowish red glandular structure situated about the 

 middle of the outer surface of the third eyelid, which furnishes 

 a thick unctuous secretion, also of a protective character. These 

 structures are all the more necessary, as in few animals is the 

 globe of the eye so well protected by bony walls as in man. 



SPECIAL GONSIDIiRATIONS. 



Comparative. — It seems to be established that some animals 

 devoid of eyes, as certain myriopods, are able to perceive the 

 presence of light, even when the heat-rays are cut off. The most 

 rudimentary beginning of a visual apparatus appears to be a 

 mass of pigment with a nerve attached, as in certain worms ; 

 though it is questionable whether mere collections of pigment 

 without nerves may not in some instances represent still earlier 

 rudiments of our eyes. 



The eye of the fish is characterized by flatness of the cornea; 

 spherical form of the lens, the anterior surface of which pro- 

 jects far beyond the pupillary opening ; the presence of a pro- 

 cess of the choroid (processus falciformis) ; and usually the ab- 

 sence of eyelids, the cornea being covered with transparent skin. 



The eye of the bird, in some respects the most perfect visual 

 organ known, is of peculiar shape as a whole, presenting a large 

 posterior surface for retinal expansion ; a very convex cornea, 

 a highly developed lens, an extremely movable iris ; eyelids 

 and a nictitating membrane (third eyelid), which may be made 

 to cover the whole of the exposed part of the eye, and thus 

 shield screen-like from excess of light ; ossifications of the scle- 

 rotic ; a structure which is a peculiar modification of the 

 choroid, of which it is a sort of offshoot and like it very vascular, 

 answering to the falciform process of the eye of the fish and the 

 reptile. From its appearance it is termed the pecten. ■ Birds, on 

 account of a highly developed ciliary muscle, possess wonderful 

 powers of accommodation, rendered important on account of 

 their rapid mode of progression. They also seem to be able to 

 alter the size of the pupil at will. Their iris is composed of 

 striped muscular fibers. 



A layer of fibrous tissue outside of the choroidal epithelium 



