308 



ANIMAL PHYSIOLOGY. 



[LESSON 96. 



of plates, of great breadth at their anterior portion ; the vessels at 

 the posterior portion gradually glide into the vessels (arteries) of 

 the middle layer of the choroid. 



1402. The veins (venae vorticosae) of the external membrane of 

 the choroid, are well seen in a preparation of them from the Dog (Fig. 

 437). Some of them are of large size, and all of them (in the prepa- 

 ration) distinguished by great roundness. 



1403. But it is in the visual apparatus of Man, organized for 

 seeing in the erect position of the trunk, that we find the most com- 

 plete protection of the -orbits, by solid osseous parietes, and the 

 most parallel direction of their axes. Shaded externally by the eye- 

 rie. 437. 



FIG. 486. 



Ciliary processes, eye of the Cat. 



Venae vorticosae, choroid of Dog. 



brows, which are moved by their proper muscles, and protected by 

 two highly movable eye-lids, which continue over the forepart of the 

 organ, the human eye presents only a small rudiment of the third 

 eye-lid, or membrana nictitans, so highly developed in most of the 

 inferior vertebrata. 



1404. The eye-balls are nearly spherical in form, with their axes 

 parallel, and perforated behind by the optic nerves ; they are sup- 

 ported on the back part by a large deposit of adipose substance (fat) , 

 and are moved by four recti (straight) , and two oblique muscles. 



1405. A longitudinal section of the human eye will be found 

 at Fig. 438. 



The function of the optic nerve is to inform the brain of all the 

 details of form, color, &c., of any object pictured upon its thin ex- 

 pansion, the retina. 



The tubes, or fibres, as they have been erroneously described, 

 which enter into the composition of an optic nerve, decussate (di- 



