532 A MANUAL OF VETERINARY PHYSIOLOGY 



eyeball depends, the choroid may be regarded as that which 

 principally attends to the vascular supply, while the retina is 

 the sensitive expansion of the optic nerve on which the picture 

 is imprinted, and thus gives rise to sensory impressions. The 

 shape and tension of the eyeball is maintained by means of its 

 humours, which are known as the aqueous and vitreous. The 

 aqueous humour occupies the space between the cornea and the 

 Jens. It is a watery fluid, poor in solids, and is in reality lymph 

 It is constantly being secreted, probably by the ciliary processes, 

 and as constantly carried away by the lymphatic channels with 

 which it communicates through the spongy ligamentum pecti- 



Fig. 158. — Vertical Section of the Eye of the Horse, Natural Size. 



c, Cornea ; /, lens ; i, iris ; cp, ciliary process ; Ip, ligamentum pectinatum ; 

 cl m, position of ciliary muscle ; si, suspensory ligament of lens ; on, optic 

 nerve, showing its curve. Note its attachment to the lower part of the globe. 



natum ; these channels empty themselves into the anterior 

 system of veins. If the anterior chamber be experimentally 

 evacuated, it is refilled in about twenty-four hours. The use of 

 the fluid it contains is to maintain the convexity of the cornea. 

 After death the process of drainage still appears to occur, though, 

 of course, there is no reproduction, the result being that in a day 

 or two the cornea flattens through loss of the aqueous humour. 



The vitreous humour is a viscid, tenacious material, contained 

 within the hyaloid membrane, which permeates its substance. 

 The vitreous contains mucin and a very small percentage of 

 solids. The use of this fluid is to maintain the intra-ocular 

 pressure, by which the proper tension of the globe is brought 

 about. The whole of the vitreous chamber is rendered dark 



