120 ESSENTIALS OF PHYSIOLOGY. 



cornea is supplied with lymph from the blood-vessels which surround 

 its margin. The veins of the choroid converge to form four or five 

 main trunks, the vence vorticosw. The blood from the retinal artery is 

 returned by the corresponding retinal vein. 



INTRA-OCULAR TENSION. 



By inserting a cannula connected with a manometer into the anterior 

 chamber of the eyeball, it can be shown that the contents of the globe 

 exert a pressure on the walls equal to about 25 mm. of mercury. If the 

 intra-ocular pressure be recorded on a revolving drum simultaneously 

 with that of the carotid artery, it will be seen that the two tracings 

 run a parallel course. Obstruction of the descending aorta, for example, 

 causes an immediate rise in both curves, and these remain at the new 

 level till the obstruction is removed, when both fall simultaneously. 

 'The chief source of fluid in the eyeball is the vessels of the ciliary 

 processes, and as the pressure of the intra-ocular fluid varies with the 

 blood-pressure, it may be assumed that it is derived from the blood- 



L vessels by a process of filtration. Normally, the addition of new fluid 

 is balanced by the draining away of an equal amount, mainly through 

 the filtration angle and the canal of Schlemm, but also to a very small 

 extent by the posterior lymphatics of the eyeball. In certain diseased 

 conditions this drainage is interfered with, and fluid accumulates in the 

 eyeball, causing a rise of pressure. This condition is known as glaucoma, 

 and if it is not promptly relieved, it results in atrophy of the retina from 

 pressure, and therefore leads to blindness. 



The mechanism of transudation of fluid from the ciliary processes 

 and escape of an equal quantity by the canal of Schlemm and posterior 

 lymphatics of the eyeball is not only of importance for the nutrition 

 of the non-vascular contents of the eyeball, but the state of tension 

 which is thereby maintained gives the eyeball the degree of rigidity 

 which is necessary if it is to serve any useful purpose as an optical 

 instrument. 



THE FUNCTION OF SIGHT. 



The function of sight, in the commonly accepted sense of the word, 

 involves (1) the formation of a real image of external objects in the 

 retina, (2) changes in the retinal end-organs, (3) the transmission of 

 the stimulus due to the retinal changes to the cortex of the occipital 

 lobe, (4) the changes in the cortex, visuo-sensory and visuo-psychic 

 areas, which result in a visual sensation, and (5) the associational pro- 

 cesses of comparison of the sensation with previous sensations by which 

 visual judgments are formed. 



