666 PHYSIOLOGY 



manometer E, and with a reservoir F, containing 0-9 per cent, salt solution. The 

 pressure in the apparatus is now raised to about 25 cm. of water. While the 

 fluid is dropping from the end of the needle, it is thrust through the lateral 

 part of the cornea, so as to lie in the middle of the anterior chamber. A bubble 

 is then introduced by the side tube, D, into the capillary tube, and the reservoir 

 adjusted to such a height that the bubble remains stationary. We know then 

 that the pressure inside the eye exactly balances the pressure of the fluid in 

 the reservoir, and we have also provided that there shall be no appreciable 

 escape of fluid from the eye or entry of fluid into the eye. If the bubble remains 

 stationary for three or four minutes we know that equilibrium is attained, and 

 we can read off the height of the intraocular pressure on the manometer E 

 connected with the reservoir. 



On making an opening into the cornea the fluid drains away 

 and the eyeball becomes soft and collapsed, the cornea being folded, 

 and the eye being naturally useless as an optical instrument. The 

 fluid which flows away, and which forms the aqueous humour and 

 also fills the interstices of the gelatinous tissue of the vitreous, con- 

 tains only a minute trace of protein, consisting, in every 100 parts, 

 of 98'7 parts water and T2 to T3 total solids, of which only 0'08 

 to 0'12 part consists of protein. If a cannula be kept in the anterior 

 chamber this fluid rapidly alters its character, becoming coagulable, 

 and containing 3 to 4 per cent, of proteins. 



The intraocular fluid is continually being renewed. The eyeball 

 receives a rich vascular supply, which forms a close network of vessels 

 and capillaries in the choroid coat, with its prolongations the ciliary 

 processes and iris. The chief seat of formation of the intraocular 

 fluid is the ciliary processes. Here there is a constant transudation 

 of fluid from the blood-vessels into the anterior part of the vitreous 

 cavity, the amount of the transudation varying with the pressure 

 in the blood capillaries (Fig. 307), being increased by any rise in the 

 capillary blood pressure or by any fall in the intraocular pressure. 

 Of the fluid poured out by the ciliary processes a very small propor- 

 tion (perhaps one-fiftieth) passes backwards into the vitreous humour 

 and gradually drains out of the eyeball by the lymphatic spaces of 

 the optic nerve. By far the larger amount passes forward through 

 the fibres of the suspensory ligament into the posterior chamber (the 

 annular cavity between the iris in front and the lens and ciliary 

 processes behind), and thence round the margin of the iris into the 

 anterior chamber. From the anterior chamber it passes into the 

 spaces of Fontana at the outer angle of the chamber, whence, under 

 pressure, it can filter slowly between the endothelial cells lining the 

 canal of Schlemm into this vessel and so drain away into the venous 

 system. A considerable resistance is offered to the passage of fluid 

 into the canal of Schlemm. Hence the constant transudation of 

 fluid from the ciliary processes raises the intraocular pressure to 

 25 mm. Hg, and a continuous production of about 6 cubic milli- 



