THE NUTRITION OF THE EYEBALL 597 



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 con- 

 tinuous production of about 6 cubic millimetres of fluid per minute suffices 

 to maintain the pressure at this height. 



If the formation of intraocular fluid be increased by rise of blood pressure, 

 the intraocular pressure must also rise until a point is reached at which the 

 increased filtration through the anterior angle is just equal to the increased 

 production resulting from the rise of blood pressure. Within wide limits 

 therefore the intraocular pressure varies with the blood pressure. This is 

 shown by the following records of both pressures in different animals 

 (Henderson and Starling) : 



DOG 

 Arterial pressure Intraocular pressure. 



128 mm. Hg. . . 26 mm. Hg. 



158 mm. Hg. . . 34 mm. Hg. 



180 mm. Hg. . . 40 mm. Hg. 



70 mm. Hg. . . 23 mm. Hg. 



Conversely the intraocular pressure may be altered by increasing or 

 diminishing the ease with which the fluid escapes through the filtration angle. 

 If the anterior angle becomes blocked as the result of inflammatory changes, 

 or other causes, the intraocular pressure rises gradually until it attains a 

 height far above normal. The eyeball to the finger feels stony hard, and the 

 increased pressure affects seriously the circulation of blood through the 

 retinal vessels, so that atrophy of the retina is produced together with dis- 

 turbance of the nutrition of the whole eyeball. This condition of raised 

 intraocular tension occurs in the disease known as glaucoma. 



The constant renewal of the intraocular fluid is important not only for 

 the maintenance of the intraocular pressure but also for the nutrition of the 

 structures, such as the lens, suspensory ligament, and vitreous humour, which 

 do not receive any vascular supply. 



