THE CORNEA, IRIS AND AQUEOUS HUMOR 811 



whence it finds its way into the lymphatics accompanying the optic 

 nerve. 



At all events, the offlow balances the production, so that the aque- 

 ous and Vitreous humors are constantly held under a pressure of about 

 25 mm. Hg. This implies that these different drainage tubes are 

 adjusted so as to place a considerable resistance in the path of the 

 escaping fluid. In spite of this fact, however, it has been estimated 

 that at least 6 c.mm. of new secretion are required per minute in order 

 to maintain the pressure at the height just stated; moreover, it has 

 been found that its quantity may be varied considerably by either 

 raising or lowering the blood pressure. It cannot be doubted that 

 this factor plays an important part in all processes of secretion, because 

 it gives rise to the secretory pressure, but it seems that it is of special 

 value for the formation of the aqueous humor. This fact suggests 

 that this fluid finds its origin in large part in transudation. 



The Iris. The circumferential border of the iris is anchored to the 

 eyeball immediately in front of the ciliary body. At this point it is 

 continuous with the choroid coat as well as with the cornea through 

 the ligamentum pectinatum. In its course through the aqueous 

 chamber, its posterior surface is brought into close relation with the 

 ciliary body and lens, while its anterior surface is everywhere fully 

 exposed to the humor filling this cavity. Its inner margin surrounds 

 an orifice, the pupil, through which the rays of light are enabled to 

 enter the vitreous chamber. This orifice is nearly circular in shape 

 and is placed somewhat nearer the nasal side of the eyeball. Under 

 ordinary conditions its diameter measures about 4 mm., but is subject 

 to constant changes in consequence of variations in the intensity of the 

 light and the range of accommodation. A fuller discussion of this 

 phenomenon will be given in a subsequent paragraph. 



When looked at from in front, the iris measures about 11 mm. across, 

 its inner margin being held at a distance of about 5 mm. from its cir- 

 cumference. Its thickness amounts to about 0.4 mm. Its body is 

 formed by a stroma, consisting of a delicate framework of connective 

 tissue, the fibers of which are in large part arranged in a radial direc- 

 tion. Anteriorly, the latter is lined with cells similar in structure to 

 those covering the posterior limiting membrane (Descemet) of the 

 cornea. Posteriorly, it is enveloped by two layers of epithelial cells, 

 containing black pigment to which the blue color of the iris is due blue 

 becaus-e transmitted through the stroma. Its different shades of 

 black, brown and gray, however, are caused by pigment cells which are 

 scattered through the substance of the stroma. 1 



The plain muscle fibers of the iris are arranged either circularly 

 around the lumen of the pupil or radially to it. The former are most 

 numerous right next to its margin, where they form a conspicuous 

 sphincter, about 0.5 mm. in width. The latter form a layer of elon- 



1 Retzius, Biolog. Untersuchungen, 1893. 



