810 THE SENSE OF SIGHT 



lens and ciliary body. Many physiologists, however, believe that in 

 adult life the iris lies in absolute contact with the lens and that the pos- 

 terior chamber is merely a potential space. The transparent liquid 

 filling this entire cavity, is known as the aqueous humor. Its quantity 

 amounts to about 0.4 c.mm. and its specific gravity to 1.0053-1.008, 

 which is the equivalent of a solution of sodium chlorid of a concen- 

 tration of rather more than 1.0 per cent.-^ Its osmotic pressure 

 is somewhat higher than that of the serum of the blood. ^ It may con- 

 tain a few leukocytes, but only 0.08-0.12 per cent, of protein. This 

 watery fluid also permeates the interstitial spaces of the gelatinous 

 substance of the vitreous humor. 



If a small manometer is connected with the anterior chamber of the eye by 

 means of a tubular needle, it will be noted that the aqueous humor is held under a 

 pressure of about 25 mm. Hg. This pressure is designated as the intraocular 

 pressure. Its very obvious function is to render the eyeball tense so that its differ- 

 ent refractive elements are fully unfolded. It need scarcely be emphasized that 

 any unevenness in the cornea or an unduly relaxed ciHary body and ligament must 

 greatly impair the usefulness of these structures for refraction. In addition, it 

 may justly be assumed that the aqueous humor forms the nutritive medium for 

 the lens, ciliary ligaments, and vitreous humor, because these structures are not 

 directly supplied with blood. In certain pathological conditions, such as glaucoma, 

 the intraocular pressure is enormously increased so that the eyeball can scarcely 

 be indented with the finger. Clinically the tenseness of the eyeball is measured by 

 means of the ophthalmotonometer. This instrument is pressed against the outer 

 surface of the eyeball until its plate-like extremity causes a certain flattening at the 

 point of contact. The pressure necessary to accomplish this end, is indicated by 

 a tension spring. 



A number of observations have been made which prove conclusively 

 that the aqueous humor is continually renewed. Thus, any operation 

 requiring an incision through the cornea, most generally leads to a loss 

 of a considerable portion of this fluid which is again reformed in the 

 course of a few days. Furthermore, it is possible to drain it off in a 

 relatively steady stream by inserting a delicate cannula through the 

 margin of the cornea. Its character is then gradually changed until 

 it contains as much as 3 or 4 per cent, of proteins and becomes coagula- 

 ble. It is commonly held that the aqueous humor is secreted by 

 the epithelium of the ciliary body and its glands. From here it flows 

 into the anterior recess of the posterior (vitreous) cavity of the 

 eyeball, whence it finds its way through the clefts in the hgamentum 

 pectinatum iridis into the angle of the anterior chamber. A portion 

 of this fluid also escapes through the meshes of the ciliary ligament into 

 the posterior chamber situated between the iris and the lens, and thence 

 round the edge of the iris into the anterior chamber. The canal of 

 Schlemm is the natural drainage tube of this space. A portion of this 

 fluid also escapes into the lymph spaces of the iris and from here into 

 the perichoroideal lymphatics. Still another portion is diverted from 

 the cihary glands into the interstitial spaces of the vitreous humor, 



1 Golowin, Archiv fiir Ophthalm., li, 1900. 



2 Hamburger, Osmotischer Druck and Jonenlehre, 1904. 



