822 



THE INTRAOCULAR PRESSURE. 



of Tenon, which lies between the sclera and Tenon's capsule. Posteriorly 

 this is continuous with a lymph-space surrounding the surface of the optic nerve in 

 the form of a sheath; anteriorly it is in direct communication with the subcon- 

 junctival lymph-spaces of the eyeball. The optic nerve has three sheaths: (i) 

 The dural; (2) the arachnoid; and (3) the pial, arising from the corresponding 

 cerebral membranes. Between these three sheaths there are two lymph-spaces: 

 the subdural, between i and 2, and the subarachnoid, between 2 and 3 (Fig. 271). 

 Both are lined by endothelial cells: fine trabeculse extending from one wall to 

 the other are similarly covered by cells. According to Axel Key and Retzius 

 these lymph-spaces communicate anteriorly with the perichoroidal space. 



The aqueous humor resembles closely the cerebrospinal fluid, and contains 

 albumin, some sugar, urea and sarcolactic acid (which is present in the vitreous 

 body). The albumin increases when the difference between the blood-pressure 

 and the intraocular pressure is augmented. Such changes of pressure and like- 

 wise intense irritations applied to the eye cause the production of fibrin in the 

 anterior chamber. 



MHHBVw 



7. I /* / 



FIG. 271. Horizontal Section through the Optic Nerve, at its Entrance into the Eyeball through the Coats of the 

 Jiye: a inner, b outer layers of the retina; c, choroid; d, sclera; e, physiological cup; /, central artery of the 

 retina m the nerve; g, its point of bifurcation; h, lamina cribrosa; /, outer (dural) sheath; m, outer (subdural) 

 lymph-space; n, inner (subarachnoid) lymph-space; r, middle (arachnoid) sheath; p, inner (pial) sheath 

 t, nerve-fiber bundles; k, connective-tissue (longitudinal) septa. 



, . The fl uid within the eye is under a definite pressure, the intraocular pressure, 



This depends ultimately upon the pressure in the arteries in the 



interior of the eye, and must rise and fall with the latter. It is determined by 



the resistance or the yielding of the eyeball with the fingers. It may be 



measured more accurately by an apparatus, the "ophthalmotonometer." Like 



tne arterial pressure, it is influenced by many circumstances. It is increased 



with every pulse-beat and every expiration and it is decreased with every inspira- 



ie elasticity of the sclera and the cornea acts as a regulator with every 



:rease m the arterial pressure, causing, like the air-chamber of a fire engine, 



e venous blood to be driven out when more arterial blood is pumped into the 



is also important for the stability of the intraocular pressure that the 



aqueous humor is secreted as rapidly as it is absorbed. Increase of the intra- 



ocular pressure makes the cornea flatter. 



The secretion of the aqueous humor takes place with comparative rapidity, a fact 

 tnat Landois ; proved by the appearance of hemoglobin in the anterior chamber of a 

 dog nail an hour after the introduction of free hemoglobin (transfusion of lamb's. 



