THE NUTRITION OF THE EYEBALL 595 



becomes excessive ; so that the whole of the fluid under normal circumstances 

 is kept within the sac and flows away only through the nasal ducts. 



INTRAOCULAR PRESSURE. The eyeball is formed of a tough in- 

 extensible capsule, the sclerotic, filled with fluid or semi-fluid contents. In 

 order that the eyeball may be sufficiently rigid to maintain the normal 

 relations of the various refractive media, and to afford a fixed point for the 

 action of the ciliary muscle, this fluid must be under pressure. On connecting 



FIG. 308. Arrangement of apparatus for measurement of intraocular pressure. 



(HENDERSON and STARLING.) 

 G is a piston-recorder for recording graphically the changes in pressure. 



a .small manometer with the anterior chamber, care being taken to prevent 

 any escape of the intraocular fluid, it is found in the normal eye that this 

 pressure is about 25 mm. Hg. 



The problem of measuring intraocular pressure is analogous to that of measuring the 

 intracranial pressure. The eyeball represents a cavity into which fluid is continually 

 being poured and from which it is being absorbed, the intraocular tension determining 

 the exact balance between the processes of secretion and absorption. It is therefore 

 necessary in determining the amount of this pressure to take care that no fluid either 

 enters or leaves the eyeball. For this purpose we can make use of the arrangement 

 represented in the accompanying diagram (Fig. 308). The steel needle A is connected 

 to a capillary glass tube, CB. This has a lateral opening, through which a bubble of air 

 can be introduced into the tube. By means of a T -piece the capillary is connected 

 with a water 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 intro- 

 duced 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. 



