THE NUTRITION OF THE EYEBALL 665 



of specialised sebaceous glands, the ' Meibomian glands.' The 

 fatty secretion of these glands is poured out at the edge of the 

 lids, keeping these and the eyelashes greasy, and so preventing 

 their being wetted by the tears. Any overflow of tears from the 

 conjunctival sac is thereby prevented, unless the secretion 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 

 inextensible capsule, the sclerotic, filled with fluid or semi-fluid 



FIG. 306. Arrangement of apparatus for measurement of intraocular pressure. 



(HENDERSON and STABLING.) 

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



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 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 accompany- 

 ing diagram (Fig. 306). 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 



