330 THE SPECIAL SENSES. 



occur even when it is a matter of memory rather than immediate 

 experience. The explanation of this mydriasis can hardly be ob- 

 tained by experiment. So far as the physiological mechanisms are 

 concerned, it may be explained either by a stimulation of the dilator 

 muscle or by an inhibition of the tonus of the constrictor muscle. 

 In favor of the latter explanation we have the fact that stimula- 

 tion of the cortex cerebri causes dilatation when the dilator paths 

 are severed. 



Intraocular Pressure. The liquids in the interior of the 

 eye are normally under a pressure, the average value of which 

 may be estimated at 25 mms. of mercury. In consequence of 

 this internal pressure the eyeball is tense and its external surface, 

 including the cornea, shows a regular curvature. It is obvious 

 that folds or creases in the cornea would entirely destroy its use- 

 fulness, so far as the formation of an image is concerned. The 

 amount of the intraocular pressure may be measured by thrusting 

 a tubular needle, properly connected with a manometer, into the 

 anterior chamber of the eye. The liquid in the interior of the 

 eyeball may be considered as tissue lymph, and like the lymph 

 elsewhere it is derived from the blood-plasma. Investigation 

 has shown that the lymph is formed in the ciliary processes, but 

 in this as in other cases there is a difference of opinion as to whether 

 the production is due to so-called secretory or to mechanical 

 causes, such as filtration.* We may suppose that the liquid filters 

 into the eye through the vessels in the ciliary processes, and, on 

 the other hand, drains off or is absorbed at the angle of the anterior 

 chamber through the vein known as the canal of Schlemm. The 

 intraocular pressure rises until, under its influence, the outflow 

 just balances the inflow. It is evident from this point of view 

 that intraocular pressure will be increased by any change that 

 will augment the production of the liquid at the ciliary processes, 

 such as a rise of blood-pressure, or by any interference with the 

 outflow, such as might arise from a blocking of the canal of 

 Schlemm. Certain pathological conditions (glaucoma) are char- 

 acterized by an abnormally high intraocular tension, the differ- 

 ence from the normal being such that it is easily recognized by 

 pressure with the fingers. 



Methods of Determining the Refraction of the Eye. The condition 

 of the eye as regards its refraction may be determined by the use of suitable 

 charts and a series of spherical and cylindrical lenses. The results by 

 such a method depend largely upon the statements of the patient, that 

 is to say, they are largely subjective. A number of instruments have been 

 devised, however, by means of which the refraction of the eye may be studied 



* For discussion and literature, see Henderson and Starling, "Proceed- 

 ings Royal Society," 1906, B. vol. 77; Hill and Flack, ibid., vol. 85, 1912, and 

 Henderson, "Glaucoma," London, 1910. 



