310 THE SPECIAL SENSES. 



lary accompaniment of the emotional states may occur even when 

 it is a matter of memory rather than immediate experience. The 

 explanation of this mydriasis can hardly be obtained by experi- 

 ment, but reasoning from analogy we know that strong emotional 

 states are usually accompanied by more or less distinct inhibitory 

 effects on motor centers, and perhaps in this case the reaction is 

 most satisfactorily explained by attributing it to an inhibition of 

 the constrictor center in the midbrain. 



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. The facts that are known seem to 

 be explicable from the mechanical point of view.* We may 

 suppose that the liquid filters into the eye through the vessels 

 in the ciliary processes, and, on the other hand, drains off at the 

 angle of the anterior chamber through the canal of Schlemm. 

 The intraocular pressure rises until, under its influence, the out- 

 flow 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 

 characterized by an abnormally high intraocular tension, the 

 difference 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. Ixxvii. 



