612 THE SENSES. 



bility from cerebral compression, are all followed by dilatation of the 

 pupil ; and the same thing takes place immediately after death. In the 

 normal reflex actions of expansion and contraction of the pupil, under 

 the varying intensity of illumination, the fibres of the sphincter are 

 those which alternately contract and relax in a manner analogous to 

 that of the voluntary muscles ; while those of the dilator are more con- 

 tinuous in their operation, and are under the control of different nervous 

 influences. 



The pigmentary layer which is continued uninterruptedly, except at 

 the entrance of the optic nerve, over the internal surface of the choroid, 

 the ciliary processes, and the posterior surface of the iris, is called the 

 system of the uvea, from its resemblance to the skin of a purple grape 

 separated from its stem ; the opening of the membranous sac at the 

 point of detachment representing the orifice of the pupil. Owing to the 

 existence of this continuous pigmentary layer, no light can penetrate 

 the eyeball excepting that which enters through the pupil; and the 

 rays, furthermore, which reach the retina at any point are arrested 

 there, and prevented from being dispersed by reflection over other parts 

 of the membrane. 



Aqueous Humor and Vitreous Body. By the transverse partition of 

 the iris, the cavity of the eyeball is divided into two portions, an anterior 

 and posterior. The portion situated in front of the iris, called the "an- 

 terior chamber," is filled with a colorless, transparent fluid, of watery 

 consistency, the aqueous humor. This fluid is to be regarded as an 

 extremely dilute exudation from the bloodvessels of the surrounding 

 parts, especially from those of the iris ; since it consists mainly of water, 

 holding in solution less than two per cent, of solid ingredients, namely, 

 sodium chloride and other inorganic salts derived from the blood, with 

 a trace of albuminous matter. It is faintly alkaline in reaction, and 

 has a refractive power but slightly different from that of water. It 

 is rapidly reproduced after evacuation by puncture of the cornea. It 

 serves to maintain the internal tension of the anterior parts of the eye- 

 ball, and to allow of the changes of figure of the iris and crystalline 

 lens, without affecting the external configuration of the cornea. The 

 posterior and larger portion of the cavity of the eyeball is filled mainly 

 by a semifluid substance, the vitreous body, so called from its trans- 

 parent and glassy appearance. Its composition is similar to that of the 

 aqueous humor, excepting for the larger proportion of albuminous 

 matter, which gives it more or less of a gelatinous consistency. Its 

 refractive power, according to Helmholtz, though slightly greater than 

 that of the aqueous humor, does not differ much from that of water. 

 It distends the principal part of the cavity of the sclerotic, supports the 

 retina which is extended over its surface, and preserves the general 

 spheroidal form of the eyeball. 



The vitreous bocty is enveloped by an exceedingly thin, colorless 

 membrane, for the most part without definite structure, and measuring, 

 according to Kolliker, not more than 4 mmni. in thickness. This is the 



