774 SPECIAL SENSES. 



from before backward, until its posterior border apparently fuses with the fibrous struct- 

 ure of the choroid. It is semitransparent and of a grayish color. Its situation is just 

 outside of the ciliary processes, these processes projecting in front of its anterior border 

 about V of an incn - Regarding the anterior border of this muscle as its origin and the 

 posterior border as its insertion, it arises in front from the circular line of junction of the 

 cornea and sclerotic, from the border of the membrane of Descemet, and the ligamentum 

 iridis pectinatum. Its fibres, which are chiefly longitudinal, pass backward and are 

 lost in the choroid, extending somewhat farther back than the anterior limit of the 

 retina. In addition, a net-work of circular muscular fibres has been described lying over 

 the anterior portion of the ciliary body, at the periphery of the iris, beneath the longitu- 

 dinal fibres. Some of these fibres have an oblique direction. 



Although there was formerly considerable discussion with regard to the structure of 

 the ciliary ligament, or muscle, there can now be scarcely any doubt of the fact that it is 

 composed mainly of muscular fibres. These fibres, anatomically considered, belong to the 

 non-striated, or involuntary variety. They are pale, present numerous oval, longitudinal 

 nuclei, and have no striae. 



It is evident, from the arrangement of the fibres of the ciliary muscle, that its action 

 must be to approximate the border of connection of the sclerotic and cornea and the cir- 

 cumference of the choroid, compressing the vitreous humor and relaxing the suspensory 

 ligament of the crystalline lens. We shall see farther on that this action enables the lens 

 to change its form, and probably it adapts the curvature of the lens to vision at different 

 distances. The nerves of the ciliary muscle are derived from the long and the short 

 ciliary. 



Iris. The iris corresponds to the diaphragm of optical instruments, except that its 

 orifice is capable of dilatation and contraction. It is a circular membrane, situated just 

 in front of the crystalline lens, with a round perforation, the pupil, near its centre. It is 

 called the uvea by some anatomists, a name that was formerly applied to the iris and 

 choroid together. 



The attachment of the greater circumference of the iris is to the line of junction of 

 the cornea and sclerotic, near the origin of the ciliary muscle, the latter passing back- 

 ward to be inserted into the choroid, and the former passing directly over the crystalline 

 lens. The diameter of the iris is about half an inch. T^he pupil is subject to considerable 

 variations in size. When at its medium of dilatation, the diameter of the pupil is from 

 to of an inch. The pupillary orifice is not in the mathematical centre of the iris but is 

 situated a little toward the nasal side. The thickness of the iris is a little greater than 

 that of the choroid, but it is unequal in different parts, the membrane being thinnest at 

 its great circumference and its pupillary border, and thickest at about the junction of its 

 inner third with the outer two-thirds. It slightly projects anteriorly and divides the space 

 between the lens and the cornea into two chambers, anterior and posterior, the anterior 

 chamber being much the larger. Taking advantage of a property of the crystalline lens, 

 called fluorescence, which enables us, by concentrating upon it a blue light, to see the 

 boundaries in the living eye, Helmholtz has demonstrated that the posterior surface of 

 the iris and the anterior surface of the lens are actually in contact, except, perhaps, for a 

 certain distance near the periphery of the iris. This being the case, the posterior cham- 

 ber is very small and only exists near the margins of the lens and the iris. 



The color of the iris is very different in different individuals. Its anterior surface is 

 generally very dark near the pupil and presents colored radiations toward its periphery. 

 Its posterior surface is of a dark-purple color and is covered with pigmentary cells. 



The entire iris presents three layers. The anterior layer is continuous with the mem- 

 brane of the aqueous humor. At the great circumference, it presents little fibrous pro- 

 longations, forming a delicate dentated membrane, called the ligamentum iridis pectina- 

 tum. The membrane covering the general anterior surface of the iris is extremely thin 



