ANATOMY OF THE EYEBALL. 51 



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, anatom- 

 ically 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 adapts its curvature to vision at 

 different distances. The nerves of the ciliary muscle are de- 

 rived 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 to- 

 gether. 



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 backward 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. The pupil is subject to considerable variations in 

 size. When at its medium of dilatation, its diameter is from 

 J- to -J- of an inch. The pupillary orifice is not in the mathe- 

 matical centre of the iris, but is situated a little to the nasal 

 side. The thickness of the iris is a little greater than that of 

 the choroid, but is unequal in different parts, the membrane 

 being thinnest at its great circumference and its pupillary 



