ANATOMY OF THE EYEBALL. 



679 



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 scle- 

 rotic and cornea and the circumference of the choroid, compressing the vitre- 

 ous humor and relaxing the suspensory ligament of the crystalline lens. This 

 action enables the lens to change its form, and 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. It 

 is a circular membrane, situated just in front of the crystalline lens, with a 

 round perforation, the pupil, near its centre. 



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 pass- 

 ing directly over the crystalline lens. The diameter of the iris is about half 

 an inch (12'5 mm.). The pupil is subject to considerable variations in size. 

 When at its medium of dilatation, the diameter of the pupil is to of an 

 inch (3'2 to 4-2 mm.). The pupillary orifice is not in the mathematical cen- 

 tre 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 differ- 

 ent 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 prop- 

 erty of the crystalline lens, called fluorescence, which enables an observer, 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 chamber is very small and exists only near the margins of the lens 

 and the iris. 



The color of the iris is 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 membrane of the aqueous humor. At the great circumference, it 

 presents little, fibrous prolongations, forming a delicate, dentated membrane, 

 called the ligamentum iridis pectinatum. The membrane covering the gen- 

 eral anterior surface of the iris is extremely thin and is covered by cells of 

 tessellated epithelium. Just beneath this membrane are a number of irregu- 

 larly shaped, pigmentary cells. 



The posterior layer of the iris is very thin, easily detached from the middle 

 layer, and contains a number of small cells rich in pigmentary granules. 

 Some anatomists recognize this membrane only as the uvea. 



The middle layer constitutes by far the greatest part of the substance of 



