654 SPECIAL SENSES 



They are pale, present a number of oval longitudinal nuclei and have 

 no striae. 



It is evident, from the arrangement of the fibres of the ciliary mus- 

 cle, that its action must be to approximate the border of connection of 

 the sclerotic and cornea and the circumference of the choroid, com- 

 pressing the vitreous humor and relaxing the suspensory ligament of 

 the 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 passing directly over the crystalline lens. The diameter of 

 the iris is about half an inch (12.5 millimeters). 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 millimeters). The 

 pupillary orifice is not in the mathematical centre of the iris but is situ- 

 ated 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 poste- 

 rior, the anterior chamber being much the larger. Taking advantage 

 of a property of the crystalline lens called fluorescence, which enables 

 an observer, by concentrating on 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 ante- 

 rior surface usually is 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 pigment-cells. 



The iris presents three layers. The anterior layer is continuous 

 with the membrane of the aqueous humor. At the greater circumfer- 

 ence, it presents little fibrous prolongations, forming a delicate dentated 

 membrane continuous with the ligamentum iridis pectinatum. The 



