294 THE CAT. [CHAP. ix. 



iris and the anterior surface of the vitreous humour. It is somewhat 

 harder within than superficially. It consists of albuminous substance, 

 and is non- vascular. It is really made up of a multitude of fibres 

 (each a flattened hexagonal prism, about 3~oV<r of an inch wide), with 

 serrated edges and a nucleus. These are so connected that the lens 

 is practically made up of concentric layers of fibres, as becomes 

 evident after immersion in alcohol. The lens is not quite spherical, 

 but is compressed from within outwards, that dimension being less 

 than its transverse diameter. 



The capsule of the lens is a transparent, glassy membrane, which 

 is very elastic, and closely invests the lens itself. Its anterior surface 

 is in contact with the iris, its posterior surface adjoins the vitreous 

 humour. From the extreme circumference of the lens it sends off a 

 membrane, the suspensory ligament of the lens, which becomes con- 

 tinuous with the choroid and with the membrana limitans of the 

 retina, where it has become the pars ciliaris retince, behind the ciliary 

 processes. It thus divides the chamber in front of the lens from 

 that of the vitreous humour. Just behind that part of it which 

 adjoins the lens, it is slightly separable from the front of the vitreous 

 humour, and when separated leaves a space which has been called 

 the canal of Petit, but this is no true persistent canal, but only a 

 yielding interspace affording room for change of place in the lens, 

 and consequent focal adjustment. 



The AQUEOUS HUMOUR is the fluid which fills the front chamber 

 of the eye between the iris and lens, and the cornea. It scarcely 

 differs in composition from water, but contains some solids in 

 solution chiefly chloride of sodium. A small portion passes 

 behind the iris, in what is called the posterior chamber of the 

 aqueous humour. 



K The direction of the eyes is determined, apart from the position of 

 the body, neck, and head, by the ACTION OF THE ORBITAL MUSCLES, 

 of which the reeti severally pull the cornea in four directions, while 

 the obliqiii roll the eyeball on its axis, and pull it a little forwards 

 and inwards. The choanoid muscle tends to pull it backwards. 

 The eyelids protect the ball of the eye from foreign substances and 

 from excessive light, and hinder the too rapid evaporation on its 

 surface, the moisture of which is secured by the lachrymal secretion, 

 the superfluous quantity of which escapes to the nose (through the 

 juncta lachrymalia and nasal duct), while the secretion of the 

 Meibomian glands tends to check its overflow beyond the margins 

 of the eyelids. 



The structure of the eye, AS AN OPTICAL INSTRUMENT, is that of 

 a camera obscura filled with water, with a circular aperture, behind 

 which is a partition, or diaphragm, with another and smaller circular 

 aperture immediately in front of a bi- convex lens. 



The lens, according to the laws of optics, concentrates the rays 

 coming from every point in front of it to other points behind it, and 

 thus throws upon any surface situated at due focal distance, an 

 inverted image of the objects in front of it. The surface behind, 



