94 AN INTRODUCTION TO ZOOLOGY 



fluid, the aqueous humour, while the posterior chamber is rilled with 

 a transparent semi-gelatinous mass, the vitreous humour. The 

 vitreous humour is bounded by a delicate sheath, the hyaloid mem- 

 brane. The part of the choroid stretched curtainwise across the eye 

 is termed the iris, and it' is perforated in the middle by an oval aper- 

 ture, the pupil. The iris also is deeply pigmented, appearing bright 

 yellow on the outside, and it acts in the same way as the diaphragm 

 of a camera lens, regulating the amount of light entering through the 

 pupil, which appears as a black spot. In the iris run two sets of 

 involuntary muscle fibres, one set are circular and the other radiating, 

 and so between them they control the size of the pupil. At the 

 line where the iris leaves the sclerotic the choroid coat is thrown 

 into a series of tightly packed radiating folds, well supplied with 

 blood-vessels, the ciliary processes. These have unstriped muscles 

 running from their outer edges to the sclerotic cartilage, thus forming 

 a ring of fibres, the ciliary muscle, as it is termed. Just behind and 

 touching the iris, and so completely separating the anterior and pos- 

 terior chambers, is a hard strongly biconvex body, the lens. It is 

 crystalline and quite transparent in the living animal, but becomes 

 opaque after death and is composed of a series of concentric fibres, 

 each derived from a single cell. The lens is held in position by being 

 contained in a thin membranous bag, the lens capsule, which is 

 continued 'out around its circumference into a tough flange, the 

 suspensory ligament, in its turn attached to the ciliary processes. 



Within the choroid we have the third or internal coat of the eye, 

 the retina. This is a neuro-epithelium, and is the part sensitive to 

 light. It lies close to the choroid in the living animal, but is readily 

 detached, save at the point of exit of the optic nerve and when the 

 eye has been cut in half often hangs loosely. The thickness of the 

 retina is approximately the same over the greater part of the posterior 

 chamber, but just behind the ciliary processes it suddenly becomes 

 much thinner, being reduced to a columnar epithelium, and the ridge 

 or step marking the point of this reduction forms a wavy line, the 

 ora serrata, running round the inside of the eyeball. The thin part 

 of the retina is continued on over the ciliary processes, where it is 

 termed the pars ciliaris retinae, and beyond this again on to the pos- 

 terior surface of the iris, but here it is represented only by its pigment 

 layer. The innermost surface of the retina turns in to run through 

 the posterior coats of the eye as the optic nerve, and just at the small 

 point where it does so the retina is not sensitive to light ; thus a 

 " blind spot " is produced. At the end of the line passing through 

 the centre of the lens, the optic axis, the retina becomes thinner- 

 forming a tiny saucer-shaped depression, usually having a yellowish 

 tinge, hence termed the yellow spot or macula lutea, which is the 



