502 PHYSIOLOGY 



lens) at the mouth of the optic cup. The two layers of the optic cup now become 

 contiguous, and the outer develops pigment, while the inner increases greatly in the 

 complexity of its structure to form the adult retina. Through a special cleft in the 

 optic cup (the choroidal cleft) enters a bud of mesoderm to form the vitreous body. 

 This carries with it blood vessels which form the central artery of the retina and those 

 which nourish the lens and iris during their development, namely the hyaloid artery 

 and its branches. These vessels are accompanied by corresponding veins. During 

 1 hese changes the mesoblast surrounding the optic cup has condensed to form the highly 

 vascular choroid and outside it the dense and hard sclera. The latter becomes at the 

 same time transparent in front to form the cornea. Behind the cornea a cleft like 

 aperture appears which develops into the anterior chamber, and thus separates tin- 

 cornea from the iris. The anterior chamber becomes lined by endothelium, and is filled 

 with fluid, the aqueous humour. The iris thus becomes composed of three layers : (1 ) the 

 posterior pigmented layer which is the continuation of the retina ; (2) the iris tissue 

 proper developed from mesoderm, and containing the two muscle layers and elastic 

 tissue ; (3) the anterior layer of endothelial cells. The iris is thus at first ;i continuous 

 sheet of tissue, but its structure is thinner at the part corresponding with the pupil, 

 thus forming the pupillary membrane. This disappears shortly before birth. The 

 ocular muscles are formed from the mesoderm in a similar way to other muscles. 

 The lids form as two buds growing out from the epiblast ; they advance till they 

 meet and then fuse together, to reopen again about the time of birth. The 

 nervous layer appears to take a very important part in the development of 

 the eye, and this is borne out by experiment ; for if the outgrowing optic cup be diverted 

 during its advance to the epiblast towards some other part of the embryo, it is found 

 that a normal organ of vision develops in this new and entirely abnormal situation. 



COMPARATIVE ANATOMY OF THE EYE. The types of light-receiving 

 organ in the animal kingdom make an elaborate study because of the variety of 

 form that is met with. We may however effect an approximate classification, 



The most primitive type of all light organs consists of a single pigmented spindle- 

 shaped cell such as is found in the epidermis of certain amphibia and ccelenterata (see 

 Fig. 251 A). In cases where the creature is transparent, the end organs may be devel- 

 oped in connection with the nervous system. The functions of such organs may be 

 to inform their possessor if a part is exposed to light, and therefore also liable to be 

 noticed and attacked by passing enemies. In the next type of light organ, a number 

 of such cells are grouped together, often to form a hollow cup in the epidermis, as shown 

 at B. These cups retain their connection with the nervous system by means of an 

 optic nerve. This type of organ (B) is found in Platyhelrainthes and in the mollusc 

 Patella. In the next type (C) the cup becomes deeper and its mouth small, the epidermis 

 round it becoming deeply pigmented. This organ therefore functions like a pinhole 

 camera, allowing its possessor to observe a rough image of external objects : it should 

 be noticed however that this image formation occurs at the expense of brightness. 

 This type ((') is found in most annelida and in the mollusc' Nautilus. The next type 

 (I)) isa moilitic.it ion of the last, in that the centre of the optic cup is filled \\ith a spherical 

 highly refractile body. This permits a larger opening to be used for the admission of 

 light without at the same time causing too much confusion in the image. This is the 

 arran.L'emrnt met with in the mollusc Helix and the arthropod Scorpio. In the in-\t 

 type of light organ (E) the highly refractile lens becomes separated from contact with 

 the retina, and the space between is filled by liquid or a mass of transparent cells which 

 form a vitreous humour. This Interval between the lens and retina allows the former 

 to produce a focussed image on the latter, so that for the first time we find an eye having 

 the property of defining external objects. This type of eye (E) is found in the 

 ccalenterate Charybdea and in the ocellus of insects. In the mollusc Sepia 

 the eye is further improved by possessing an adjustable iris. The eye of Pecten 

 (P) has another interesting feature, namely that the optic nerve spreads out over, and 



