EYE 



275 



at first a hollow invagination, reminding one of a primitive sen- 

 sory organ of the Anamnia or of the primitive olfactory pit. On 

 the closure of the aperture of invagination, it forms a vesicle, the 

 thinner outer wall of which gives rise to the 

 so-called lens-epithelium, while the cells of 

 the thicker outer wall elongate to form the 

 transparent fibres of which the greater part 

 of the lens is composed. 



The remaining space within the optic 

 vesicle becomes filled by tissue l which ex- 

 tends through the so-called choroid fissure 

 (p. 273), and gives rise to the vitreous body or 

 hu'inour (Fig. 201, B). Blood-vessels also 

 extend into the vesicle in the same manner, 

 and others arise at its periphery, where a 

 definite vascular and pigmented membrane, 

 the choroid, is formed from the surrounding 

 mesoderm. 



Internally to the lens, the choroid gives 

 rise to the ciliary folds, while more extern- 

 ally it passes in front of the lens to form 

 the iris, which retains in the centre a circular 

 or slit-like aperture, the pupil, through which 

 the rays of light pass (Fig. 203). The 

 amount of light admitted is regulated by Ce < 



the dilator and constrictor (sphincter) muscles p lt , 202. CHIASMA OF 

 of the iris, which are able to increase or THE OPTIC NERVES. 



lessen the size of the pupil ; the iris thus Semi diagrammatic. 



i , i c A, most leleo.stei ; B, 



serves as a screen to regulate the amount of " - - 



light which enters the eye. 



Not only are the size and form of the 

 pupil inconstant, but the lens is also capable Chi, chiasma of the 

 of undergoing considerable change in relative bundle of nerves lying 

 position (e.g. Fishes, Amphibians, Snakes) or ^ l 



in form, becoming more flattened or more 

 convex, as the case may be (e.g. Mammals, 

 Birds, Lizards. Chelonians) : the former condition occurs when 

 distant, the latter when near objects are looked at. This 

 delicate accommodating apparatus in higher forms is regulated 

 by a ciliary muscle (tensor choroidecu) supplied by the oculomotor 

 nerve, which arises in a circle all round the eye from the point of 

 junction of the iris and sclerotic and is inserted along the peri- 

 pheral border of the iris. 



Externally to the vascular layer of the choroid is a lymph- 

 sinus with pigmented walls (lamina fusca), and externally to this, 

 again, is a firm, fibrous, partly cartilaginous, or even ossified 



1 By some embryologists this tissue is said to be ectodermic, and not meso- 

 dermic in origin, except as regards the evanescent embryonic blood-vessels. 



T 2 



Herring ; 0, Lacerta 

 agilis ; D, Agama ; E, 

 Mammal. 



S, S l , lateral fibres, 

 which do not cross. 



