530 COMPAEATIVE ANATOMY. 



In the Saurii, Cheloniij and Aves, the anterior portion of the 

 sclerotic, which abuts on the cornea, is supported (Fig. 296, &•') by a 



circlet of flat pieces of bone (sclerotic ring). 

 In all Mammals except the Monotremata 

 the sclerotic is formed of connective tissue; 

 it is very thick in the Cetacea (Fig. 299, .v). 

 The choroid is made up of several 

 layers, which, as a rule, have the same 

 characters as in Man. Anteriorly it gives 

 rise to the folded ciliary processes ; these 

 are feebly developed in the Selachii and 

 * Gano'idei (Sturio), and absent in most of 



Fig 299. Eye of Balaena ^^ Teleostei ; the choroid is then continued 



mysticetns. Horizontal , . '. . . . 



section (after W. Sommer- Oil as the iris, which bounds by its inner 

 ing). margin the pupil, which varies in form. 



The tapetum lucidum is a special modi- 

 fication of the choroid; this forms a spot of varying size, which 

 is generally greenish or bluish in colour, and has a metalhc lustre ; 

 it is sometimes pi'oduced by groups of spicular crystals placed in the 

 cells of the tapetum (Selachii), or by a fibrous tissue (Carnivorous 

 Mammals and Ruminants). It is owing to its presence that the eye 

 can be seen in the dark. 



A vascular plexus, which lies outside the choroid of Fishes, forms 

 the so-called choroid gland. In the anterior portion of the choroid 

 there is a muscular layer, which forms the ring known as the ciliary 

 ligament. The musculature is continued hence into the iris, in 

 which there are radial and circular fibres. In Fishes, Amphibia, 

 and Mammals, this musculature is composed of smooth fibres ; in 

 Eeptiles and Birds, of transversely striated ones. 



The retina, which is placed on the choroid, extends forwards as 

 far as the commencement of the ciliary body, where it ceases to be 

 developed. The optic nerve is distributed, and ends, in it. The optic 

 fibres occupy the innermost layer of the retina, which is merely sepa- 

 rated from the vitreous body by a thin membrane. It is followed by 

 a number of layers, of varying structure, the last and outermost of 

 which is made up of rod-like and cone-like structures, the bacillar 

 layer. These end-organs, which are similar to the rods of the 

 invertebrate eye, are, therefore, turned away from the opening of 

 the eye in the Yertebrata; the Vertebrate eye is therefore 

 distinguished from the optic organs of the Invertebrata 

 by a very essential point, which must not be left out of con- 

 sideration when we are discussing their genetic relations. 



Connected with the development of the secondary optic vesicle is 

 the formation of a special organ, which makes its way into the 

 vitreous body at the point at which the optic nerve passes into the 

 retina; it has no connection with the choroid, but forms a vascular, 

 darkly-pigmented, process. A structure of this kind is found in 

 the eyes of many Teleostei, and is known as the processus f alciformis 

 (Fig.296, j7). Its end, which in many Fishes is distinguished by a layer 



