Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 530 COMPAEATIVE ANATOMY. In tlie Saurii, Chelonii, and Aves, the anterior portion of the sclerotic, which abuts on the cornea, is supported (Fig. 296, s) 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, s). 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 ;
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 530 COMPAEATIVE ANATOMY. In tlie Saurii, Chelonii, and Aves, the anterior portion of the sclerotic, which abuts on the cornea, is supported (Fig. 296, s) 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, s). 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 <» Ganoi'dei (Sturio), and absent in most of Fig. 299. Eye of Baiaena th Teleostei. tho ch0roid is then continued mysticetns. Horizontal , . . . .. , , ... section (after W. Summer. ou as the iris, which bounds by its inner iug). 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 metallic lustre; it is sometimes produced 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 Reptiles 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
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