448 THE EYE IN EVOLUTION 



The lack of any scleral support, cartilaginous or bony, results in a 

 spherical globe. 



The choroid is of the standard vertebrate type, usually thinner than that 

 of man, and may contain a tapetum. The ciliary body has a variable 

 topography, but the ciliary muscle, often vestigial, is always composed of 

 plain muscle fibres. A peculiarity is that the anterior surface of the iris 

 is partially covered by a mesodermal leaf additional to that found in other 

 Vertebrates. The angle of the anterior chamber is continued by a cleft of 

 varying depth, extending into the ciliary region bridged across by delicate 

 strands of uveal tissue. 



The lens — usually lenticular in shape but round in aquatic species — is 

 suspended freely from the ciliary processes by a well-developed zonule and 

 is deformed in accommodation {when this function is present) by the 

 elasticity of its capsule, being stretched or relaxed by the ciliary muscle. 



The retina with few exceptions is duplex in type and of typical 

 vertebrate architecture. 



Most of these characteristics are seen in some form or another in 

 other classes of Vertebrates : in only three features does the placental 

 eye differ characteristically from all others : — 



1. In the development and fate of the hyaloid system of vessels, 

 the persisting renmants of which frequently supply an intra -retinal 

 system of vascularization. 



2. In the formation of a mesodermal layer of the iris superficial 

 to the structures found in other Vertebrates. 



3. In an accommodative mechanism depending on a relaxation 

 of the tension normally maintained upon the capsule of the lens. 



It is unnecessary in a volume of this type to describe the detailed 

 morphology of the placental eye which conforms closely with that of 

 man — to which an entire subsequent volume will be devoted. It will 

 suffice to describe those features which show marked variations from 

 the general scheme (Figs. 554 to 563). 



The General Shape and Size of the Olobe, In shape the placental 

 eye is spherical, a necessity with its fibrous, unbuttressed sclera. As a 

 rule the cornea continues the scleral curve, although sometimes there 

 is a shallow corneo -scleral furrow with a protruding cornea having a 

 smaller radius of curvature, as in man ; alternatively, while the 

 peripheral zone of the cornea maintains the curve of the sclera, its 

 apex may be more acutely curved, as is seen in Carnivores. In 

 Cetaceans the shape of the globe is fish-like ^ with a short antero- 

 posterior axis ; it is interesting that the Pinnipedes, less wholly adapted 

 to an aquatic existence than the Cetaceans, have a spherical globe. 

 In some nocturnal prosimian Primates such as the lemuroids (galago 

 and Nycticebus) and Tarsius the shape is almost tubular (Fig. 743). 



1 p. 276. 



