460 THE EYE IN EVOLUTION 



intimately supplied with choroidal capillaries ; to a certain extent, also, the 

 irregular arrangement of the visual cells in the hills and craters may act as an 

 accommodative device.^ 



It is interesting that Rohen (1954) found in the dog thick longitudinal 

 mviscular layers in the walls of the posterior ciliary arteries and in the arteries 

 of the posterior part of the choroid which he interpreted as a vascular shunt - 

 apparatus regulating the flow of blood into the choroid. Such a mechanism he 

 failed to find in the cat, rabbit, rat or guinea-pig, or in man. 



The Ciliary Region. The size and topograph}^ of the ciHary region 

 in Placentals vary considerably, the dominating factor being the pre- 

 sence or absence of an accommodative mechanism. Derived from noc- 

 turnal ancestors few Placentals, particularly of the lower species, have 

 any marked degree of accommodative activity ; this, indeed, is found 



Fig. 582. — The Ciliary Body of Primates. 



The inner aspect of the anterior part of the eye showing the ora serrata, 

 the pars plana, ciliary processes and posterior surface of the iris. 



only in the squirrels (Sciuridae), the large Carnivores and the Primates. 

 On this essentially depend the size of the ciliary body, its muscular 

 development, the prominence of the ciliary processes, a.nd the con- 

 figuration of the angle of the anterior chamber. In most small-eyed 

 primitive tyj^es with comparatively large lenses (Insectivores, Rodents, 

 etc.) the ciliary body is small and narrow with miniature processes ; 

 in the slu'ews it is a simple roll without processes, as in snakes.^ In 

 large-eyed Placentals, it assumes the prominent triangular shape with 

 well-developed processes such as are seen in man. It is noteworthy, 

 however, that from the aspect of joure anatomy, in many species a 

 considerable degree of asymmetry exists ; thus in animals with an 

 ovoid cornea (and pupil) the circular ciliary body encroaches far into 

 the iri.< nasally and temporally, rendering the horizontal segment of 

 1 p. 643. - p. 386. 



