8 VERTEBRATE PHOTORECEPTORS 



This region is called the corona ciliaris (pars plicata). The 

 vascular cihary processes are covered over by two layers of 

 epithelium. The inner heavily pigmented layer is a continua- 

 tion of the pigment epithelium of the retina. The outer 

 epithelial layer is a continuation of the sensory portion of 

 the retina. These two layers constitute the so-called pars 

 ciliaris retinae. They continue over the posterior surface 

 of the iris as the pars iridica retinae. 



Within the ciliary body is the ciliary muscle which, in 

 man, is divisible into three regions: a) meridional fibers 

 (Briicke's muscle), h) radial fibers, and c) circular fibers 

 (Miiller's muscle). 



This muscle, which is concerned with accommodation, 

 varies considerably in shape and size. When the circular 

 fibers are strongly developed or in the contracted state, the 

 muscle appears shorter (hypermetropic type) . When weakly 

 developed or relaxed, the muscle appears larger and flatter 

 (myopic type). 



The ciliary body varies greatly among vertebrates. In 

 fishes and amphibians the ciliary processes are rudimentary. 

 They are well developed in reptiles and in birds. In the latter 

 the processes are large and numerous (pigeon, 104-108; 

 horned owl, 130; kestrel, 158, v. Hess, 1909). The fine 

 accommodating power of birds is well known. In mammals 

 apparently the processes vary considerably in size. For ex- 

 ample, they are said to be small in the rabbit; large in the ox. 



The ciliary muscle, also, shows marked variations in size. 

 It is extremely small in fishes, and contains only a few mus- 

 cular fibers. Amphibians possess only a rudimentary bundle 

 of meridional fibers. Birds have a very large muscle which 

 is striated. In some instances the muscle inserts into a bony 

 ring around the sclero-corneal margin. According to Duke- 

 Elder, this muscle in mammals exhibits an ascending scale of 

 development. It is least developed in rodents, larger in 

 herbivors, still more developed in carnivors and reaches its 

 highest development in primates. Although striated in rep- 

 tiles and in birds, this muscle in mammals (as in fishes) is of 



