THE VERTEBRATE EYE 9 



the smooth type. It is interesting in this connection to point 

 out that the cihary muscles of nocturnal lemurs (Galago and 

 Nycticebus) are strikingly large. Both these forms, however, 

 have large lenses. When one compares the relative size of 

 the ciliary muscle with the size of the whole eye, one finds 

 that in Galago it is relatively four times as large as in the 

 rhesus monkey (Macaca mulatta) and in man, and nearly 

 twice as large as in the marmoset (Detwiler, 1939). In 

 Nycticebus this muscle is larger than in man, yet the eye as 

 a whole is only slightly more than one-third as large (Figures 

 8, 9, 10, 11). If the size of the muscle can be taken as signi- 

 ficant, then it would seem that the accommodation mechan- 

 ism of the lemur is superior to that of man. 



Although nocturnal animals in general possess large lenses 

 and correspondingly large cihary muscles, a noted exception 

 is seen in the gecko where the lens is enormous, but where the 

 ciliary muscle is extremely small. It is apparent then that a 

 large lens does not necessarily connote the presence of a cor- 

 respondingly large ciliary muscle. 



Iris, The iris, which is attached to the anterior surface 

 of the ciliary body, forms a delicate diaphragm across the 

 anterior part of the eye. It possesses an opening, the pupil, 

 which is usually somewhat eccentrically placed towards the 

 nasal side. The pupillary border rests upon the lens. The 

 iris does not have the shape of a fiat diaphragm but inclines 

 forward from the ciliary body in the form of a low truncated 

 cone. 



The iris houses two muscles: the m. sphincter pupillae 

 and the m. dilatator pupillae. Both have the same embry- 

 onic origin, being derived from^the pigment epithelial layer 

 of the retina. In the former, the epithelial cells are com- 

 pletely converted into smooth muscle fibers; in the latter, 

 the transformation is incomplete so that these cells can justly 

 be called ^'myoepithelial cells." 



Since the size of the pupil is constantly varying with 

 respect to the amount of light and changing accommoda- 

 tion, the width of the iris is likewise very inconstant. 



