SAUROPSIDAN MUSCLES OF ACCOMMODATION 269 



Sauropsidan Muscles of Accommodation — ^With the advent of the 

 Sauropsida the eye underwent a considerable revolution, especially as 

 regards the mechanism of accommodation. None of the great changes 

 involved is even hinted at, in any extant amphibians, and their production 

 therefore cannot be traced. In the reptiles, pro- and retractor lentis mus- 

 cles are finally abandoned, though a new mesodermal lens-moving mus- 

 cle, the transversalis, makes its appearance in turtles and lizards and is 

 concerned with swinging the lens sidewise in the eye, toward the nose, 

 thus aiding in the convergence of the two visual axes for the purposes of 

 binocular vision (Fig. Ill, p. 278). 



The sauropsidan method of accommodation involves an actual periph- 

 eral squeezing of the lens, the power coming from a ciliary muscle which, 

 compared with the puny one in the Ichthyopsida, is massive indeed. The 

 whole ciliary body is conspicuous and elaborate. Its muscle fibers, and 

 those of the iris as well, differ greatly from those of fishes and amphib- 

 ians in that they are of the striated type, histologically, instead of smooth. 

 How profound a difference this may make physiologically, we do not 

 really know. Striated muscle elsewhere in the body differs from smooth 

 muscle in being ordinarily voluntary, in having no inherent rhythm of 

 contraction, in greater rapidity of action, and in its propensity for easy 

 fatigue. But the smooth muscles of vertebrate eyes are not quite like 

 those of the rest of the body. The dilatator iridis is not a fully-differenti- 

 ated muscle at all, though it and the sphincter iridis are physiologically 

 and pharmacologically indistinguishable (despite their ectodermal origin) 

 from somatic smooth muscles or from the ciliary muscle, which has more 

 in common with somatic muscles embryologically. Human accommo- 

 dation is notoriously fatigable, this being the usual basis of 'eyestrain'; 

 but whether the residence of the fatigue is the muscle itself, we do not 

 know. 



We hardly know what to expect from the striated sauropsidan homo- 

 logues of these contractile structures. Would they, like the striated 

 muscle of the heart, contract rhythmically if denervated? This has not 

 been tested. Are they voluntary? The iridic muscles of birds certainly 

 seem to be — but so do the smooth-muscled irides of a few mammals. Are 

 they unusually fatigable as compared with the corresponding mammalian 

 muscles? Of this, we know nothing. 



If any one of the usual differences between smooth and striated mus- 

 cles does exist here, to serve as a 'reason' for the change, it would seem 

 to be the greater rapidity of contraction of striated muscle. If striated 



