286 ADAPTATIONS TO SPACE AND MOTION 



work, it will be found, is always done upon cats or perhaps rabbits — 

 neither of which has any circular fibers whatever. 



The whole ciliary body may be so oriented as to put the ciliary muscle 

 at an advantage or at a decided disadvantage, because of great inter- 

 specific variations in the shape of the mammalian eyeball which in turn 

 are due to considerations which happen to be more important to the eyes 

 concerned than accommodation. Thus in the prosimians the ciliary body 

 may be tubular like the eye itself (Fig. 84b, p. 213), while in sirenians 

 and whales it may lie in a plane continuing that of the iris (Fig. 140b, 

 p. 409; Fig. 141a, p. 413). 



Like the vitreous humor, the ciliary processes in the terrestrial 

 mammals and man are functionless vestiges so far as mechanical impor- 

 tance is concerned. Any such importance disappeared as soon as the pro- 

 cesses lost their former approximation to the lens, for the accomplishment 

 of which the reptiles evolved the processes themselves, the scleral ossicles, 

 and the annular pad. They do serve as convenient attachments for some 

 of the zonule fibers, but would seem not to be indispensable in this con- 

 nection. They have persisted presumably because, as with ciliary folds 

 and iris folds, their great contribution to the aqueous-secretory surface 

 is valuable for the regulation of the intra-ocular pressure. Franz sharply 

 distinguishes between two types of processes in different species: a 

 rugose, vascular kind (e.g., man — see Fig. 6c, p. 14) and a thin, rel- 

 atively avascular kind (e.g., cat). The meaning of these differences is 

 not surely known, but they imply a difference in secretory capacity. 



According to Lindsay Johnson, wild mammals normally show a slight 

 hypermetropia (up to one diopter) , which is better for animals which do 

 no close work with hands than myopia would be. Myopia is normal only 

 for mandrills and other baboons, which is comprehensible considering 

 that these are the only sub-human primates which have abandoned the 

 trees for a life on open ground, where food objects are smaller. A little 

 hypermetropia is even better than emmetropia for most mammals of any 

 size, for two reasons : (a) because with increasing age the lens hardens 

 and its index of refraction rises, making an emmetropic eye become 

 somewhat myopic as time goes on. An initial hypermetropia will delay 

 this change to a greater age of the animal, by allowing 'slack' for the rise 

 in refractive power before that rise results in a myopia; and (b) because 

 since a hypermetropic eye must accommodate a little even at long object- 

 distances, the tonus of the accommodatory muscles is always fully devel- 

 oped and the apparatus is alert for the performance of any needed 



