242 



THE HUMAN SPECIES 



In man and all Vertebrates (except amphioxus and the 

 myxinoides) a characteristic of the eye is the more or less mov- 

 able globe upon the forepart of which the sclerotic coat is trans- 

 formed into the transparent, refracting cornea with its varying 

 convexity. Within the eyeball are two other refractive media, 

 the lens and the vitreous body ; accommodation is provided 

 for in the iris and its muscles, whilst the retina lying in front of 

 the pigmented choroid acts as the light-receptor (see Fig. 1 20) . 

 Physiologically as well as anatomically the same general 

 agreement can be seen. All mammals (including man) possess 

 the means of adjusting the optical mechanism and the faculty 



of distinguishing points of 

 varying magnitude and their 

 relative positions. In addi- 

 tion to visual acuity and sense 

 of space they have all a so- 

 called visual field, the power 

 of perception in the three 

 dimensions, and of appreci- 

 ating solid figures. More- 

 over, they are endowed with 

 colour-perception and light- 

 sense, i.e., the distinguishing 



FIG. r 2 o. Diagram of the anterior portion f light-intensity (quantite- 



of the eye. (Thome.) C, cornea;], tive) and light- waves of vary - 

 iris; Cc, ciliary body ; ZZ, ora serrata 

 (ciliary muscle) ; S, sclerotic ; Ch, cho- 



Cc 



ing lengths (qualitative). 



roid; R, retina; Lh, suspensory liga- 

 ment ; *, Petit's canal. 



As regards the trans- 

 parency and composition of 

 the refractive media man stands above most Vertebrates, except 

 beasts of prey and birds, especially in the refracting power of 

 the lens. The pupil, which is as a rule round, whether dilated 

 or contracted, assumes in certain animals an oval or chink 

 shape owing to a peculiar arrangement of the fibres of the iris 

 when contracted. The less the intensity of the external light 

 the greater must be the dilation of which the pupil is capable 

 in order to admit as- much light as possible (mice, bats). 



Accommodation, the focussing of the eye to objects near 

 at hand, is brought about in fishes by the falciform process 

 with its bell-shaped muscular swelling (campanula), which draws 



