124 ZOOLOGY. 



An opposite defect is myopia, by which distant objects are 

 rendered indistinct, or not visible. The habitual use of a 

 strong lens has been thought equal to the production of this 

 disease. The myops has the sight improved with age, and 

 he may in time fall into the opposite condition, namely, 

 become presbyopic. By glasses, man endeavours to remedy 

 both defects. 



237. Insensibility to the rays of light is called gutta 

 serena. All parts of the retina are equally susceptible of 

 impressions, but it is the centre or in the axis of vision that 

 this sensibility is most acute.* The various points of the 

 retina may be exhausted of their sensibility by too strong a 

 light or by gazing intensely for a long time at one object. 

 The impression also which every object makes on the retina, 

 continues for a certain time after the object has been re- 

 moved ; thus it is that a body moving in a circle with great 

 rapidity resembles a ring or hoop, and that a wheel turning 

 with great rapidity resembles a disc. 



238. The section of the optic nerve produces immediate 

 blindness. But injuries of the thalami nervorum opticorum, 

 or of the corpora quadrigemina, also effect the same; and thus 

 it is evident that these bodies have the most intimate relation 

 with the optic nerves and the function of vision. 



When the cerebral hemisphere of one side has been de- 

 stroyed in a living animal, it is the opposite eye which loses 

 its power; and this is partly explained by the anatomical 

 fact of the decussation of the optic nerves ; by this decussa- 

 tion, the right retina derives most of its fibres from, the left 

 side of the brain, and vice versa. 



239. Motor Organs of the Eyeball. The apparatus 

 of vision is composed of the eyeball and its appendages ; of 

 these, let us first examine the muscles. 



240. The muscles enabling us to direct the eyeballs, and 

 consequently the axis of vision, upon any point, are six in 

 number. They are attached posteriorly (with one exception) 

 to the osseous orbit, around or near to the entrance of the 

 optic nerve, and by their other extremities to the sclerotic or 

 fibrous tunic of the eyeball (Fig. 81). The globe of the eye 

 rests on a cushion of a fatty cellular substance, and thus these 

 muscles can readily move it in all directions. The nerves, 



* It is a remarkable fact that at the point where vision is most distinct, 

 the pulpy retina is wanting in man and in the apes of the Old World. The 

 appearance is called the foramen centrale retinae. K. K. 



