222 On the accommodation of the Eye to distances. 



by the contraction of the fibres at the roots of one portion of the ciU- 

 ary processes while the other fibres are relaxed. 



The design of creative Wisdom in varying the structure of the eye 

 to the purposes to which it is to be applied is very conspicuous in the 

 sheep. The muscle on the choroid coat is larger above than that 

 which is below ; its fibres take a different direction that it may more 

 efiectually compress the ciliary veins, and Petit's canal is broader al- 

 lowing the crystalline more extended motion ; in this manner enabling 

 it more effectually to select its food and soon to watch against danger 

 without lifting its head fi:'om the ground : or according to the degree 

 of light, enabling it to remove the picture from the lucid tapetum to a 

 part that is darker. The retina terminates where the muscle begins 

 lest its action should interfere with the impressions received upon it. 

 It is slit asunder in fishes apparently for no other purpose than avoid- 

 ing the motions of the cord to which the triangular muscle is attached. 



When we look from one object to another we are conscious of an 

 eJGfort and that some time is requisite for distinct vision ; apparently 

 allowing the vascular processes to be filled or emptied by compressing 

 or not compressing the veins that return from them. The winding 

 direction of the carotid before it gives off the ophthalmic artery, the 

 smallness of the ciliary vessels and their bendings and anastamoses 

 before reaching the processes prevent the blood from passing per sal- 

 tum but conduct it in a steady uninterrupted course. In the cat an 

 animal that has to watch her prey often for a very considerarble time, 

 the artery that supplies the eye after making a curve, gives off a num- 

 ber of very small branches which make several convolutions before 

 some of them again unite and penetrate the sclerotica. By this 

 structure the eye may continue for a long time without losing its ob- 

 ject. Momentary relaxation of the ciliary processes may be permit- 

 ted without having again to bring the organ to the same focus. 



That the theory advanced by Kepler and supported by Porterfield, 

 that the eye is accommodated to different distances by the position of 

 the crystalline nearer to, or more remote from the retina is the true 

 one, and that the change is effected in the manner described is evi- 

 dent from the structure of the apparatus. 



