822 THE SENSE OF SIGHT 



the fish's eye may be made extremely myopic by surrounding it with air, while 

 that of a terrestrial animal may be rendfered hypermetropic by placing it in 

 water. 



The birds are noted for their exceptionally accurate and rapid accommodation. 

 Their eyes are normally set for distant objects, and their accommodation for near 

 objects is made possible not by changing the position of the lens, but by increasing 

 its convexity and hence, its refractive power. The lens is enabled to change 

 its shape by lessening the tension under which it is ordinarily held. This relaxation 

 is effected by pulling the sclerotic-corneal junction backward. Inasmuch as the 

 suspensory ligament of the lens is attached to this area of the eyeball, this retrac- 

 tion must relax it, thereby permitting the lens to become more convex on account 

 of its inherent elastic power. A special muscle, known as Crampton's muscle, 

 controls this retraction. It is also of interest to note that the fibers composing it, 

 are of the striated variety and are, therefore, under a more direct and exact control 

 of the higher centers than the ciliary muscle of the mammals. This structural 

 peculiarity accounts for the rapidity of the accommodation in birds which enables 

 them to swoop down from great heights to catch their prey. In addition, the birds 

 of prey possess the power of increasing the convexity of their cornea. A special 

 muscle is provided for this purpose. 



The Accommodation of the Human Eye. — It has been shown by 



Helmholtz that the accommodation of the mammaHan eye is effected 

 by an alteration in the convexity of the lens, chiefly of its anterior 

 part. Naturally, an increase in its convexity gives rise to an increase 

 in its refractive power and hence, to an accommodation for near ob- 

 jects. Two theories have been formed in explanation of this phe- 

 nomenon, namely: 



(a) The greater curvature of the lens on near vision is due to the fact that it is 

 subjected to a greater tension by the components of the zonula Zinnii.^ 



(6) The greater convexity of the lens on near vision is caused by the fact that 

 the tension under which it is ordinarily held is diminished at this time. 



The second view is the one commonly accepted to-day. It is 

 usually designated as the detention theory of Helmholtz. ^ It is 

 believed that the contraction of the ciliary muscles causes the ciliary 

 body and adjoining choroid to be pulled forward. In consequence 

 of this displacement, the ciliary ligaments are loosened, permitting 

 the lens to bulge forward. Besides, it must be evident from figure 

 429 that this forward movement of B must give rise to a relaxation es- 

 pecially of those ligamentous bands which extend between the posterior 

 surface of the ciliary body {CB) and the anterior marginal zone of the 

 capsule of the lens (L). As a result of this detention of its peripheral 

 area, the mass of the entire lens adjusts itself and assumes a more spher- 

 ical shape. Its anteroposterior diameter is increased thereby. 



While it is a matter of common observation that even lenses with 

 fluid contents tend to assume a spherical outhne,^ it is to be noted that 

 the lens of the mammalian eye is invested by an elastic capsule. 

 It is this investment which is chiefly responsible for the aforesaid 



^ Schon, Pfliiger's Archiv, lix, 1895, 427; also: Tscherning, Optique physiol- 

 ogique, Paris, 1897. 



2 Physiol. Optik, ii, 136. 



3 Schweigger, Archiv fur Augenheilkunde, xxx, 1895, 276. 



