5i8 



PRINCIPLES OF GENERAL PHYSIOLOGY 



by the contraction of a ring of muscle, the ciliary muscle, this tension of the 

 ligaments is released, like that of a stretched cord of india-rubber would be if the 

 attachment of one of its ends were pulled nearer to that of the other end. In 

 consequence of the release of tension, which admits of degrees, the lens assumes 



Cor. 



Cor.ps 



.A^^ft-rt' '-' *mto^"" 



^^fe, 



R.c.n. -- 



li... I. 



r; .4-V| 



. : W;; : ^' 



^- '"trf- Sl>1 - 



' ' """:" Ol> '"'' 



| u 



i " '!fc';. ; ' ; 'i 



Fio. 162. 



EYE OF PECTEN MAXIMUS. Longitudinal section. 

 150 times. 



Magnified 



Cor., Cornea. 



Cor.ps., Pseudo-cornea. 



/., Iris. 



/... l.i'ii-. 



>'/'.. Si pt inn. 



D.c.L, Distal cell layer. 

 R.e p.. Pseudo-rod cells. 

 B.C., Rod cells of retina. 

 R., Rods of retina. 



Arg., Argentea. 

 T., Tapetuni. 

 Scl., Sclerotic. 

 Op.n., Optic nerve. 

 Op.n.i., Inner branch of optic nerve. 

 Op.n.o., Outer branch of optic nerve. 

 R.c.n., Nuclei of rod ci-11-. 

 A'.,S7., Kye stalk. 



(Reproduced from \V. .T. Dakin's monograph on "Pecten," by 

 permission of the Liverpool Marine Biology Committee.) 



more or less the form it takes when free, that is, a more spherical one ; hence it is 

 able to focus near objects on the same plane on which it previously brought distant 

 ones to a focus. 



At this point, I may stop for a moment to mention that this mechanism of 

 accommodation was first made clear by Helmholtz, to whom we owe a very large 

 part of our knowledge of the eye as well as of the ear. Although we have already 



