554 On the Muscle in the Eyes of Fishes. 



form ligament, which passes through a slit in the retina, and 

 penetrates the vitreous humour. It contains blood-vessels 

 and nerves, and is attached to the capsule of the crystalline 

 at its inferior surface, sometimes by a simple elevation, or by 

 a fold a little more opaque ; at other times by means of a 

 grain or tubercle, transparent, and harder than the vitreous 

 humour in which it is placed." " Jurin has named this the 

 ganglion of the crystalline." Porterfield states that some 

 have asserted there is a distinct instrument in fishes for regu- 

 lating the focus of the lens, though he gives no credit to the 

 assertion. 



I admit that my publication was premature, and that it 

 contains several inaccuracies which I shall endeavour to cor- 

 rect in another edition of my ' Treatise on the Eye.' 



On the western shores of the Atlantic there are many fishes 

 uncommon on the coast of Europe. In some of these the 

 muscle is so large that there cannot be a doubt of its charac- 

 ter ; while in others it is so small, that unless exhibited on a 

 larger scale its function might seem uncertain. Its position 

 and attachments ; its absence when there are other instru- 

 ments for acting on the lens ; and the origin of its nerve ; are 

 all in favour of the opinion which I advanced. 



As when the crystalline lens is a sphere, and of course for 

 the reflection of light presenting a medium of the same extent 

 and density whichever way it be turned, more instruments 

 than one for the regulation of its focus would be superfluous; 

 so when the lens is greater than a sphere, as in the cuttle- 

 fish, or less than a sphere, as in the porpoise, the shark, and 

 in animals which live in air, the relative extent of the passage 

 of light through the denser medium would be changed if act- 

 ed upon only at one point, and the rays would not be brought 

 to & focus. Accordingly we find that when the lens is greater 

 or less than a sphere, there are from sixty to eighty instru- 

 ments arranged round its circumference, and attached to the 

 capsule. These instruments, (the ciliary processes), are fine 

 membranes, very vascular, and surrounded by muscular fibres. 

 The arteries which supply them pass to the ciliary body at 

 the equator of the eye, and send off a branch to each mem- 

 brane. When the returning veins are compressed by the 

 muscular fibres, the processes become extended, and the lens 

 is drawn forward. To the action of the single muscle there 

 seems to be no other opposing force than the elasticity of the 

 fine membranes of the vitreous humour : . in like manner the 

 elasticity of the membranes adhering to the posterior surface 

 of the lens, draw it back when the muscular fibres surround- 

 ing the processes are relaxed. During the paralysing effects 



