74 DR*KNOX on the Comparative Anatomy of the Eye. 



and such is the power possessed by the parrot over this mem- 

 brane, that some have supposed the iris to be in this bird a vo- 

 luntary muscle. But there is no necessity for resorting to such 

 a supposition, for it is nearly as voluntary in man as in the par- 

 rot*. 



The ciliary nerves are remarkable for their size and number 

 in birds, in the quadrumana, and in man ; they are very consi- 

 derable in the deer, remarkably distinct, readily traced into the 

 ciliary muscle, and send branches round the iris at about an 

 equal distance between the margin of the pupil and base. This 

 distribution is very remarkable in the hawk and owl, and may be 

 easily seen without dissection. A similar distribution prevails 

 in all the birds I have dissected, and even in many quadrupeds ; 

 in the former, the ciliary nerves pass through the ciliary muscle 

 in large trunks, and having reached the space or cavity existing 

 between the ciliary muscle and base of the iris, there form, or 

 are connected with, an extensive plexus of nerves, which passes 

 all round the eye-ball, and has been mistaken for the ciliary 

 nerves themselves. From this plexus arise a vast number of 

 very delicate nerves, which are distributed chiefly to the ciliary 

 muscle, and form partly on its inner surface, and partly within 

 its substance, a complete net-work of nerves. The meshes of this 

 net are large, and, at the junction of some of the nervous 

 branches composing it, delicate ganglia or swellings are perceiva- 

 ble. The iris seemed to me to receive but a few nerves from 

 the great plexus, and that by far the greater proportion of those 

 almost innumerable fibres which are distributed to it, pro- 

 ceeded from a few large trunks of nerves which pass immediate- 



* Were we to suppose, that, in the cat, owl, and parrot, the movements of the 

 iris are dependent on volition, it would prove an unsurmountable objection to the 

 doctrines which teach that the nerves of volition never pass through ganglia. 



