of the Canal of PETIT. 251 



cepting its base or point of insertion into the sclerotic : this seems 

 (as we would expect) to be of a cartilaginous nature. I have 

 found the annulus albus to be very lully developed in the dol- 

 phin, and repeated dissections have left no doubt in my mind of 

 the extreme inaccuracy there is in viewing this body as a nervous 

 ganglion *. 



Now, this opaque circular stripe, which may be seen in the iris of man, of the 

 quadrumana, and many other mammiferous animals, (very remarkably in the otter), 

 and in almost all birds, is pompously called the Sphincter Muscle of the iris. 

 But to demonstrate the radiating fibres, a little artifice is resorted to, violating 

 at the same time, all the rules of philosophic anatomy. The iris of the ox, or 

 of some other large quadruped, is selected, in which the parallel membranous striae, 

 so abundant on the choroidean portion of the iris, are remarkably striking, and 

 these are called the Radiating Muscles of the Iris. These striae are not distinct 

 in man, and in several other animals, in whom the iris is extremely irritable ; its 

 choroidean portion becoming proportionally delicate (. 



* WALTER says, that, in the human eye, the retina does not receive any arteries 

 from the central artery of the retina : " Retinam nullas accipere arterias a centrale 

 arteria." He proves, that, by filling the ophthalmic artery with coloured injection, 

 nearly the whole veins of the eye-ball are also filled ; and it was by comparing such 

 vascular preparations with those in which the veins only were injected, that he final- 

 ly adopted the above opinion. Succeeding anatomists do not seem to me to agree 

 with him relative to the distribution of the central artery of the retina. 



I have made relative to these matters very considerable researches, but not on the 

 human subject. I find that when the eye of a quadruped, as the ox, sheep, deer, &c. is 

 removed from the head, and injected by the branch of the ophthalmic artery, pro- 

 ceeding to the eye-ball, the injection very readily fills, not only the arterial system, 

 but also the whole of the vasa vorticosa, the vessels of the capsule of Petit, and a 

 great portion of those supplying the iris ; but to succeed perfectly, the injection must 

 be thrown in with considerable force, and some of the vessels are generally ruptured. 

 I have not succeeded so well as I could have wished in those preparations intended 

 to exhibit the veins only. Hence I am not able to speak positively as to the relative 

 development of the veins, compared with the arteries, in the capsule of Petit ; it is not 

 improbable, however, that they observe the same proportion as in the other parts of 



f See Memoir read before the Royal Society of Edinburgh , entitled, " On the Comparative Anatomy of 

 the Eye." Page *a et seq. of thi volume. 



