THE TAPETUM AND IRIS. 15 



they are yellowish, and their nuclei pellucid ; but by reflected light they 

 have a beautiful blue colour, and the nuclei appear as small dark points. 



Mr. Gumming* has found that the human eye, when observed under 

 favourable circumstances, appears almost as luminous as the eye of the cat, 

 dog, and other animals provided with a tapetum, to which this luminous 

 appearance has been hitherto supposed to be limited. For the purpose of 

 observing this in the human subject, the person whose eye is to be 

 examined should be placed in a dark room, four or five feet from the half- 

 closed door, with his face towards a light held at an equal distance outside 

 the door. By such a contrivance the reflection may usually be perceived 

 by an observer standing between the screen and the light, and occupying a 

 position as near as possible to the direct line between the source of the 

 light and the eye examined. It varies in appearance from a red livid 

 glare to a bright golden red or burnished brass tint. In some indivi- 

 duals the phenomenon is much more manifest than in others ; and in all, 

 the brilliancy of the reflection is proportionate to the intensity of the 

 light used in the experiment. Mr. Gumming is of opinion that the 

 reflection takes place not from the retina, but from the choroid and its pig- 

 ment. But Mr. Bowman f is disposed to consider it as proceeding from the 

 hyaloid membrane and retina, as well as from the choroid. 



The same luminous appearance of the human eye under favourable 

 circumstances has been since noticed also by M. Briicke.J He observes 

 that this phenomenon is less manifest in old than in young or adult 

 persons, a circumstance which he attributes to the greater quantity of 

 choroidal pigment in the eyes of old than of young persons, to the less 

 perfect transparency of the optic media of the eye, and to the more 

 contracted state of the pupil commonly observed in old people. With 

 respect to the source of this red glare, M. Briicke is of opinion that, in 

 man, as in animals, it proceeds entirely from the blood in the vessels of the 

 choroid and retina. 



Iris. The result of experiments recently performed by Signer Guarini, 

 taken in conjunction with those obtained by Valentin, and Dr. J. Reid,|| 

 appear to leave no doubt that the movements of the iris are regulated by 

 nervous influence derived from two different sources ; the act of contraction 

 whereby the aperture of the pupil is narrowed, being excited by the third 

 pair of cerebral nerves, that of dilatation whereby the size of the pupil is 

 enlarged, being dependent on branches from the cervical spinal nerves, 

 which pass through the superior cervical ganglion of the sympathetic. 

 Irritation of the third nerve, for example, causes contraction of the pupil, 



* Medico-Chirurg. Transactions, 1846. 



-|- Physiol. Anatomy of Man, by Dr. Todd and Mr. Bowman. Part iii. p. 51. 



Miiller's Archiv. 1847, p. 225. 



Annali Univ. di Med. 1844, and Gazette M6dicale, 26 Avril, 1845. 



|| Muller's Physiol. Second Edition, vol. i. p. 827. 



