PHYSIOLOGY OP THE COATS OF THE EYE. 241 



firmation of this, he remarks, that it is dark in the monkey, and in all 

 animals that look upwards, and in all birds exposed to the sun's rays; 

 whilst the owl, that never sees the sun, has no black pigment. It 

 doubtless possesses the function assigned to it by Sir Everard. 



The use of the shining spot on the outside of the optic nerves of quad- 

 rupeds, called tapetum.) has been an interesting theme of speculation ; 

 and has given rise to much ingenious, and to not a little ridiculous, 

 hypothesis amongst naturalists. The absence of the black pigment ne- 

 cessarily occasions the reflection of a portion of the rays from the mem- 

 brana Ruyschiana ; and it has been presumed, that these reflected rays, 

 in their passage back through the retina, may cause a double impres- 

 sion, and thus add to the intensity of vision. Another view has been, 

 that the reflected rays may pass outwards through the retina without 

 exciting any action, to be thrown on the object in order to increase the 

 distinctness of the image on the retina, by an increase of its light. Dr. 

 Fleming, 1 who usually exhibits much philosophical acumen, and phy- 

 siological accuracy, thinks it not probable, that both surfaces of the 

 retina are equally adapted for receiving impressions of external objects, 

 and is of opinion, that the rays, in their passage inwards, alone produce 

 the image. M. Desmoulins 2 has, however, adduced many facts and 

 arguments to show, that the tapetum really does act the part of a mir- 

 ror; and, by returning the rays through the retina, subjects it to a dou- 

 ble contact. He affirms, that in nocturnal animals, and in many fishes 

 and birds, which require certain advantages to compensate for the con- 

 ditions of the media in which they are situate, the tapetum is of great 

 extent, and always corresponds to the polar segment of the eyeball or 

 to the visual axis ; that in many animals, as in the cat, the pigment 

 is wholly wanting ; and that it is only necessary for the vision of diur- 

 nal animals. He farther remarks, that, in man, it diminishes accord- 

 ing to age, and in advanced life becomes white; and he ingeniously 

 presumes, that this is a means employed by nature to compensate, in 

 some measure, for the gradual diminution of the sensibility of the re- 

 tina, the choroid beneath reflecting more and more of the rays in pro- 

 portion as the pigment is removed from its surface. 



The views of M. Desmoulins are the most satisfactory of any that 

 have been propounded, and they are corroborated by the experiments 

 of Gruithuisen, Esser, and Tiedemann, 3 which show, that the luminous 

 phenomena never occur in the eyes of nocturnal animals when light is 

 totally excluded. Gruithuisen observed it in the dead as well as the 

 living Animal. Tiedemann perceived it in a cat, which had been de- 

 capitated for twenty hours ; and it did not cease until the humours had 

 become turbid. The views of these observers impress us the more 

 forcibly, when we compare them with certain fanciful speculations, 

 as that of M. Richerand, 4 who supposes, that the use of the tapetum is 

 to cause animals to have an exaggerated opinion of man ! As if the 



1 Philosophy of Zoology, i. 192, Edinb., 1822. 

 9 Magendie's Journal de Physiologie, iv. 89. 



3 Traite Complet de Physiologie de 1'Homtne, &c., traduit par A. J. L. Jourdan, p. 550. 

 Adelon, Physiologie de 1'Homme, 2de edit., i. 443. See, also, Sir E. Home's Lectures on 

 Comp. Anat., iii. 243. 

 VOL. I. 16 



