ACTION OF THE RETINA. 245 



quadrigemina, and the three pairs of motor nerves of the eye, gave no 

 signs of general sensibility. On the other hand, the general sensi- 

 bility of the conjunctiva is well known. It is such, that the smallest 

 particle of even the softest substance excites intense irritation. This 

 general sensibility M. Magendie 1 found to be totally annihilated by the 

 division of the fifth pair of nerves within the cranium; after which, 

 hard-pointed bodies and even liquid ammonia made no painful impres- 

 sion on the conjunctiva. Nictation was arrested ; and the eye remained 

 dry and fixed like an artificial eye behind the paralysed eyelids. The 

 sight, in this case also, was almost wholly lost ; but by making the eye 

 pass rapidly from obscurity into the vivid light of the sun, the eyelids 

 approximated; and, consequently, slight sensibility to light remained; 

 but it was slight. 



In this sense, then, as in the senses of hearing and smell, we have 

 the distinction between a special nerve of sense, and one of general 

 sensibility : without the latter, the former is incapable of executing its 

 elevated functions. 



The expansion of the retina occupies at least two-thirds of the cir- 

 cumference of the eyeball. It is of obvious importance, that it should 

 have as much space as possible; and, in certain animals, in which the 

 sense is very acute, the membrane is plaited so as to have a much 

 larger surface than the interior of the eyeball; and thus to allow the 

 same luminous ray to impinge upon more than one point of the mem- 

 brane. This is seen in the eyes of the eagle and vulture, and in noc- 

 turnal animals. The inconceivable acuteness of the sense of sight in 

 birds of prey has been already referred to under the sense of smell. 

 It was then stated, that the strange facts regarding the condor, vulture, 

 turkey-buzzard, &c., which meet in numbers in the forest, when an 

 animal is killed, ought rather, perhaps, to be referred to acuteness of 

 the sense of sight than of smell. Sir Everard Home 2 affords an addi- 

 tional illustration of this subject. In the year 1778, Mr. Baber, and 

 several other gentlemen were on a hunting party in the island of Cas- 

 simbusar, in Bengal, about fifteen miles north of the city of Marshe- 

 dabad: they killed a wild hog of uncommon size, and left it on the 

 ground near the tent. An hour after, walking near the spot where it 

 lay, the sky being perfectly clear, a dark spot in the air, at a great 

 distance, attracted their attention; it appeared to increase in size, 

 and move directly towards them ; as it advanced it proved to be a 

 vulture flying in a direct line to the dead hog. In an hour, seventy 

 others came in all directions, which induced Sir. Baber to remark, 

 "this cannot be smell." 



How inconceivably sensible to its special irritant must this mem- 

 brane be in the human eye, when we consider that every part of an 

 extensive landscape is depicted upon its minute surface ; not only in its 

 proper situation, but with all its varied tints ! and how impracticable 

 for us to comprehend, how the infinitely wider range of country can be 

 so vividly depicted on the diminutive eye of the vulture, as to enable it 

 to see its prey from such remote distances. 



1 Op. cit., i. 494. * Lectures on Comparative Anatomy, Lond., 1814-1828. 



