Peculiarities rf Strii^iire in the E-jCS oj ^ladriipeds and Birds. 477 



mouth, as the lion and cat tribe; the third by the mouth only, as all ruminating animals. 

 Thcle three difierent modes require the food being brought fmni different diftances from the 

 eye, and it is curious that the mufcles of the eye are different in all the three tribes. 



hi the monkey tribe the mufcles of the eye are exactly the fame as in the human. In 

 the lion tribe they are double in number, and the four intermediate mufcles are loft in the 

 fclerotic coat at a greater dillance from the cornea than the others. In the ruminating 

 tribe there are four mufcles, as in the human eye ; but there is alfo a mufcle furrounding the 

 eye-ball attached to the bottom of the orbit, round the hole through which the optic nerve 

 palfes, and loll upon the fclerotic coat immediately before the broadeft diameter of the globe 

 "of the eye. The upper portion of this mufcle is rather the iongcft, the infertion being 

 nearly in a circular line at right angles at the axis of vifion, but not to the axis of the 

 eye from the entrance of the optic nerve. 



In quadrupeds in general, the ball of the eye is broader in proportion to its depth than in 

 the human fubjeft. In the bull the proportion is 1 {• J inch , to i/'j.. The cornea is larger 

 and more prominent, its real thicknefs not eafy to be afcertained, becaufe, like that of the 

 human eye, it readily imbibes moifture after death. When dried, it is thinner than the fcle- 

 ,rotlc coat in the fame ftate. In ruminating animals, the cornea, though circular, has an 

 oval appearance, arifing from an opake portion which is covered by a membrane. The ci- 

 liary procefles, as in the human eye, are conneiSled with the choroide coat ; but they are 

 larger, and are united at their origin with the iris. 



This ftrufture of the eye in quadrupeds, as far as it differs from that of the human eye, 

 appears calculated to increafe the power of adjufting it to near objefls ; and from the mode 

 of life which thcfe animals purfue, fuch additional powers appear neceflary to enable them 

 to procure their food with eafe. 



That birds, procuring their food by their beak, muft require an adjuftment to fee very 

 near objefts, and that a degree of precifion and facility, with regard to remote objefts, is no 

 lefs neceflary from their fituation during flight, are obvious circumftances. Mr. Home men- 

 tions fome inftances, in which birds are fuppofed to have feen diftant objefts with peculiar 

 diflinftnefs. It is related, that vultures and other birds very fpcedily repair to the \:lacc 

 where a dead animal or other prey is expofed to them ; and though there is nothing, per- 

 haps, in thefe fafls which fuppofe an acutcnefs of light greater than that of other animals, 

 yet it cannot be doubted, upon the whole, that thefe animals fee with very great precifion at 

 vaft diftnnces. 



The eyes of birds are larger in proportion than thofe of other animals, and broader in 

 proportion to their depth, with a more prominent cornea than in the quadruped. The cor- 

 nea in the goofe is not united to the fclerotic coat by the terminating of one abruptly in 

 the other ; but the two edges are bevelled off and laid over each other for nearly v'(,lli of 

 an inch. This circumftance, as alfo the bony rim or apparatus furrounding the bafis of 

 the cornea, wlilch is peculiar to this ciafs of animals, and dcfcribed by Mr. Smith, was well 

 known to Haller. Mr. Home defcribcs it more particularly, and ftates its ufc in the focal 

 adjuflment to be nearly the fame as that dcfcribed by Mr. Smith. 1 he ciliary proccflesj 

 arc larger and longer in birds than in other animals whofe eyes are of the fame fizc ; and the 

 marfupium or membranous procefs peculiar to the clafs of birds, is ftiewn by him to be 

 very fimilar to that of the ciliary procefles 5 but ftronger in all its parts, and, like them, con- 



1 ncded 



