96 NATURAL HISTORY 



Ideas of external things are conveyed to the soul of 

 man by means of the five senses, seeing, hearing, feel- 

 ing, tasting, and smelling. The organs by which the 

 senses act are the nerves, which are small thread-like 

 fibres distributed over the whole bodyj and all of them 

 connected with the brain. 



The eyes seem to be formed very early in the human 

 embryo. In the chicken also, of all the parts that are 

 double, they are the soonest produced ; and I have ob- 

 served upon the eggs of several sorts of birds, as well 

 as upon those of lizards, that the eyes were much 

 larger and earlier in their expansion, than any other 

 parts of two-fold growth. Though in viviparous ani- 

 mals, and particularly in man, they are, at first, by no 

 means so large in proportion as in the oviparous classes, 

 yet they obtain their due formation sooner than any 

 other parts of the body. This is also the case with 

 the organ of hearing. The little bones that assist in 

 constructing the internal parts of the ear are entirely 

 formed before any of the other bones have acquired any 

 part of their growth or solidity. Hence it is evident, 

 that the parts of the body which are furnished with 

 the greatest quantity of nerves are those which appear 

 first, and which first attain to perfection. 



Mr Chesselden having couched, for a cataract, a 

 lad of thirteen years of age, who had from his birth 

 been blind, and thus communicated to him the sense 

 of seeing, was at great pains to mark the progress of ' 

 his visual powers. This youth, though hitherto inca- 

 pable of seeing, was not, however, absolutely and en- 

 tirely blind. Like every other person, whose vision 

 is obstructed by a cataract, he could distinguish day 

 from night, and even black from wliite, or either from 

 the vivid colour of "scarlet. Of the form of bodies, 



