84 
CiAcciOj on the Nerves of the Cornea. 
tissue has different degrees of firmness in the nerves of 
different animals,, and that in the same nerve its firmness is 
gradually diminishing from the trunk to its terminal branches. 
If this supposition is not allowed^ the facts which we are 
observing must remain either unaccounted for^ or we must 
admit that] the nerves at their termination lose the common 
investment^ only retaining the special one to each separate 
fibre. Although the latter supposition has many degrees of 
probability in its favour^ and explains very satisfactorily the 
continual change of position which the nerve-fibres undergo, 
as the trunks, dividing and subdividing, reach their ultimate 
distribution ; yet there are some facts which positively de- 
monstrate that, in some animals, the finest nerve- branches 
are provided with the same common covering as the trunks, 
from which they originate. Thus I have observed that, in 
the cornea of the sparrow, the individual nerve-fibres that 
compose the trunks and all the branches into which they 
divide, scarcely undergo any change of position. In this 
small bird the nerves of the cornea in the trunks, as well as 
in all the branches, exhibit the general appearance of large 
twigs, which, by dividing and subdividing, gradually diminish 
in size. So great is the firmness of the connective tissue 
which holds together the individual nerve-fibres, composing 
the nerves distributed to the cornea of this bird. 
Number, size, and relative position of the primitive nerve- 
fibres composing the nerves of the cornea; their division 
and nature. 
The number of the primitive fibres which compose the 
nerve-trunks of the cornea^ is found to vary according to their 
size. But sometimes we observe in animals of different kinds 
nerve-trunks of the same size, containing various numbers of 
primitive fibres. This depends upon the different diameters 
of the nerve-fibres, as some of them are thicker than others. 
According to my observations in the mouse, the primitive 
nerve-fibres are larger than those of the frog and man ; and 
in the sparrow they are much finer. 
Before the nerve-trunks of the cornea begin to branch, the 
primitive fibres composing them undergo a very little change 
of position, but as soon as their branching begins the change 
of position takes place, and increases as the division of the 
nerve- trunks goes further on. In different animals this 
change of position does not occur to the same extent. Thus, 
for instance, in the frog and eel the nerve-fibres change their 
relative position very frequently and extensively, but less in 
