The Evolution of the Morse. 
121 
to a hoof is of the very slightest character, especially iu the donkeys 
and zebras, where, indeed, it has none. But the most serious objec- 
tion is the situation of the one that is most constant — that on the 
fore limb — where it is placed, not on the hand, as it would be, 
if it represented the thumb, but upon the arm, at some distance 
above the wrist-joint. Lastly, such a hypothesis is quite un- 
necessary, for they obviously belong to a numerous class of 
special modifications of particular parts of the cutaneous surface, 
which occur in very many animals, the use of which is in most 
cases remarkably obscure. Bare spots, thickened patches or cal- 
losities, and tufts of elongated, or modified hair, often associated 
with groups of peculiar glands, are very common in many parts 
of the body, but especially the limbs, of many Ungulates, and to 
this category undoubtedly the '•chestnuts" of the horse belong. 
If they teach us nothing else, they afford a valuable lesson as to 
our own ignorance of the meaning and the use of a structure so 
cons2)icuous to observation, and in an animal whose mode of life, 
almost more than any other, we have had the fullest opportunity 
of becoming intimately acquainted with. That they have some 
significance or utility cannot be doubted ; but I must admit that 
it is at present beyond our powers to guess what it is, or to 
account for their presence upon any of the hitherto recognised 
principles of causation of animal modifications. 
The nostrils of the horse are, as is well known, large, and 
very dilatable, allowing of the admission of a greater or less 
amount of air according to the demands of respiration. Owing 
to the structure of the soft palate, and its relation to the upper 
end of the larynx, breathing takes place entirely through the nose. 
"When men, dogs, and many other animals, owing to any great 
exertion, begin to pant, and require an additional quantity of 
air to that which is ordinarily taken in by the nose, the mouth 
comes to the aid of that channel, and is widely opened ; but the 
horse, under the same circumstances, can only expand the mar- 
gins of the nostrils, for which action there is a very efficient 
set of muscles, acting upon the cartilaginous framework which 
supports them and determines their peculiar outline. Imme- 
diately within the margin of the upper part of the nostril is a 
structure of very considerable interest, which is generally sup- 
posed to be peculiar to the horse and its immediate allies, as 
the ass, the use of which is entirely unknown. It is a blind 
pouch, about three inches in depth, conical in form, though 
slightly curved, and lying in the cleft seen in the' dried skull 
between the nasal and the premaxillary bones. It is a diver- 
ticulum from the nasal passage, with which it freely communi- 
cates below, and is lined by a continuation of the same mucous 
