372 Remarks on the Physiology of the Egg. 
through.the medium of spiracula, dispersed over the ex- 
terior ivvolucrum. The description of the folliculus aéris 
just delivered is taken from that in the ege of our common 
hen. The same apparatus exists in the eggs of all birds, 
and contains a similar air: its capacity, however, does not 
seem to vary either with the size of the egg, or of the bird 
to which it belongs ; but [ think T have discovered a beau- 
tiful law by which its extent is modified. 
I have uniformly found, as far as my contracted inquiries 
have led me, that the folliculus aéris is of greater magni- 
tude in the eggs of those birds which place their nests on 
the ground, and whose young are hatched fledged, and 
capable of exerting their muscles as soon as they burst from 
their shell, than in the eggs of those whose nests are ge- 
nerally built on trees, and whose progeny are born blind 
and forlorn. Thus the folliculi in the eggs of fowls, par- 
tridges, and moor-hens are of considerable extent, whilst 
those in the eggs of crows, sparrows, and doves are ex- 
tremely contracted. The chick, therefore, of fowls and. 
partridges has a more perfect plumage, and a greater apti- 
tude to locomotion, than the callow nestlings of doves and 
sparrows. Such an instance of the agency of oxygenation in 
the promotion and increase of muscular power is not solitary 
in physiology ; for the history of ruminating animals will 
furnish us with a parallel example. ‘ Their cotyledons,” 
observes the author of Zuonomia, “ seem to be designed 
for the purpose of expanding a greater surface for the ter- 
mination of the placental vessels, in order to receive oxy- 
genation from the uterine ones: thus the progeny of this 
class of animals are more completely formed before their 
nativity than that of the carnivorous classes. Calves 
therefore and lambs can walk about in a few minutes after 
their birth; while kittens and puppies remain many days 
without opening their eyes.” If any further testimony be 
necessary to show that the augmentation of muscular energy 
is the result of a nice combination of oxygen with the ani- 
mal organs, many interesting facts might be adduced in 
confirmation of its truth. We generally find the strength 
of an animal proportionate to the extent of its chest : hence 
an attention to the * anmimosum pectus” has been attended 
with the improvement of our breed of cattle; and it is in 
consequence of a great extent of pneumatic receptacle that 
birds are enabled to bear the prodigious muscular exertion 
of flight. Is it not probable, too, that the repeated suspira- 
tions of the fatigued are instinctive exertions to procure a 
greater proportion of oxygen, by which their muscular 
energy 
