of the Skull and the Skeleton. 349 
of the muscles. And so when the humerus of an active burrow- 
ing animal, like the mole, is compared with a humerus where 
the limb is merely used as a prop and does not meet with the 
like lateral resistance, it is found that it is so enormously 
expanded laterally as to be nearly as broad as it is long, instead 
of presenting a simple cylindrical shaft. But the best examples 
will be found in animals which use the limbs differently. Thus 
in the frogs, which use the hind limbs chiefly in leaping, it is 
found that they are longer than the fore limbs: this, too, is 
characteristic of the kangaroo and jerboas, of struthious birds, 
and of man; nor can an example be cited where an animal uses 
its hind limbs more than the fore limbs without their attaming 
to a greater length,—bhecause, as we saw at the outset, to use a 
limb is to bring to bear on it the pressure of all its muscles and 
the carcase, which were seen to be the stimulants to growth. 
Thus, too, birds of powerful flight have the fore limbs developed 
enormously ; while in those which do not fly, and therefore 
where but little pressure can be brought to the bones, these 
limbs are extremely small. 
Thus it has been attempted to prove by various arguments 
that pressure and tension is a cause of growth in bones. 
To show that the same cause which developes bones originally 
calls them into existence, it is only necessary to reverse the 
argument, and show that the less the pressure the less the 
ossification, until at last, where pressure and tension cease, the 
bones are lost. 
But there are a few simple facts which, exhibiting the forma- 
tion of osseous particles where they are normally absent, are 
worth mentioning: one is ossification of the heart, and another 
the union of fracture in the costal cartilages by bone, just as in 
birds they become ossified normally ; and a third is the signifi- 
cant fact that ossification in the foetal cartilage first appears 
around the artery which supplies it—that is, at the first place 
where pressure can be exhibited. And it seems indisputable 
that if there had been no inflammatory pressure the heart would 
never have ossified, and that, but for the pressure of the artery, 
the fcetal cartilage would not have been converted into bone. 
This, therefore, I take to be proved ; and we shall presently see, 
when considering the ribs, that pressure is capable of producing 
not only growth, but new bones also. 
It remains now to show that the developmental force (if such 
a power exist distinct from vitality, of which I see no easy proof) 
is the same in effect as pressure, and must be regarded as 
only an inherited result of pressure and tension. Thus Dr. 
Humphrey tells us that in the foetal cartilage the curves and 
processes are already modelled which afterwards characterize 
