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 attaining 

 to a greater length, — because, 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 pressm'e 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 fcetal 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 foetal 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 efitct 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 



