350 Mr. H. G. Seeley on a Theory 
the bones. Now this may either be a result, as by all analogy 
would seem natural, of the formation of the foetal muscles 
which are attached to it, or may be referred to the same force 
(if such is assumed to exist) which gives the individual his form. 
But the foetal cartilage is a minute model of the adult bone. I 
therefore cannot but conclude that the same forces which deve- 
loped the adult bone also developed the foetal cartilage, and that 
the pressure of the uterus and the tension of the muscular fibres 
could not have failed to produce the same result at one period 
of life as similar pressure and tension do at another. 
Having thus glanced at the nature of growth in the abstract, 
as seen in a single ossification, we will now briefly examine the 
conditions exhibited by the more complex bones, which show 
several distinct osseous parts. Thus, as is well known, the 
humerus or femur or the bodies of vertebrze consist, as a rule, of 
three pieces, each of which ossifies from a distinct centre, and is 
therefore in that sense a distinct bone. Now, since it has been 
seen that all ossification takes place under the influence of pres- 
sure and tension, we have no other forces at command to which 
to attribute the formation of these terminal parts called epi- 
physes. 
The turtle shows no epiphyses in its limbs; and in a section 
of a femur of a young crocodile, kindly made for me by Mr. J. 
W. Clark, I was unable to distinguish epiphyses ; and it is well 
known that these sluggish animals do not subject the bones to 
enormous pressure in their crawling motion: but when the ac- 
tivity becomes greater and the pressure is increased, then epi- 
physes appear, as in the frog, where they long remain separate. 
And in the case of a limb-bone, it is worth considering that 
when the limb comes to the ground, it receives a blow at each 
of its ends, equivalent to the weight it supports, and varying 
with the power with which the limb strikes the ground. Here, 
then, it is seen that special pressure, if powerful enough and 
maintained, developes special ossification, just as the ordinary 
pressure of the atmosphere, the muscles, and the weight of the 
body developed the original bone. And hence it is found that 
in the phalanges, metatarsals, and metacarpals there is commonly 
but one epiphysis, because, from the way m which the bones are 
applied to the ground, the pressure takes place at one extremity 
only. 
iebebver there can be no doubt that atmospheric pressure, 
which holds the bones together so well, must also be a powerful 
stimulant to ossification. 
The ligaments, too, by their resistance all help the epiphysial 
formation. 
And when it is seen that the trochanters appear under the 
