348 Mr. H. G. Seeley on a Theory 
come larger still. In this class, as in the subclass Saurornia, 
founded on the Pterodactyles, the process goes on till even the 
marrow disappears in the bones most used in motion, and their 
wonderfully thin walls become filled with hot air from the 
lungs. 
Now it becomes necessary to consider in what manner pres- 
sure from muscles and other internal forces can act on the bones 
so as to produce growth; and here I would draw the illustration 
from pathology. Inflammation, in- effect, is pressure; and 
whenever inflammation extends to the periosteum, that struc- 
ture is excited to a morbid rapidity of action, and the bone 
immediately beneath is thickened: hyperostosis is defined to be 
a thickening and condensation of the shaft from inflammation. 
Nor is the pathology of the heart and the lung less suggestive 
where it shows, as is well known, that muscle may be inflamed, 
indurated, and changed into cartilage, which undergoes a partial 
change into bone, though, from the nature of the case, the last 
change can never advance very far, except in the lung, which 
may be replaced by muscle and well-developed bones. 
Therefore, seeing that the effect of motion is a succession of 
falls, every one of which gives a powerful blow to the bones, and 
that no muscle can be moved without both pulling and pressing 
bones, we have an irritating cause, similar in kind though less 
in degree to that which results in abnormal growth. And 
accordingly it is found that the greater the activity (that is, the 
nearer the approach to an inflammatory condition) the more 
extensive will be the ossification. 
Thus in the wild animal, which uses its muscles more vigo- 
rously than the tame animal, the ridges and processes for the 
attachment of muscles are more developed. In the limbs a 
trochanter appears as a separate ossification, where powerful 
muscles are attached. The marsupial muscles, which are small 
in man, become largely developed in the Didelphia, and create 
the marsupial bones. 
Now it remains to show that the intensity of growth depends 
on the amount of the pressure and tension in the direction of 
the increase. Dr. Humphrey tells us that bones are densest in 
those parts which are subject to the greatest mechanical stress, 
and hardest in those persons who are strongest and most active. 
Here the intensity of ossification clearly depends on the pres- 
sure. And, again, it is observed that bones are most curved in 
those persons whose muscular strength is greatest—that is to 
say, where the pressure resulting from muscular action is the 
greatest ; while weak persons, on the contrary, have compara- 
tively straight bones. And thus it is seen that, even in the 
individual, the form of the bone varies with the relative power 
