of the Skull and the Skeleton. 347 
animal after birth, that the developmental force and the other 
forces jointly tend to produce growth in the same direction, 
which depends on the morphology of the animal, coimciding in 
results, they will be admitted to be different forms of the same 
force, originally due to the same cause, namely motion. In 
this way I shall first try to account for the growth of bone. 
The forces acting on bones must be from within or from with- 
out ; and therefore every such force will in its effect be either of 
the nature of an impact or of an explosion. The forces acting on 
the animal are the media in which it lives (as air and water), the 
air it breathes and the food it eats; while the mechanical forces 
acting within the animal will be the muscles, vessels, viscera, &c. 
All these can only produce alternations of pressure and tension 
and rest. These, therefore, are the stimulants to growth. But 
growth is an enlargement in which the particles expand and in- 
crease externally. And as this cannot be favoured, but rather 
resisted, by pressure, it becomes evident that the actual increase 
must take place when the pressure is removed. Therefore, since 
rest must be purely negative as a force, the stimulant to growth 
is pressure and tension. 
The first and most obvious source of these powers is the 
muscles; while the more they are used to propel the animal 
through the resisting air or water, the greater will be the equi- 
valent of general pressure on the bones. 
Thus if we take a limb-bone (the humerus, for instance), it 
will be found most extended in the direction between the radius 
and the scapula, in which it has to support the weight of the 
carcase ; and if the ends of the bone are examined, where mus- 
cles are attached or press tightly, it will be found that growth 
has extended outward more rapidly than in the body of the 
shaft, where there is no direct tension, but only a lateral pres- 
sure. 
But the resistance of the atmosphere produces a different 
series of modifications. It appears to be a mechanical principle, 
that if pressure be applied to the outside of a cylinder, it will in 
effect be relatively equivalent to reducing the pressure within. 
And it is found, from observations in paralysis and other affec- 
tions, and in the aged skeleton, that when, from failing vitality, 
motion is less and the muscles become less powerful (that 
is, when the pressure is reduced), the bone to some extent 
dies, and, from the dead part being carried away, becomes 
smaller and lighter. Now, the effect of motion through the air 
is relatively to diminish the pressure in the interior of the bones. 
And therefore it is found, in the sluggish Sloth, that the limb- 
bones are solid, that the active Mammals have large medullary 
cavities, while in the more active class of Birds the cavities be- 
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