380 ANATOMY. 



the older bone hollow. From this then we see that the greatest part 

 of the bone is solid, and afterwards becomes hollow; therefore it is 

 reasonable to think that the first ossification was solid, and then became 

 hollow. 



The bones are such parts that do not grow or increase in any part 

 while that part is exposed to the air ; therefore Nature has taken care 

 to cover bones that are sound as soon as possible [if they are by any 

 accidental circumstance exposed]. 



Bones are commonly hard in proportion to their length ; for, in pro- 

 portion to their length they are obliged to make the greater resistance. 

 Any lateral bias, wrench, blow, &c. more easily breaks a long bone 

 than a short one. It in all cases acts with a longer lever. 



Animals whose bases of support and motion are formed of hard and 

 solid parts, have them so joined as to allow of motion between them; 

 which joints, in the formation of their parts, are so related to the appli- 

 cation of the powers of motion, as to constitute what are called the 

 mechanical powers or levers. A lever has the moving power, the centre 

 of motion, and the resistance. And as these may be so placed with respect 

 to each other as to vary their relative position as much as three numbers 

 can, so are the joints in an animal variously formed. The joint of the 

 foot upon the tibia is of all the three sorts. It is a lever of the first 

 kind when we push anything with our toes ; of the second kind when 

 we raise anything with them ; and of the third when we raise our body 

 upon them. 



In mechanics, without resistance there could be no such thing as 

 motion ; but in animals, where there is a self-moving power, this can 

 act without any resisting point. But this self-moving power cannot be 

 applied to other parts, even in the animal, without a fixed point of 

 resistance. However, these self-moving powers, by their simple action 

 within themselves, may produce immediately the effect, without one 

 point resisting more than another; as [in the instance of] a worm 

 simply contracting and bringing its two ends together, or contracting 

 laterally, so as to push itself out, by which means it is elongated ; but 

 even here the one half of the worm may be said to be the fixed point 

 or point of resistance to the other ; but a worm can fix any part of its 

 body to whatever it lies upon, and move towards that point. A circular 

 muscle would appear to have no fixed point, only the power of contrac- 

 tion, or shortening itself; and from the figure the muscle is thrown 

 into, it produces its effects ; each portion becomes a fixed point to the 

 other all round ; therefore every moving body which acts mechanically, 

 acts from some resisting power which may be called the fixed point, or 

 centre of motion. But animals, more especially the compound ones, 



