346 PHYSIOLOGICAL DEVELOPMENT. 



rigid parts that are resisting. It needs but to remember the 

 sudden collapse and fall which take place when the muscles 

 are paralyzed, or to remember the inability of a bare skeleton 

 to support itself, to see that the struts without the ties can- 

 not suffice. And we have but to think of the formless mass 

 into which a man would sink when deprived of his bones, to 

 see that the ties without the struts cannot suffice. To trace 

 the way in which a particular bone has its particular thrust 

 thrown upon it, may not always be practicable. Though it 

 is easy to perceive how a flexor or extensor of the arm causes 

 by its tension a reactive pressure along the line of the 

 humerus, and is enabled to produce its effect only by the 

 rigidity of the humerus ; yet it is not so easy to perceive how 

 such bones as those of a horse's pelvis are similarly acted 

 upon. Still, as the weight of the hind quarters has to be 

 transferred from the back to the feet, and must be so trans- 

 ferred through the bones, it is manifest that though these 

 bones form a very crooked line, the weight must produce a 

 pressure along the axis of each: the muscles and ligaments 

 concerned serving here, as in other cases, so to hold the 

 bones that they bear the pressure instead of being displaced 

 by it. Not forgetting that many processes of the bones have 

 to bear tensions, we may then say that generally, though by 

 no means universally, bones are internal dense masses that 

 have to bear pressures — pressures which in the cylindrical 

 bones become longitudinal thrusts. Leaving out exceptional 

 cases, let us consider bones as masses thus circumstanced. 



When giving reasons for the belief that the vertebrate 

 skeleton is mechanically originated, one of the facts put ii 

 evidence was, that in the vertebrate series the transition from 

 the cartilaginous to the osseous spine begins peripherally 

 (§ 257) : each vertebra being at first a ring of bone sur- 

 rounding a mass of cartilage. And it was pointed out that 

 this peripheral ossification is ossification at the region of 

 greatest pressures. Now it is not vertebrae only that follow 

 this course of development. In a cylindrical bone, though 



