336 PHYSIOLOGICAL DEVELOrMENT. 



to bear a pull in the direction of its length ; and these struts 

 and ties are differently formed to adapt them to these 

 different strains. Further, it should be remarked that though 

 the rigidity of the f'-amework depends on the ties which are 

 flexible, as much as on the struts which are stiff, yet the ties 

 help to give the rigidity simply by so holding the struts in 

 position that they cannot escape from the thrusts which fall 

 on them. Kovv the like relation holds with a difference 

 among the bones and muscles — the difference being, that hero 

 the ties admit of being lengthened or shortened and the struts 

 of being moved about upon their joints. The mechanical re- 

 lations are not altered by this however. The actions are of 

 essentially the same kind in an animal that is standing, or 

 keeping itself in a strained attitude, as in one that is changing 

 its attitude — the same in so far that we have in each a set 

 of flexible parts that are pulling and a set of rigid jDarts that 

 are resisting. It needs but to remember the sudden collapse 

 and fall that 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 cannot 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 re- 

 active 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 haunch are similarly acted upon. Still, as the weight 

 of the hind quarters has to be transferred fronj the pelvis to 

 the feet and must be so transferred 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 



