LECT. XVI. PASSIVE ORGANS OF LOCOMOTION. 303 



thighs to the trunk were cut across in a suspended subject, 

 without the limb suffering the least change in its position ; 

 but when a hole was made into the cotyloid cavity, the 

 limb immediately fell, and it could be made to fall, or be 

 prevented from doing so, merely by opening or keeping 

 closed this aperture. By calculating the pressure exercised 

 by the atmosphere against the plain section of the cotyloid 

 cavity, we find that it is equivalent to 11-970 kilogrammes 

 [about 26 Ibs. 6 oz. avoirdupoise;] that is to say, it slight- 

 ly exceeds the mean vreight of the leg. 



Passive Organs of Locomotion. The passive organs of 

 locomotion, like every other part of the human machine, 

 present a constant application of the principles of mechanics, 

 in order to realize a very complicated result. Suppose we 

 said to an engineer, we want a moveable column composed 

 of a certain number of cylindrical pieces, united together 

 by their extremities, and of which the length must be varia- 

 ble: the thickness of the column should be such that it can 

 support the weight with which it will be loaded, and it 

 should be capable of resisting lateral shocks. The extremi- 

 ties of the different pieces of the column shall so terminate, 

 that the forces to put it in motion may be applied there; 

 and, finally, the column must be capable of executing a 

 great number of partial movements without deranging its 

 simplicity and elegance. The engineer would certainly 

 consider it a problem difficult, if not impossible, to solve. 



Boms. The bones are formed of a mixture of gelatine, 

 and of phosphate and carbonate of lime in different propor- 

 tions. If these vary, great changes in the degree of tenacity 

 and elasticity of the bones in consequence take place. 

 Independently of their composition, the more or less fibrous, 

 and more or less spongy, texture of bones, serve to modify 

 their weight and resistance. These properties also vary 

 according to the dimensions of the bones. Euler proved 



