18 PHYSIOLOGY OF MDSCLES AND NERVES. 



stretch the knee, previously bent, so that the upper 

 and the lower thigh-bones are in a straight line ; or it 

 can raise the whole extended leg yet higher and bring 

 it nearer to the pelvis. But the points of origin and 

 of attachment of muscles may exchange offices. When 

 both legs stand firmly on the ground, the above-men- 

 tioned muscles are unable to raise the thigh ; instead, 

 on shortening, they draw down the pelvis, which now" 

 presents the more mobile point, and thus bend forward 

 the whole upper part of the body. In order, therefore, 

 to understand the action of the skeleton, the separate 

 bones of the skeleton and their connection must first be 

 studied. 



4. All bones are classified according as they are 

 flat, short, or long. Fiat bones, as their name indicates, 

 are expanded chiefly in two directions ; they form thin 

 plates. Short bones are expanded almost equally and 

 but slightly in all three directions. In long bones, 

 finally, the expansion in the longitudinal direction con- 

 siderably exceeds that in the other two directions. The 

 extremities, the arms and legs, are chiefly formed of 

 these long bones. The arm, for instance, consists of 

 the long bone of the upper arm, to which are attached, 

 first, two other long bones (called the elbow bone and 

 the radius), which together form the fore-arm ; and 

 secondly, by means of several shorter bones, wdiich con- 

 stitute the WTist, the hand itself; this latter consists of 

 the five bones of the palm and the five fingers, of which 

 the first has two, the others each have three divisions. 

 In all these bones, with the exception of those of the 

 wrist, a l^ng middle part, or shaft, with two thickened 

 ends, are noticeable. As this shaft is hollow, these 

 bones are also spoken of as cylindrical. The expanded 



