18 PHYSIOLOGY OF MUSCLES AND NERVES. 



stretch the ki , previously bent, so that the u]>] i 



and the lower thigh-hones an- in a >traidit line; or it 

 can rai-e ihc whole extended leg yet higher and brin^- 

 it nearer to the pelvis. J'nt the points of origin and 

 of attachment of muscles may exchange olh'ces. 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. Flat bones, as their name indicates, 

 are expanded chielly in two directions ; they form thin 

 plates. Short bones are expanded almost equally and 

 but slightly in all three directions. In long bon 

 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 dhow bone and 

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

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

 stitute the \\rist. the hand itself; this latter consists of 

 the fi\e bones of the pa 1 m and the live fingers, of which 



theiirsi has two, the others each have three di\ i-ions. 



In all these bones, with the exception of (ho>e of the 



wrist, a 1 >ng middle part, or shaft, \\ith two thickened 

 nds, are noticeable. A- this shaft is hollow, the-e 

 bones are also spoken of as cylindrical. The expanded 



