SUPPORT, MOTION, AND SENSATION 



337 



the particular function which they perform. The frog's leg, for 



example, is relatively incapable of more than a flexing motion or a 



straight swing of the limb, whereas the human arm responds to flexing, 



rotating, or swinging, 



origin of 



according to the way 

 in which it is moved. 

 Human musculature is 

 much more complex 

 than that of a frog be- 

 cause it has many more 

 diverse functions to per- 

 form. 



Evidently there is a 

 definite relationship 

 between the types of 

 motion which are possi- 

 ble from the standpoint 

 of skeletal structure 

 and the development 

 of muscles that make 

 such movements effec- 

 tive. Actual movement 

 results from the con- 

 traction of muscles and 

 is stimulated into activ- 



raixsole. 



musctle 



terjoCon cf 

 Achilles ' 



lir— .insertion 

 f\ of muscle 



man 



Comparison of the arrangement of the muscles 

 and supporting skeleton oi an insect's and verte- 

 brate's leg. (Former after Berlese.) 



ity by nerves. Since the muscles, nerves, and skeleton are closely 

 correlated parts, their degree of usefulness depends to a marked 

 degree upon the proper development and functioning of all the 

 parts. 



Smooth or Involuntary Muscles. This tissue is characterized 

 by the absence of striations and the presence of a single nucleus in 

 each cell or fiber. It is the type of muscle which carries on most of 

 the internal movements of the body. The walls of the intestines 

 are lined by layers of circular and longitudinal involuntary muscles. 

 The muscles in the walls of blood and lymph vessels, the tracheal 

 tube, reproductive ducts, the ureters, and tlie skin are also of this 

 type. Typically sluggish in contraction, they are the principal kind 

 of muscles found in the lower animals. 



Skeletal or Striated Muscles. In this category fall all of the 

 muscles which are under the control of the central nervous system and 



