22 THE MECHANISM OF LIFE 



extension of the other. Extension is not merely a passive relaxa- 

 tion, but is the result of a nervous impulse just as much as is 

 contraction. 



Note also that the humerus and scapula are fixed bones with 

 regard to the flexion and extension of the forearm, but the 

 humerus is itself a movable part, and the fixed points to which 

 its muscles are attached are in the scapula. The latter is also 

 movable, and then the fixed bones are the vertebra and ribs. 

 The hand is a movable part, and the fixed bone to which its 

 muscles are attached is the radio-ulna. Thus " fixed " and 

 " movable " bones are relative to the parts that are to be moved. 

 The reader must also note that the movement of any part of a 

 limb is never so simple as we have indicated, for several pairs of 

 antagonistic muscles are generally in action at the same time. 



Circular musc/es 

 contha-cTed. 



Circular muscles 



H^CavifY of' the 

 /nresTine 



Longitudinal muscles 



Longitudinal 

 muscles contracfecL 



Fig. 6. — Diagram of the Muscles of the Intestinal Wall. 



Non-Skeletal Mechanisms. — There are many movable parts in 

 the body for which there are no skeletal supports. Thus the 

 heart consists of a very complicated series of muscle bundles that 

 contract and expand automatically. Simpler cases are those of 

 the intestine and bloodvessels. In the former we have two series 

 of muscles, one which is made up of bundles of fibres running 

 circularly in the wall of the bowel, and the other being made up 

 of fibres that run lengthways. When the former contract the 

 diameter of the intestine is decreased, and when they relax and 

 the longitudinal muscles contract the length of a segment of the 

 bowel is decreased and the diameter is correspondingly increased. 

 In this way waves of contraction of calibre of the intestine are set 

 up, and these propel the food contents from place to place. The 

 mechanism of contraction of an artery is very similar, except that 

 the longitudinal muscles are not present. The wall of the artery 

 is elastic, and when the circular muscles contract the calibre is 

 diminished, but when they relax the elasticity of the wall, and 



