CHAPTER YI. 



1. Forms of muscle ; 2, Attachment of muscles to the bones; 3. 

 Elastic tension ; 4. Smooth muscle-fibres ; 5. Peristaltic motion ; 

 6. Voluntary and involuntary motion. 



1. In examining the action of muscle in the previous 

 chapters we have invariably dealt with an imaginary 

 muscle the fibres of which were of equal length and 

 parallel to each other. Such muscles do really exist, 

 but they are rare. When such a muscle shortens, each 

 of its fibres acts exactly as do all the others, and the 

 whole action of the muscle is simply the sum of the 

 separate actions of all the fibres. As a rule, however, 

 the structure of muscles is not so simple. According 

 to the form and the arrangement of the fibres, anatomists 

 distinguish short, long, and flat muscles. The last- 

 mentioned generally exhibit deviations from the ordinary 

 parallel arrangement of the fibres. Either the fibres 

 proceed at one end from a broad tendon, and are directed 

 towards one point from which a short round tendon 

 then effects their attachment to the bones (fan-shaped 

 muscles) ; or the fibres are attached at an angle to a 

 long tendon, from which they all branch off in one 

 direction (semi-pennate muscles), or in two directions 

 like the plumes of a feather (pennate muscles). In the 

 radiate or fan- shaped muscles the pull of the separate 

 parts takes effect in different directions. Each of these 



