84 GENERAL ZOOLOGY 



sarcomeres. Thus a single sarcomere would include the parts of 

 each myofibril, within two adjacent membranes of Krause, lying 

 at the same level. Usually, however, the longitudinal cohesion is 

 greater than the lateral with the result that some striated muscle 

 fibers cannot be made to cleave transversely into disks. Figure 

 50 illustrates the appearance of a longitudinal section of skeletal 

 muscle of a larval salamander. 



Action of Skeletal Muscle.— Skeletal muscle is voluntary in 

 its action, i.e., its action is under the control of the central 

 nervous system, the brain and spinal cord. This is in sharp 

 contrast to the involuntary activity of the plain or unstriated 

 muscle cells of the walls of the alimentary tract and blood 

 vessels. Cardiac muscle, though striated, is also involuntary 

 in its action. 



