246 THE BIOLOGY OF THE FROG 



CHAPTER XIV 



THE MUSCULAR SYSTEM 



The function of muscle is the production of movement 

 through contraction. The muscles of the frog retain their 

 vitahty for a long time after they have been removed from 

 the body, and they are consequently well adapted for 

 physiological experiments. The large gastrocnemius, or calf 

 muscle, of the frog is so favorable a one for investigation 

 that much of what is known of the general physiology of 

 muscular activity is derived from a study of this object. 

 When a muscle contracts, it increases in thickness as it de- 

 creases in length. Contraction may be brought about, as 

 is readily demonstrable with a fresh muscle, by a variety 

 of causes, since it follows upon the application of nervous, 

 thermal, mechanical, chemical, or electrical stimuli. The 

 response to stimulation in voluntary muscle takes place very 

 quickly and increases within certain limits, with the increase 

 of the intensity of the stimulus. In involuntary muscle the 

 response is much slower. 



Most of the muscles are attached by one or both ends to 

 bones. The attachment in some cases is direct, in others 

 it is by means of a tendon, which is a band of very tough, 

 inelastic connective tissue. The outer surface of a muscle 

 is covered by a connective tissue membrane, or fascia, 

 which is more or less elastic. The tendons of many muscles 

 are formed by a continuation of the fascia, which becomes 

 thicker toward the end of the muscle, where it graduates into 



