THE PHYSIOLOGY OF MUSCULAR WORK 225 



that it passes into visible contractions of an involuntary nature ; 

 shivering thus arises, and in most cases appears to be a protective 

 response to the effects of cold. It is true that it may occur apart 

 from external cold during nervous excitement, but even in this 

 case observations may show that the temperature of the skin has 

 been lowered by a sudden and vigorous contraction of its blood 

 vessels. 



Voluntary contraction is the shortening of the muscles pro- 

 duced by an effort of the will. This is only a general definition, 

 for movements such as walking may be performed unconsciously. 

 During ordinary movements the change of state is not confined to a 

 single muscle ; others are also involved, it may be in active contrac- 

 tion or in relaxation. The important researches of Sherrington l 

 upon reciprocal innervation have shown how the muscles can in- 

 fluence the activity of one another by means of their motor and 

 sensory nerves : the contraction of one muscle brings about the 

 relaxation of its antagonist. The muscle possesses in its muscle 

 spindles a special sensory ending which is stimulated by the 

 stretching produced by the contraction of its antagonistic muscle ; 

 in this way not only can reflex effects be produced upon the muscle, 

 but also upon the vaso-motor and respiratory mechanisms. 



A simple movement, such as removing the finger from a key 

 at a given signal, may or may not, according to the individual's 

 type of reaction, be preceded by a movement of the opposite kind. 

 This may be called the antagonistic form of reaction in contrast 

 to the ordinary form in which the lifting of the finger is carried 

 out at once ( M ). 



Such a movement of the finger can only be repeated about eight 

 to twelve times a second by the ordinary man, but a pianist by 

 practice and by using muscles in relays can press the keys at a 

 faster rate. In short, sharp, and rhythmical movements of the 

 fingers the rate may be as rapid as forty a second. 



The nature of a voluntary contraction, whether it be a single 

 twitch or a spasm, requires consideration. It has been studied 

 in two ways, by a determination of the note of the sound produced 

 by the contraction of the muscle, and by graphic records. The 

 muscle note corresponds to forty vibrations a second, but the diffi- 

 culty in such a determination by the ear lies in the fact that it 



1 Article Spinal Cord, "Textbook of Physiology," edited by Schafer, vol. ii., 

 1900. 



