THE CONQUEST OF NATURE 



ject to the influence of the will, and under ordinary 

 conditions contract in response to the voluntary nervous 

 impulses. Certain sets of them, indeed, as those 

 having to do with respiration, have developed a ten- 

 dency to rhythmical action through long use, and 

 ordinarily perform their functions without voluntary 

 guidance. Their function may, however, become 

 voluntary when attention is directed toward it, and is 

 then subject to the action of the will within certain 

 bounds. Should a voluntary attempt be made, how- 

 ever, to prevent their action indefinitely, the so-called 

 reflex mechanism presently asserts itself. All of which 

 may be easily attested by anyone who will attempt to 

 stop breathing. All systems of voluntary muscles 

 are subject to the influence of habit, and may assume 

 activities that are only partially recognized by conscious- 

 ness. As an illustration in point, the muscles involved 

 in walking come, in the case of every adult, to perform 

 their function without direct guidance of the will. 

 Such was not the case, however, in the early stage of 

 their development, as the observation of any child 

 learning to walk will amply demonstrate. In the case 

 of animals, however, even those muscles are so under 

 the impress of hereditary tendencies as to perform their 

 functions spontaneously almost from the moment of 

 birth. These, however, are physiological details that 

 need not concern us here. It suffices to recall that the 

 voluntary muscles may be directed by the will, and 

 indeed are always under what may be termed sub- 

 conscious direction, even when the conscious attention 

 is not directed to them. 



[46] 



