252 



HUMAN BIOLOGY 



altering them, although they are reflex. This touch attains 

 its closest in instances where as in man the expiratory 

 movement is regulable by the higher and so-called volitional 



Fig. 2. Schematic cross section of spinal cord, illustrating control which 

 may be exerted by a receptor over effectors on two sides of body. One motor 

 neurone may, through peripheral branching, supply many muscle fibers. 



centers in order to serve the vocal organ for speech, man's 

 language having become a supreme mode of expression for 

 his mental experience. 



Turning next to acts in which we employ the muscles 

 which are famihar to us as moving our bony frame 

 in trunk and Hmbs, much of the reflex activity of the nervous 

 system is devoted to exploiting this field for many purposes 

 of Hfe. Regarding these muscles it is to be noted that although 

 we have already Hkened them to motor machines at the 

 disposal of nerve centers driving them from without, they 

 are in fact instruments not purely passive under that drive, 

 for they possess receptors of their own intrinsic within them- 

 selves, and can report and send messages on their own 

 behalf into the central exchanges. They have some voice in 

 their own conditions of service, and the messages by which 

 they thus express themselves are termed proprioceptive. 

 The proprioceptive reflexes are peculiarly remote from 

 mental experience and lie quite beyond our self-examination 

 by any eff"ort of introspection. But they habitually exert a 

 self-regulation upon the muscular activity not only during 



