THE WORK OF MUSCLES AND GLANDS 41 



In varying degrees the different portions of the alimentary 

 canal exhibit the same power, not ceasing to shorten and 

 to extend when observed entirely outside the body of the 

 animal. While we emphasize this remarkable tendency 

 to rhythmic activity, we must hasten to add that tissues 

 showing such capacities are nevertheless subject to some 

 nervous control. Thus the heart beats primarily because 

 of the peculiar nature of its own substance, but varia- 

 tions of rate and strength are constantly occurring as a 

 result of the influence of the nervous system. In this 

 connection it must be pointed out that such influence is not 

 necessarily so applied as to excite increased activity, but 

 may be inhibitory, that is, reducing the rate and force of 

 the spontaneous contractions. A large place is now given 

 to the inhibitory functions of the nervous system, and we 

 shall meet with other examples of the restraint which it 

 imposes upon various organs. A little reflection makes us 

 realize that much of the highest work of the brain must be 

 in the line of inhibition. A man is distinguished by the 

 acts from which he refrains quite as much as by those 

 which he performs. 



Muscular Tone. It will be well before we go farther to 

 make clear what is meant by tone (tonus, tonicity) in 

 connection with the behavior of contractile tissues. 

 Muscle is said to exhibit tone when it is not completely 

 relaxed. Tone is thus a mild, sustained contraction. It 

 seems rarely to be absent altogether, but may vary much 

 in degree. Tone in the skeletal muscles gives them a cer- 

 tain firmness and maintains a slight, steady pull upon their 

 tendons. This is not likely to result in actual movement, 

 because these muscles usually fall into antagonized 

 groups, one of which opposes another. A heightened tone 

 in the muscles of the arm may not change its position, 

 since the force tending to bend it may be offset by an 

 equal tension adapted to straighten it. Changes of tone 

 in the walls of the hollow viscera, as the stomach, have a 

 much more evident effect, since they alter the size of the 

 cavity. One must discriminate carefully between stretch- 



