CONTRACTILE TISSUES 63 



do with a tissue that is fundamentally automatic, the 

 influence of the nervous system may be either to increase 

 or to diminish the degree of activity. The idea that the 

 nervous system excites action is familiar, the conception 

 that it may also abate activity must be borne in mind. 

 Such restraint is called inhibition. 



Tone. An organ with smooth muscle in its walls may 

 be much more fully relaxed at one time than at another. 

 When it appears exceptionally large we need not con- 

 clude that it is forcibly distended by its contents as we 

 should assume of a rubber bag. The stomach, for in- 

 stance, may be much more capacious at one time than 

 at another and yet it may exert no more pressure upon 

 the food inside when it is very large than when it is small. 

 The adaptations exhibited by the hollow organs toward 

 their varying contents are referred to as tone changes. 

 If the stomach is dilated to accommodate a meal we say 

 that its tone has been lowered. The student must 

 clearly distinguish between such changes and actual 

 stretching. Tone may be defined as a residue of con- 

 traction or incomplete relaxation. The conception is 

 one which we shall constantly be called upon to entertain. 

 The equivalent words, tonus or tonicity, may be used. 



The facts of tone variation are well illustrated by the 

 behavior of the urinary bladder. When this container 

 has just been emptied, its cavity is practically ob- 

 literated. At another time it may hold a pint. But 

 everyone knows that the urgency of the call to empty the 

 bladder is not by any means proportional to the quantity 

 of its contents. The desire may be strong when the 

 organ is very small. If we had to do with a simple 

 elastic sac the internal .pressure would necessarily cor- 

 respond with the degree of distention but the walls are 

 living tissue and can contract and relax independently 

 of the amount of liquid enclosed. Tone has to be recog- 

 nized also as an essential property of the arteries, es- 

 pecially those of the smallest order. Variations of ar- 

 terial tone will call for careful attention at another time. 



