274 PHYSIOLOGY 



exactly analogous to the summation of contraction in a voluntary 

 muscle. It may be noticed, however, that the first three or four 

 stimuli are ineffective, and that there is in this case a summation 

 before any contraction has occurred, a summation of stimuli. Each 

 stimulus, in fact, alters the state of the contractile tissue and makes it 

 more ready to respond to the next stimulus, so that the stimuli become 

 more and more effective. If time is allowed for the muscle to relax 

 between successive stimuli, this summation is evidenced by a con- 

 tinually increasing height of contraction, the so-called ' staircase.' 

 It will be remembered that the same initial increase of effect was 

 observed when voluntary muscle was excited by continually recurring 

 stimuli (v. Fig. 69, p. 234). 



We shall meet with other examples of this summation of stimuli 

 when dealing with the physiology of the central nervous system. It- 

 is indeed a fundamental phenomenon in the physiology of excitation. 



CHEMICAL STIMULATION. Strong salt solution excites con- 

 tractions just as in the case of skeletal muscle. Many drugs, such as 

 physostigmin, ergot, salts of lead and barium, digitalis, may act 

 directly on smooth muscle and cause contraction. As one would 

 expect, however, from the greater independence of the smooth muscle, 

 the action of these drugs varies from organ to organ, muscle-fibres, 

 which apparently are histologically identical, reacting diversely 

 according to their origin. 



MECHANICAL STIMULATION. Smooth muscle may react to a 

 local pinch or blow with a local or a general (propagated) contraction. 

 The most important form of mechanical stimulation is that produced 

 by tension. The effect of increasing the tension on smooth muscle 

 may be twofold : causing in the first place relaxation and in the 

 second excitation with increased contraction. These two effects 

 may be illustrated by taking the case of the bladder. If this viscus 

 (which is surrounded by a complete coat of smooth muscle) has all its 

 connections with the central nervous system severed, it is when empty 

 in a state of tonic contraction. If fluid be injected into it rapidly 

 there is a great rise of pressure in its cavity, due to the forcible disten- 

 sion. If, however, the fluid be injected slowly the bladder muscle 

 relaxes to make room for it, so that a considerable amount of fluid may 

 be accommodated in the bladder without any great rise of pressure. 

 This process of relaxation has its limit. If the injection of fluid be 

 continued the walls begin to be stretched passively, and this increased 

 tension acts as a stimulus causing marked rhythmic contractions of 

 the whole bladder. 



In the same way the response of a smooth muscle to an electrical 

 stimulus is much increased by previous increase of the tension on the 

 muscle fibres. 



