66 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



movement, and shocks of medium strength, if given at short intervals, may 

 cadi cause a larger contraction than its predecessor, until a certain height of 

 contraction has Ween reached, beyond which there is no further increase pos- 

 sible. We shall consider these so-called "staircase contractions" more care- 

 fully later (see page 112). When irritations follow each other very rapidly 

 the whole character of the contraction is changed, and the muscle, instead of 

 making rapid single ton tractions, enters into the condition of apparently con- 

 tinuous contraction known as tetanus, during which it shortens considerably 

 more than it does when making single contractions. Increase in irritability 

 plays only a comparatively small part in the production of this remarkable 

 phenomenon, which we shall study more carefully when we come to the 

 mechanical problems involved in muscular contractions. 



Rapidly repeated- stimuli, though at first favorable to activity of a muscle, 

 soon exert an unfavorable influence by causing the lessened irritability which 

 is associated with fatigue. 



When a nerve is excited there is a change in its electrical condition, and 

 the extent of the change is generally believed to be an indication of the 

 extent to which the protoplasm of the nerve has become active in response 

 to excitation. Waller, 1 taking the amount of change in the electrical 

 condition of the nerve as an evidence of the ability of the protoplasm to 

 react under varying conditions, found that repeated excitation increases the 

 aetivitv of the nerve as it does of the muscle. Repeated excitation of a 

 nerve at suitable, regular intervals causes a staircase-like increase in the 

 strength of the electrical response, the record resembling that got by stair- 

 case contractions of muscles (see page 112). Moreover, if the electrical con- 

 dition of the nerve is tested by a series of excitations of equal strength 

 before and after it is subjected to a tetanizing current, the strength of the 

 variations is found to be increased. 



If a second stimulus follows the first too soon, it may be wholly ineffec- 

 tive ; at least this has been found to be the case with certain forms of proto- 

 plasm. It has been shown that heart muscle has a "refractory period," as it 

 is called, responding very imperfectly to stimuli applied to it just before 

 and during its systole.- Apparently much the same is true of the nerve. 

 Boycott, 3 using contraction of muscle as a test, and Gotch and Burch, 4 using 

 the current <>f action as a test, have lately discovered that for a brief period 

 after the nerve has been stimulated it is incapable of responding to a second 

 Stimulus. The length of the period of lessened excitability is greatly influ- 

 enced by temperature; at 1 <'.. with maximal stimuli, the "critical period" 

 may lie 0.007-0.008 second ; at higher temperatures it is shorter. 



(I>) Influences which favor the maintenance of the Normal Physiological 

 Oondiiion <>f Nerve <m<l Muscle. — Effect of Blood-supply m, Nerve and Muscle. 

 — The vascular system is a path of communication between the several organs 



1 Waller: Lectures on Physiology, first scries, 1897, p. C>8. 



- Cashing: Journal of Physiology, 1897, vol. xxi. p. '_'] 1. 



5 Boycott : Ibid., 1899, v<<\. sxiv. p. 144. * Gotch and Burch: Ibid., p. 410. 



