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A MANUAL OF VETERINARY PHYSIOLOGY 



will describe a curve still higher than the preceding, and so on, 

 until a maximum is reached when it can go no higher. Such a 

 piling up of contraction on contraction is known as summation of 

 contractions (Fig. 118). It will be remembered that one character- 

 istic of heart muscle is the absence of summation ; the fibres of 

 the heart yield their best possible contraction to a single stimulus, 

 be it weak or strong, single or multiple. 



Tetanus.- — If an induction current be applied to a muscle or 

 its nerve, a rapid succession of stimuli is thus introduced, and 

 there is no time for complete relaxation to occur between each 

 successive stimulus. This may be seen in the lower curve of 

 Fig. 119, in which ten stimuli per second were passed into the 

 muscle, and partial relaxation only will be observed to have 

 occurred between each of them. In the middle curve twenty 



stimuli per second were 



used, and the amount of 

 relaxation is represented by 

 a slightly wavy line ; in the 

 upper curve thirty stimuli 

 per second were employed, 

 and the curve shows no re- 

 laxation ; the muscle is in 

 the condition of electrical 

 tetanus, and tetanus, there- 

 fore, consists of the summa- 

 tion of a series of short con- 

 tractions with an insufficient 

 interval for intervening re- 

 laxation.* The number of 

 stimuli per second required 

 to provoke tetanus depends 

 on the condition of the muscle, the nature of the muscle, and the 

 class of animal ; for example : 



In the bird, 100 stimuli per second. 



In man, 40 stimuli per second. 



In the rabbit, 20 to 40 stimuli per second for the pale voluntary 



muscles. 

 In the rabbit, 10 to 20 stimuli per second for the red voluntary 



muscles. 



Fig 



118. — Superposition of Contrac- 

 tions (Stewart). 



is the curve of contraction due to the 

 first stimulation ; 2 is the curve of the 

 second contraction, superadded to 1 by 

 applying the second stimulus at the 

 moment when the first contraction had 

 nearly reached its maximum. 



In insects, whose muscles can contract a million times an 

 hour, 300 stimuli per second are required to produce tetanus. 

 A fatigued muscle is more readily tetanised than one which is 



* The tetanus of the physiologist must not be confused with the tetanus 

 of the pathologist ; the latter is a bacterial affection producing a poison 

 which causes spasm of many of the voluntary muscles of the body 

 especially of the limbs and jaw. 



