SUMMATION OF STIMULI -09 



the maximum summation effect occurred in curve 6, where the height 

 of the second apex is 17 cm. as compared with 16'5 for the second 

 apex of 5 ; 14-25 cm. for that of 4 ; 13 cm. for that of 3 ; and 9 cm. 

 for that of the first apex. As a general statement, it is found that the 

 height of the second contraction is greatest when the instant of stimu- 

 lation falls at the commencement of the last third of the period of 

 contraction of the previous twitch. In connection with the height of 

 the second twitch it is seen from curves 3 and 4 that when the com- 

 mencement of the second contraction falls in the lower half of the re- 

 laxation of the first the height of the second contraction, as measured 

 from the level at which it starts, is higher than the height of the 

 first. 



With regard to the time of the second contraction one other point 

 of interest is found, viz. that the apex time of the second contraction 

 is less than the apex time of the first, where by apex time is meant 

 the total time elapsing from the instant of stimulation till the highest 

 point of the curve is reached. 



THE GENESIS OF TETANUS 



A study of the effect of applying two stimuli to a muscle or its 

 nerve, the time interval between the two being varied, naturally 

 leads to a consideration of the effect which will be obtained by ex- 

 tending the number of stimuli. 



To gain an answer to this it is necessary to have some form of apparatus 

 which will give a series of stimuli at regular intervals, and further allow of 

 an alteration of that interval. There are many forms of apparatus that will 

 effect this, but the simplest consists of a flat steel spring which can be clainped 

 in different positions, and whose free end is provided with a platinum wire 

 by means of which a circuit may be closed with each vibration of the spring. 

 The rate of vibration with a given spring depends, solely, upon the length of 

 spring which is allowed to vibrate, and increases as the spring is shortened. 

 To keep up the vibration of the spring it is usual to introduce an electromagnet 

 which attracts it as soon as the circuit is closed. The method of fitting up 

 the apparatus is shown in fig. 60. A c is the flat steel spring which can be 

 firmly held in any position by the clamp c. At a a platinum wire is soldered 

 to the free end of the spring, and this can close a circuit by dipping into the 

 mercury m held in a vulcanite cup. e is an electromagnet held above the 

 vibrating reed. In order to get regular vibration of the reed the electromagnet 

 should be fixed at a point about two-thirds of the length of the spring away 

 from the clamp {i.e. nearer to a than its position, as shown in tbe figure). To 

 fit up the apparatus connect one terminal of the battery b with the mercury 

 cup by means of the binding screw t 1 . Connect the clamp of the reed by its 

 binding screw t to one terminal, E 1 , of the electromagnet and the second, 

 T 2 , to one terminal, p 1 , of the primary coil. The second terminal of the coil 

 is then connected to the battery. Supposing now that the platinum wire a 



