J. F. GASKELL 83 



of a strength 'of 1 in 10,000 is shown in Fig. 3. A beat of an average 

 interval of 14.5 seconds was accelerated to a beat of 9.4 seconds. 

 Later this segment stopped in systole, and the neighboring segments 

 increased their rate of beat. Adrenahn therefore causes a marked 

 acceleration and, if in sufhcient strength, it further causes complete 

 contraction which aboKshes rhythm altogether. The vascular muscle 

 is sensitive to the action of adrenahn while the voluntary muscle is 

 not; and the action on the vascular muscle is strictly comparable to 

 that on the vertebrate heart. 



In order to carry still further the close physiological relationship 

 which had become apparent between the muscles of the vessels of the 

 leech and that of the vertebrate heart, the actions of atropine and 

 muscarine were also tried. Atropine was found to cause an accele- 



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Fig. 3. The action of adrenalin borate on a lateral vessel of the curarized leech. 

 At the signal mark one drop of a solution of adrenalin borate of a strength of 1 

 in 10,000 was injected into the tissues around the vessel. A beat with an average 

 interval of 14.5 seconds was accelerated to a beat with an average interval of 

 9.4 seconds. Time marking in seconds. 



ration which gave the fastest rhythm ever observed, the average 

 interval between beats dropping to 4.2 seconds, a rate of over fourteen 

 beats per minute. The efficiency of the beat also became maximal. 

 Muscarine in strong solution caused complete cessation of beat in 

 diastole; in dilute solution it caused weakening and slowing of the 

 beat. The complete cessation of the beat caused by muscarine can 

 be removed by a subsequent injection of atropine, if the atropine is 

 injected soon after the beat has ceased. The vessel gradually re- 

 sumes a rhythmical contraction. If, however, the muscarine has 

 been allowed to act for some time, atropine has no longer the power 

 to recover the beat. These actions of atropine and muscarine are 

 strictly comparable to their actions on the vertebrate heart. 



The leech Hirudo medicinalis thus possesses a very definite type of 

 vascular muscle clothing certain of its vessels which is rhythmically 



