THE RHYTHMIC AND PERISTALTIC MOVEMENTS 107 



nerve cells, with which it is in connexion. As long as the 

 muscle remains in that condition it could not be expected to ex- 

 hibit any rhythmic power, even if it possessed such power in a 

 marked degree. Carlson himself has shown that such is the 

 case, for, in the heart deprived of its nerve cord, a rhythm, some- 

 times very regular, can be induced by plunging the heart into a 

 solution of sodium chloride. The manner in which the salt solu- 

 tion brings about this myogenic rhythm is very suggestive, for 

 Carlson states that first the excitability improves so that each 

 stimulus causes a better contraction, then instead of a single 

 contraction a single stimulus will cause a number of contractions, 

 and finally with the increasing number a stage is reached where 

 a stimulus is unnecessary and spontaneous beats take place. 

 The whole phenomenon is reminiscent of the stages passed 

 through by the strip of the auricular muscle of the tortoise on 

 its way to spontaneous contraction. 



It seems to me that Carlson's observations of the rhythm 

 of the heart of Limulus are in strict accord with the rhythmic 

 phenomena exhibited by the vertebrate heart, if we assume that 

 in each case the nerve cell does not send out rhythmic dis- 

 charges to the muscle, but keeps the muscle in such a condition 

 of tension that it can contract rhythmically. The intrinsic nerve 

 cell behaves to the muscle of the heart in the same manner as 

 the extrinsic nerve cell of the sympathetic ganglion behaves to 

 the muscle of the artery, the difference in result being due to the 

 physiological difference between the two muscles, thus causing in 

 the one case rhythmic contractions, in the other tone. 



The observations which have been made on the lymph hearts 

 of the frog are very instructive in connexion with this question. 

 The ordinary rhythm is undoubtedly neurogenic in the sense 

 that it is dependent on the central nervous system. The nerves to 

 the lymph hearts are motor nerves, and their stimulation causes 

 tetanus of the striated muscle of the heart ; again, as in skeletal 

 muscle or in the Limulus heart muscle, the strength of contrac- 

 tion depends on the strength of stimulus : the " all or none " 

 principle does not apply. The excised heart remains motionless 

 in winter but in summer irregular spontaneous beats occur. A 

 weak tetanizing current sent through the excised heart causes a 

 more or less rhythmical activity during the stimulation. From 

 these observations it is clear that the musculature of the lymph 



