324 W. J. CROZIER 



The role of the cloaca in pulsation, and especially of its anterior 

 end, could be demonsrated very clearly. Isolated cloacal ends 

 immersed in sea water containing a^^o caffeine presented this 

 condition: During the early stages of the history of a piece in 

 such a solution the anal sphincter pulsated at a normal rate of 

 10 movements in 70 to 80 seconds, but the cloaca was tightly 

 contracted excepting immediately in the region of the anus 

 (fig. 21). If now in such a preparation the contracted anterior 

 end of the cloaca was exercised, all rhythm very promptly ceased. 

 This points to the conclusion that the stimulus to pulsation 

 originates at the anterior end of the cloaca with its associated 

 radial muscles and is conducted posteriorly along the cloaca even 

 when the cloaca itself is not pulsating. The fact that in certain 

 salt solutions (e.g., NaCl m 5/8) the cloaca continued to beat 

 after the anus had ceased, serves to strengthen this idea. The 

 excised cloaca alone in sea water pulsated, though in a somewhat 

 irregular manner. In recovery from the immediate effects of 

 amputation, the cloaca and anal sphincter, and especially the 

 former, began to pulsate before the body-wall portion of the 

 complex. 



It is taken for granted in this discussion that the coordinating 

 mechanism is essentially nervovis in character, and this view is 

 supported by the experimental results. It has been suggested, 

 however, that there is a chemical basis of coordinated pulsation 

 in the case of the vertebrate intestine; Weiland ('12) claims to 

 have extracted from the mammalian digestive tube a substance 

 which, acting on Auerbach's plexus, leads to coordinated rhyth- 

 mic movement. It must be admitted that the analysis of peri- 

 stalsis upon the basis of the tonus idea (cf. Cannon, '11) does not 

 account for the inception of the stimulus to contraction, and that 

 therefore some further link is needed in the chain of pulsation 

 processes. 



d) Summary. The course of events in a pulsation cycle may 

 be pictured as beginning with the opening of the anterior end of 

 the cloaca, due to the relaxation of its circular muscles and the 

 contraction of the associated radiating fibers. The stimulus 



