THE CIRCULATION OF BODY FLUIDS 113 



contraction of the branchial hearts followed by that of the 

 systemic auricles results. There is thus reason to believe 

 that there exists in cephalopods, as in vertebrates, a complex 

 reflex mechanism co-ordinating the constituent parts of the 

 circulatory system. 



The dependence of ventricular rhythm on the stretching 

 force of the auricular beat in Octopus is a phenomenon which 

 is characteristic of the ventricular muscle of other mulluscs, 

 e.g. Anodon, Pecten, Helix, Aplysia, etc. The empty heart 

 of the mollusc does not beat. This fact depends upon a wide- 

 spread characteristic of plain muscle, studied by Straub, 

 Buddington, and others in denervated preparations of the 

 integumentary muscle of the earthworm. Stretching suffices 

 to induce rhythmical contraction in isolated rings of the seg- 

 mental muscles of Lumbricus. In the mammalian bladder, 

 distension is the normal stimulus for contraction ; and Carey 

 (1921) has recently found that by introducing fluid into a dog's 

 bladder under considerable pressure, the walls of the latter 

 gradually become thicker, come to simulate microscopically 

 those of the heart and pulsate rhythmically at a rate of two 

 hundred per minute. Carlson (1906) has utilised the fact that 

 the empty heart does not beat in seeking for evidence of 

 cardiac augmentor nerves in his investigations on numerous 

 molluscan genera. Inhibitory cardiac nerves appear to exist 

 in all groups (lamellibranchs, cephalopods, and pulmonates) 

 with the possible exception of marine gasteropods {e.g. Aplysia). 

 There is a well-developed augmentor nerve supply in Aplysia. 



Straub (1904) has made a careful study of the eflFect of 

 pressure on ventricular rhythm in Aplysia. Between the 

 pressures 4-20 mm. of water the pulse volume is almost 

 directly proportional to the pressure applied, and as the auricle 

 supplies a pressure of about 30 mm. (water), the heart normally 

 works under approximately the optimum conditions for ven- 

 tricular contraction. In gasteropods, however, the peripheral 

 circulation is poorly developed ; a few larger vessels empty 

 into ill- defined lacunae which finally converge upon the gill- 

 sinuses. The total resistance of the peripheral circulation 

 is so small that the blood-pressure is not of the same order 



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