22 COMPARATIVE PHYSIOLOGY 



in a later chapter. Excitation in nerve depends on surface 

 phenomena which are characteristically sensitive to changes in 

 the ionic composition of the external medium ; it is highly 

 probable therefore that the effect of any given ion may be 

 independently related to the initial and final stages in the 

 contraction cycle ; and we should therefore be wrong in 

 drawing from the different behaviour of striped and plain 

 muscle to electrolytes the conclusion that the contractile 

 mechanism is fundamentally different. This is clearly the 

 case with the calcium ion. Diastolic arrest of the heart of the 

 frog (Daly and Clark) in the absence of Ca ions is not due to 

 any failure of the excitatory mechanism : the electrical response 

 continues. On the other hand, diastolic arrest of the heart of 

 the lobster (Hogben) by excess of calcium ions is associated 

 with cessation of the electrical change, and is probably due to 

 the depressant action of calcium on the excitatory process. 



Further Reading 



Hill (191 3). The Absolute Mechanical Efficiency of Muscle. Journ. 

 Physiol. 46. 



(19 14). The Heat Production in Prolonged Contraction. Ibid. 47, 



(1922). The Mechanism of Muscular Contraction. Physiol. Reviews, 



2. 



Hartree and Hill (1923). The Anaerobic Processes involved in Muscular 

 Activity. Journ. Physiol. 58. 



Hogben (1925). Studies on the Comparative Physiology of Contractile 

 Tissues I. Quart. Journ. Exp. Physiol. 15 (for bibliography). 



Evans and Underhill (1923). Studies on the Physiology of Plain Muscle. 

 Journ. Physiol. 58. 



Mines (191 2). On the Relations to Electrolytes of the Hearts of different 

 Species of Animals. Journ. Physiol. 45. 



(19 14). On Functional Analysis by the Action of Electrolytes. Ibid. 



47. 



