566 



Comparative Animal Physiology 



tions scattered locally over its musculature, the condition of "delirium cordis." 

 Anodal polarization results in relaxation of the heart. 



The muscle of moUuscan and arthropod hearts is usually striated, some- 

 times transversely and sharply, sometimes spirally and vi'eakly.^'^' In general 

 the muscle Bbers of invertebrate hearts are shorter and less branched than 

 those of vertebrate hearts. Molluscan and arthropod hearts differ strikingly 

 from vertebrate hearts in their responses to electrical stirnulation."''^- "- The 

 muscle of these hearts is excitable at all phases of the cycle, but the threshold 

 is high in systole, approaching absolute refractoriness in early systole. 



Fig. 211. Electrical stimulation at different times in the cardiac cycle of the frog heart. 

 After Marey from Fulton."^ 



/ — f — ; — I — 8 — I — mmmi 



Fig. 212. Responses of Linmlus heart to stimulation of lateral ner\'es. Numbers indicate 

 number of stimuli; note increasing response and final tetanus. From Carlson."^' 



and recovery of excitability is gradual during the contraction (Fig. 212). A 

 strong stimulus applied during systole can elicit a contraction of greater ampli- 

 tude than the normal heart beat. Further, an extra contraction is not followed 

 by a corresponding compensatory pause. Most molluscan and arthropod 

 hearts can be tetanized. These hearts, therefore, do not contract in an all- 

 ot-none fashion; a normal contraction is never maximal. Certain intensities 

 of shock applied to molluscan hearts during systole can result in diminished 

 contraction;''-' the explanation of this phenomenon is not clear. Carlson ob- 

 served the above behavior in hearts of Mytilus, Cardiitm, Ariolimax, Pali 

 nurus, Limulus and other species. Similar responses to electrical stimulation 

 have been noted in the lobster,"^ cockroach,'-^' •'' Limulus,^''- and Anodonta and 



