610 



HEART. 



Ass 50 



Crab 50 



Butterfly 60 



Goat 74 



Sheep 75 



Hedgehog 75 



Frog 77 



Marmot _ 90 



Locust 90 



Ape 9o 



Dormouse 105 



Cat .. 110 



Duck 110 



Rabbit 120 



Menoculus Caster 1 20 



Pigeon . . . .^ 1 30 



Guinea-pig 140 



Hen . 140 



JBremus terrestris 1 40 



Heron 200 



Menoculus pulex 200 



For the effects of the respiration upon the 

 contractions of the heart, and the influence of 

 the circulation of dark blood upon its irrita- 

 bility, see ASPHYXIA. 



The cause of the motion of the heart. The 

 motion of the heart, and the constancy and 

 regularity of its movements, are circumstances 

 so remarkable that they could not fail early to 

 excite a deep interest among medical philo- 

 sophers when they had once turned their 

 attention to the explanation of vital phenomena. 

 When we contemplate the heart commencing 

 its movements at an early period of fcetal 

 existence, and never resting from its apparently 

 unceasing toil until the latest moments of life, 

 and when we remember the uniform and regu- 

 lar manner in which all its actions are accom- 

 plished all conspiring for the proper per- 

 formance of the deeply important functions 

 assigned to it, we are at first impressed with the 

 idea that it is regulated by laws different from 

 similar textures of the body, and altogether 

 peculiar to itself. It must have been under 

 the influence of similar impressions that the 

 older medical philosophers approached this 

 subject, and it is in this manner only that we 

 can account for many of the strange specula- 

 tions on the heart's action which they have left 

 recorded. 



We find one sect attempting to explain it by 

 a peculiar innate fire. Sylvius, the head of the 

 chemical sect, had recourse for its explanation 

 to an effervescence excited by the intermixture 

 of the old and alkaline blood with the acid 

 chyle and acid pancreatic lymph.* Descartes 

 supposed that a constant succession of explosions 

 occurred in the heart from steam generated 

 there, which propelled the blood through the 

 body. Stahl got at once out of the difficulty 

 by affirming that the heart was more particu- 

 larly under the guidance of the anima or soul. 

 But we cannot here dwell longer on these ob- 



* In the same manner Borelli says, " Constat 

 ex dictis immedhitam causam motivam cordis esse 

 ebullitionem fermativam tartarei sncci sanguine! 

 excitatam a commistione succi spiriluosi a iiervis 

 instillati." De Motu Animalium, p. 97. 



solete and to us in the present time almost 

 incredible opinions, and the only use to which 

 they are now applicable is to serve as beacons 

 to keep us, in all our inquiries into the pheno- 

 mena of living bodies, within the strict path of 

 facts and observation, and to forcibly impress 

 upon us into what strange and fatal errors even 

 the brightest intellects may fall, when they 

 leave the inductive method of investigation, 

 and wander into the alluring but dangerous 

 regions of hypothesis. And the effects of these 

 errors are only the more to be dreaded as they 

 are often clothed in the most seductive in- 

 genuity. It ought also still more forcibly to 

 inculcate upon us the important truth, which, 

 though generally in our mouths, is not unfre- 

 quently forgotten in practice, that as the 

 material world and all which it contains have 

 been placed by the Author of Nature under 

 arbitrary and fixed laws, it is impossible to ex- 

 tend our knowledge of these by theorizing in 

 the closet, and that this can only be effected by 

 the patient interrogation of Nature herself. 



It was not until the time of Senac and 

 Haller that accurate notions began to be enter- 

 tained on the nature of the heart's action. 



The cause of the movements of the heart is 

 distinctly referable to the same laws which 

 regulate muscular contractility in other parts of 

 the body, only modified to adapt it for the per- 

 formance of its appropriate functions. Like 

 all the other muscles it is endowed with irrita- 

 bility, which enables it to contract upon the 

 application of a stimulus. The ordinary and 

 natural stimulus of the heart is the blood, 

 which is constantly flowing into its cavities. 

 The greater irritability of the inner surface over 

 the outer is evidently connected with the 

 manner in which the stimulus is habitually 

 applied. When the blood is forced on more 

 rapidly towards the heart, as in exercise, its con- 

 tractions become proportionally more frequent; 

 and when the current moves on more slowly, 

 as in a state of rest, its frequency becomes pro- 

 portionally diminished. If the contractions of 

 the heart were not dependent upon the blood, 

 and their number regulated by the quantity 

 flowing into its cavities, very seiious and in- 

 evitably fatal disturbances in the circulation 

 would soon take place. 



As the heart continues to contract often for a 

 very considerable time after the venae cavae 

 have been tied, and after the blood has ceased 

 to pass through its cavities, or after it has been 

 removed from the body, this has been supposed 

 by some to indicate that there is something in 

 the heart's structure or in its vital properties 

 which enables its movements to proceed inde- 

 pendent of all other circumstances. But in all 

 these cases a stimulus has been applied in some 

 form or other to the heart. If the heart has 

 been allowed to remain in its place, though the 

 circulation of the blood may have come to a 

 stand, part of it may yet remain in the different 

 cavities of the organ ; or if the pericardium has 

 been opened, the impression of the external 

 atmosphere may act as a stimulus. The expe- 

 riments of Walther and Haller formerly men- 

 tioned upon the comparative irritability of the 



