^5 OF THE CIRCULATION OF THE BLOOD. 



final parturient effort, but in local contractions that frequently occur during the 

 later months of gestation (simulating the movements of the foetus), are more 

 satisfactorily accounted for by considering them as a discharge of accumulated 

 power, than in any other mode (110). And we have seen that muscles which 

 are ordinarily excited to rhythmical movement through the medium of the nerv- 

 ous system, may execute these movements of themselves, when this source of 

 stimulation has been cut off, and their motility has accumulated through inaction 

 ( 318, note). It is not very difficult, then, to conceive, that the ordinary rhyth- 

 mical movements of the heart may be due to a simple excess of this motility, 

 which is continually being supplied by the nutritive operations, and is as con- 

 stantly discharging itself in contractile action. And that this is the true view 

 of the case, is further indicated by the phenomena attending the cessation of the 

 heart's action. For if a stimulus be applied to it soon after it has ceased to 

 execute spontaneous movements, this stimulus is followed, not (as in ordinary 

 muscles) by a single contraction followed by relaxation, but by a succession of 

 contractions and relaxations; thus indicating that a higher degree of motility 

 than that of an ordinary muscle still persists in its tissue. 1 Gradually, however, 

 the number of repetitions becomes smaller and smaller, until the application of 

 the stimulus excites but a single contraction; thus indicating that the motility 

 of the heart has been reduced, by the cessation of the nutritive operations, to 

 that of an ordinary muscle. 3 If we pass from this comparison of the Heart with 

 other muscles, to the general phenomena of rhythmical movement in the Animal 

 and Vegetable kingdoms, the proof furnished by analogy that the immediate 

 source of its action lies entirely within itself, becomes much stronger ; but this 

 part of the subject has been treated of elsewhere. 3 



500. This view of the case is not in the least inconsistent with the fact, that 

 the ordinary rhythmical actions of the heart may be considerably modified, both 

 as to their rate and their force, by stimuli of various kinds brought to bear upon 

 its tissue, either through the nervous system, or by direct contact. Of the 

 former we have an example in the influence of the emotions; and of the latter 

 in the violent action excited by an unusual rush of blood towards the heart, in 

 consequence of sudden muscular exertion. 



501. When the Heart is exposed in a living animal, and its movements are 

 attentively watched, they are seen to follow each other with great regularity. 

 In an active and vigorous state of the circulation, however, they are so linked 

 together, that it is not easy to distinguish them into periods; both Auricles con- 

 tracting and also dilating simultaneously, and both Ventricles doing the same. 

 The systole or contraction of the Ventricles corresponds with the projection of 

 blood into the arteries; whilst the diastole or dilatation of the Ventricles coin- 

 cides with the collapse of the arteries. The contraction of the Ventricles, and 

 that of the Auricles, alternate with one another ; each taking place (for the most 

 part, at least), during the dilatation of the other. But there is a period during 



1 This is a phenomenon which has no parallel among any of the manifestations of proper 

 reflex action. 



2 [M. Duval, teacher of Anatomy at Brest, has seen the heart pulsating in a decapitated 

 criminal twenty minutes after death, and continuing with perfectly regular action for an 

 hour, at the rate of forty-four pulsations in the minute. The movements were especially 

 observed in the left auricle. 



The divided ends of the carotid arteries were also projected in successive jerks, at the 

 same time ejecting small quantities of blood. The jugular veins bled continuously. 



It should be stated that galvanism was applied to the spine, before the chest was 

 opened, producing movements of expiration and inspiration, although this would scarcely 

 be considered as explanatory of this continuance of the heart's action, of which a much 

 more satisfactory reason is given by the author. It may have been the exciting cause of 

 renewed action after entire cessation. ED.] 



8 See \ 110; also "Princ. of Phys., Gen. and Comp.," CHAP, xix., Am. Ed. 



