CIRCULATION. 69 



" the capability of being easily excited to movements of contraction 

 alternating with relaxation." This property is an endowment strictly 

 belonging to the heart, and is not derived from any connexion with 

 the nervous system, since it has been shown to continue after all 

 connexion has been severed. It is retained much longer in the cold- 

 blooded than in the warm-blooded animals ; and in the very young 

 animals than in the old. 



This movement of the heart cannot be accounted for by the sti- 

 mulus of the blood, since it will continue when the heart is empty ; 

 nor yet by the stimulus of air, since it persists even in vacuo; and 

 it has been shown above that they are independent of the nervous 

 system ; they must therefore depend on the vis insita, though the 

 exciting cause cannot be positively determined. 



If the heart of a living mammiferous animal or bird is laid bare, 

 the two ventricles are seen to contract together, and the two auricles, 

 with the commencement of the pulmonary veins and of the vence 

 cavse also simultaneously, the contraction of the auricles and that of 

 the ventricles not being synchronous. The contraction of all the 

 cavities is followed by their dilatation; the contraction is called the 

 systole; the dilatation, the diastole. 



The auricles are but little concerned in the propulsion of the blood, 

 they being mere sinuses or receptacles. The systole of the ventricle 

 corresponds with the projection of the blood into the arteries, causing 

 the pulse; whilst the diastole, coincides with the collapse of these 

 vessels. 



When the ventricles contract, the blood is prevented from return- 

 ing into the auricles by the tricuspid valve, on the right side; and by 

 the mitral valve on the left. When they dilate, the blood is prevented 

 from re-entering their cavities, by the semilunar valves at the mouth 

 of the aorta and pulmonary artery. The dilatation of the ventricles 

 may be distinguished into two stages : the first immediately succeeds 

 their systole, and manifests itself in the recession of the heart's apex 

 from the walls of the chest ; the second stage is attended with the 

 enlargement of the heart in all directions, and is synchronous with 

 the auricular contractions. It is between these two stages that the 

 interval of repose takes place. 



The diastole of the heart, according to Cruveilhier, is an active 

 force, but of its cause no precise account is given. It is supposed 

 to be owing to the presence of the yellow fibrous tissue, interwoven 

 with the muscular substance, upon whose elasticity the first stage of 

 the ventricular diastole is supposed to depend; the second stage being 

 caused by the ingress of blood produced by the auricular systole. 



The impulse ^ the heart must not be confounded with the arterial 

 pulse. The heart's impulse is the shock communicated by its apex 

 to the walls of the thorax in the neighbourhood of the fifth and sixth 

 ribs. 



