USES AND ACTION OF THE HEART. 37 



descriptions a most important omission is made the omission of the action of the 

 heart, an organ plainly connected by position and mechanism with the phenomena we 

 have under consideration. 



121. Physiologists have Jong seen, in opposition to the popular opinion, that the 

 heart can oulv exert a very subsidiary action. Plants are wholly destitute of such an 

 organ, vet their juices circulate, and there are multitudes of animals which are in the 

 same predicament. In insects, for example, for reasons for which we can give on these 

 principles a clear explanation, no such central organ of impulse appears. In fishes, the 



-emit- circulation is carried on without a heart, and in cold-blooded animals move- 

 ment in the capillaries takes place after the heart has been cut out Even in man, after 

 death, the arterial tubes are found for the most part empty, and it is inconceivable that 

 this should have happened through any possible agency of the heart itself, bat meets 

 with a very ready explanation upon the principles we have been giving. In the case of 

 acardiac monsters, also, which are not uncommon, the absence of a heart seems to have 

 exerted little agencv on development and growth. Moreover, when we inquire into 

 the condition of the circulation in the earliest periods of existence, we find that, far 

 from the heart being the first to appear as a central point, and its various vessels to branch 

 forth from it, the vessels themselves are the first to appear, and the heart, then, is subse- 

 quently developed. 



122. What, then, is the true office of the heart ? How is it that it comes to form 

 so essential a portion of this mechanism ! Had the CREATOR predetermined that in 

 the construction of the circulatory apparatus of man no organ of the kind should be 

 employed, let us consider the final difficulties which must have arisen. The systemic 

 circulation originates in the deoxydation of the blood, the pulmonary in its oxydation. 

 We can conceive that advantage might have been taken of the excess of mechanical 

 force arising at the points where these chemical changes were going on, for that model 

 was successfully followed in the systemic circulation of fishes ; but in the case of ani- 

 mals which do not live in the sea, but breathe atmospheric air, there are serious diffi- 

 culties in the way. It is very apparent that there is no necessary connexion between 

 the ohemical changes taking place in the lungs and those taking place in the system. 

 The rate of oxydation in the lungs depends on a variety of causes, and the rate of de- 

 oxvdation is also determined by its own proper causes. There is no necessary con- 

 nexion between them ; and if, on either of these points of change, an excess of action 

 took place, the result of it must have been a disturbance of the equilibrium of the whole 

 circulation. At some central point, therefore, the current going to the respiratory ma- 

 chine and that going to the system must be intercepted and intercepted by an appa- 

 ratus which could hold both in check, and time the movements of the one to the move- 

 ments of the other. In animal structures, morions of this kind are universally accom- 

 plished by the help of muscular contractions, and hence arose the idea of a heart, which, 

 by periodic muscular contractions, should serve to adjust the flowing currents to one 

 another, and prevent engorgements or deficiencies in any part of the route taking place. 



123. From this arrangement, also, another important advantage arose. The arterial 

 and venous tubes in the neighbourhood of the heart attain a considerable diameter. 



