2 78 RESPIRATION 



sures in the capillaries, the whole physiology of the circulation 

 appears in a new light. It is not the heart nor the bulbar nervous 

 centers which govern the circulation rate, but the tissues as a 

 whole; and they govern it with an accuracy and delicacy com- 

 parable to the accuracy and delicacy with which they govern 

 breathing. The heart and vaso-motor system are only the executive 

 agents which carry out the bidding of the tissues, just as the lungs 

 and nervous system do in the case of breathing. 



It appears that the immediate function of the heart is not to 

 regulate the circulation rate, but simply to pass on at a greatly 

 increased pressure the blood supplied to it. The problem of the 

 regulation of the circulation under normal conditions seems in the 

 main to resolve itself into that of the regulation by the tissues of 

 the amount of blood supplied to the heart; and this regulation 

 depends, as we have just seen, to an overwhelming extent on a 

 linked control by the oxygen pressure and hydrogen ion concen- 

 tration in the systemic capillaries. 



Just as in the case of regulation of breathing, so also in the 

 case of regulation of the circulation, the dominant facts have been, 

 and still are, obscured by masses of detail which, in their un- 

 connected form, simply confuse the mind and lead to wholly 

 mistaken judgments. It is difficult to pick a way through all these 

 details, but the salient points concerning the immediate control of 

 the heart's action must now be referred to. 



We owe mainly to Gaskell the demonstration that the muscular 

 fibers of the heart may continue to contract rhythmically apart 

 from nervous control and even when they are separated from one 

 another, just as the rhythmic activity of the respiratory center 

 continues apart from peripheral nervous control. When, however, 

 different parts of the heart are separated from one another, the 

 frequency of the contractions in the different parts is different, 

 the ventricular contracting less frequently than the auricular 

 parts. In lower vertebrates the order of frequency in contractions 

 is sinus venosus, auricle, ventricle, and bulbus arteriosus. More- 

 over the individual fibers in each separated part contract normally 

 in unison with one another so long as they are not separated. In a 

 normal intact heart, however, not only do the individual fibers in 

 sinus venosus, auricles, ventricles, and bulbus arteriosus contract 

 in unison, but so also do all the parts of the heart. 



The explanation of this contraction in unison has been furnished 

 by the physiological and clinical investigations of the last few 

 years. As was shown by Lewis with the help of the string gal- 



