BY DR. BURDON-SANDERSON. 217 



the same blood by exhaustion. The remainder of the mixture 

 contains, in addition to the excess of carbonic oxide, nitrogen 

 and carbonic acid gas, derived from the blood, but the propor- 

 tions of these gases discharged are very variable. As regards 

 ox} 7 gen, the method has yielded, in the hands of Bernard, re- 

 sults of the greatest value. It has the immense advantage 

 that it can be carried out without a mercurial pump, and for 

 pathological purposes is sufficiently accurate. 



CHAPTER XVI. 

 THE CIRCULATION OF THE BLOOD. 



IN commencing the study of the circulation of the blood, it 

 is desirable to direct our attention first to that part of the 

 circulatory apparatus in which the phenomenon presents itself 

 in its simplest form. In systematic physiological treatises the 

 heart is usually described first; but for our present purpose, 

 considering that the heart is an organ of very complicated 

 structure, that it is constantly influenced by ever-varying 

 conditions of the vessels on the one hand, and of the nervous 

 centres on the other, it is much better to begin with the 

 arterial system. 



PART I. THE ARTERIES. 



At the commencement of the period of relaxation of the 

 heart z. e., of the period which intervenes between one con- 

 traction and its successor the progressive movement of the 

 blood in the aorta all but ceases. At that moment, and during 

 the remainder of the f ime which precedes the bursting open 

 of the aortic valve, the pressure exercised by the wall of the 

 vessel on its contents is the only cause of the continuance of 

 the blood-stream. During each ventricular systole the aortic 

 pressure is reinforced by the motion communicated to the 

 blood by the contracting ventricle. Consequently, if, for the 

 sake of facilitating our understanding of the matter, we 

 assume the heart to be a mere pump, acting regularly, and 

 discharging at each stroke an invariable quantity of liquid, we 

 have the force by which the circulation is carried on at any 

 moment expressed by the tension of the arteries, and varying 

 with that tension; or if, on the other hand, we assume the 

 tension of the arterial system to remain constant, then the 

 quantity of work done varies with the mean velocit3 r of the 



