RESPIRATION 



279 



vanometer, each normal contraction starts in what is known as 

 the Keith-Flack node, an island of primitive sinus venosus tissue 

 in the right auricle. Thence it is conducted by primitive muscular 

 tissue to the auricles, and by a bundle of similar muscular tissue, 

 the bundle of Kent or His, to the ventricles. This primitive tissue 

 is distributed (as the fibers of Purkinje) over the ventricles, and 

 has a conduction rate far faster than the rest of the muscular tis- 

 sue of the heart. Thus all parts of the ventricles contract almost 

 simultaneously, and shortly after the almost simultaneous con- 

 traction of all parts of the auricles ; while the pace of the whole 

 heart is set by the contractions starting in the Keith- Flack node. 

 Impairment or total failure in the conduction from auricle to 

 ventricle, or from fiber to fiber in auricle or ventricle, explains 

 many of the peculiarities met with in heart affections. 



So long as the contractions of the ventricles are complete, the 

 volume of blood discharged at each beat must depend on the ex- 

 tent to which the right ventricle fills in diastole. This, in turn, 

 depends on the rate at which blood is let through from the arteries 

 to the veins. The difference between arterial and venous pressure 

 is so great that accessory factors such as the pumping movements 

 of respiration can hardly have more than a very minute average 

 influence on the circulation, though they have a marked tempo- 

 rary influence. It is therefore the rate at which the systemic 

 blood is allowed to pass through the tissues into the venous system 

 that determines the amount of blood pumped by the heart; and, 

 as already pointed out, the rate at which blood is allowed to pass 

 through the tissues is determined by their metabolic requirements, 

 and particularly by the amount of blood required to keep the 

 diffusion pressures in them of oxygen and carbonic acid approxi- 

 mately steady. 



It is evident that in the carrying out of this regulation, both by 

 the heart and the blood vessels, the nervous system plays a very 

 important part, just as in the case of regulation of breathing; but 

 the main fact must never be lost sight of that the primary factor 

 in determining the rate of circulation is neither the heart nor the 

 nervous centers specially connected with the circulation, but the 

 metabolic activities of the tissues. At bottom the regulation of the 

 circulation is a chemical regulation, just as in the case of the 

 breathing. 



The frequency and strength of the heartbeats are moderated 

 through the central nervous system, first by the well-known in- 

 hibitory impulses passing to the heart through the vagus nerve, 



