THE VASCULAR MECHANISM. IQI 



carotid artery. When the salt is injected, it increases the conductivity 

 of the blood, and as soon as the solution reaches the left carotid artery 

 the electric current passes more easily through the artery, the compen- 

 sation is upset, and the needle of the galvanometer is deflected. 



The time thus measured represents, however, merely the time in 

 which a blood corpuscle can complete the shortest possible circuit in the 

 circulatory system ; and the average circulation time of the blood as a 

 whole is probably twice or thrice as long as that found by these methods. 



SECTION II. 

 THE PHENOMENA OF THE NORMAL HEART BEAT. 



(1) Changes in Form. Observation of the heart, exposed in an 

 anaesthetised animal, shows that the beat begins with contraction of the 

 great veins near the heart, and is followed immediately by the contraction 

 of both_,auriclQs (atria), including their appendages. After a brief 

 interval, known as the auriculo-ventricular interval, the ventricles 

 contract synchronously, assuming the form of a short truncated cone. 

 During their contraction the ventricles become shorter from above 

 downwards, and, as the position of the apex remains almost unaltered, 

 the auricles are pulled down towards the apex, and the aorta and 

 pulmonary artery are stretched longitudinally. At the same time, the 

 cross section of the base of the ventricles alters, becoming more nearly 

 circular and smaller. When the contraction of the ventricles ceases, 

 the whole heart remains for a short time at rest (diastole). 



(2) The Sequence of Events within the Heart. During diastole the 

 blood is flowing steadily into the right auricle from the great veins, and 

 also from the auricle into the right ventricle, into which the cusps of 

 the tricuspid valve are hanging. When the auricle contracts, it empties 

 most of its contents into the ventricle, any reflux of blood along the 

 great veins being prevented by the simultaneous contraction of the 

 muscular rings round their termination. The ventricle, which is now 

 full of blood, almost immediately contracts, and the cusps of the tricuspid 

 valve, which had already been carried towards each other by eddies set 

 up behind them as the blood flowed from the auricle into the ventricle, 

 are driven firmly into apposition by the pressure of the blood, their 

 thin borders being tightly pressed together so that no blood can escape 

 into the auricle. The contraction of the papillary muscles keeps the 

 chordae tendineee taut, thereby preventing any inversion of the valves 

 under the ventricular pressure. 



The ventricle is now a closed cavitv, and remains so until the 



