THE CIRCULATION OF THE BLOOD 273 



appears to be a remnant of primitive muscle tissue at what was formerly the 

 junction of the sinus venosus and the auricle. In its structure it resembles 

 the auriculo-ventricular (Tawara's) node, in that it consists of peculiar 

 muscle-fibers, nerve-cells, and nerve-fibers enclosed by connective tissue. It 

 is also provided with an abundant blood-supply. In the human heart', the 

 muscle-fibers of this remnant are striated, possess well marked and elongated 

 nuclei and are plexiform in arrangement. From the node the muscle-fibers 

 extend downward along the sulcus terminalis for about two centimeters. 

 The thickness of the bundle is about two millimeters. Superiorly the node 

 appears to be connected with or continuous with fibers in the superior vena 

 cava; inferiorly it is connected with the true auricular fibers. The dissection 

 of this node shows that the terminal branches of the vagi and sympathetic 

 nerves are in histologic relation with the nerve-cells. The situation, struc- 

 ture and relations of this neuro-muscle node appear to justify the assump- 

 tion that it is directly concerned in the initiation of the heart-beat. 



THE MECHANICS OF THE HEART 



Methods of Observation. The movements of the heart, as well as 

 many phenomena connected with the flow of blood through its cavities, have 

 been determined by observation of, and experimentation on, the exposed heart 

 of a mammal e.g., dog, cat, rabbit supplemented and corrected by experi- 

 ments on the heart in its normal relations. Valuable information as to the 

 heart-beat and the influences which modify it has been obtained from experi- 

 ments made on the isolated heart of the turtle, frog, and allied animals. 



If the thorax of a dog, completely anesthetized, is opened and artificial 

 respiration established, the heart will be observed in active movement inside 

 the pericardium. If this sac is divided and turned aside, the heart will be 

 fully exposed to view. At the normal rate of movement of the heart 

 characteristic of the dog it will be almost impossible to determine either the 

 succession of events or their duration. But by observing the heart under 

 different conditions at different rates of movement and with instrumental 

 aids, physiologists have succeeded not only in analyzing the movements, but 

 in describing their sequence and in estimating their time duration. ^ 



Phenomena Observed. From many observations and experiments it 

 has been determined that the heart at each beat presents two distinct move- 

 ments which alternate with each other in quick succession. One is the 

 movement of contraction, or the systole, by which the blood contained within 

 its cavities is ejected into the arteries pulmonic artery and aorta; the 

 other is the movement of relaxation, or the diastole, followed by a pause 

 during which the cavities again fill up with blood from the venae cavae and 

 pulmonic veins. 



The contraction of any part of the heart is termed the systole; the relaxa- 

 tion, the diastole. As each side of the heart has two cavities the walls of 

 which contract and relax in succession, it is customary to speak of an auricu- 

 lar systole and diastole, and a ventricular systole and diastole. As the two 

 sides of the heart are in the same anatomic relation to each other, they 

 contract and relax in the same periods of time. 



It has also been ascertained that the contraction of the auricles and 

 ventricles as well as their subsequent relaxations, though occurring with 



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