CIRCULATION OF THE BLOOD. 



313 



municated to the membrane, sending a wave of air through the 

 tube and causing the lever to move at every impulse ; the point of 

 the lever makes its record on a revolving drum or kymograph, 

 covered with smoked paper (Fig. 1 66). This may be varnished 

 with shellac for preservation. Fig. 164 shows a cardiogram taken 

 in this way. 



The same instrument in a modified form is used to obtain a 

 record of the endocardiac pressure. In this case India-rubber 

 bags communicating with the recording tambours are introduced 

 through the jugular vein into the cavities of the right auricle 

 and ventricle. This method is of service only in an animal with 

 large vessels, such as the horse. 



Movements of Blood during Systole and Diastole. Before con- 

 sidering other movements of the heart it will be well to study the 

 course of the blood while contraction and relaxation of the mus- 

 cular tissue of this organ are taking place. 



The venous blood, returning from the head and upper extrem- 

 ities by the superior or descending vena cava, and from the portion 

 of the body below the heart by the inferior or ascending vena 



FIG. 163. Diagram of Marey's 

 cardiograph : A, knob attached to 

 flexible membrane tied over end 

 of metal box ; the knob is placed 

 over the apex-beat ; c, folded edge 

 of membrane; Bis the tube commu- 

 nicating with a recording tambour. 



FIG. 164. Cardiogram taken with 

 Marey's cardiograph : a, auricular 

 systole ; v, ventricular systole ; d, 

 diastole. The arrow shows the di- 

 rection in which the tracing is to 

 be read (Stewart). 



cava, flows into and through the right auricle, passing into the 

 right ventricle through the right auriculoventricular orifice. The 

 tricuspid valve at this time is open, and offers no obstacle to the 

 passage of the blood. Blood is also at the same time flowing into 

 the left auricle and ventricle from the lungs. More blood enters 

 the auricles than can at once pass out into the ventricles ; conse- 

 quently some blood accumulates in, and gradually fills, the auricles, 

 although at the same time as much blood is flowing into the ven- 

 tricles as the auriculoventricular orifices will permit to pass, nearly 

 filling these cavities, and floating up the segments of the mitral 

 and tricuspid valves until they are nearly closed. This is the 

 condition at the end of the pause. At this moment begins the 

 auricular systole. Near the ends of the veins which discharge 



