THE CIRCULATION. 229 



The pressure in the auricles is lowest at the period of diastole, 

 and since, then, the pressure in the veins is greater than the pres- 

 sure in the auricles, there is a flow of blood into the auricles, which 

 gradually becomes less. When the ventricles dilate, another fall in 

 the auricular pressure takes place and another rush of blood follows. 

 The opening of the great veins is contracted, and this act, preceding 

 the contraction of the auricle, drives the blood from the veins into 

 the auricles. When the auricles contract, the blood cannot flow back 

 into the veins to any great extent. 



The Ventricles. The ventricles represent the parts that are 

 really active in the cardiac circulation. The strength of the con- 

 tractions proper to the two ventricles reveals itself in the thickness 

 of the muscular walls, the fibers of which are inserted into fibrous 

 rings. These latter are the veritable skeleton of the heart. Mano- 

 metric observation presents us with proof of the force of the 

 ventricular contractions. 



GENERAL COURSE OF THE CIRCULATION. 



Since the main points of the anatomy of the heart have been 

 touched upon, it might be well at this stage roughly to consider the 

 circuit of the blood through it and its vessels. The vascular system 

 is a closed apparatus consisting of a central pump with its vessels 

 leading to every part and organ of the economy. All vessels lead- 

 ing away from the heart are arteries; those leading toward it are 

 veins. 



The entire circuit of the blood is divided into two principal 

 portions, which are distinctly separated from one another both 

 anatomically and functionally. The one conveys the blood to and 

 from the lungs during the process of aeration; so that to it has 

 been affixed the term pulmojiary circulation. The other has for its 

 function the distribution of the blood to all parts and organs of 

 the economy in general, thereby receiving the name systemic cir- 

 culation. 



Beginning with the left ventricle, the blood is conveyed to the 

 aorta, from which branches are distributed to every part of the 

 body, through the capillaries to the veins, to be eventually returned 

 as dark, impure blood to the right auricle. This, the greater cir- 

 cuit, has been termed the systemic circulation. During the course 

 of this circulation it has been found that the blood from the capil- 

 laries of some of the abdominal viscera is gathered together into a 

 single vessel, the portal vein, which again subdivides to form a 



