THE ARTERIES. 719 



THE GENERAL PLAN OF THE CIRCULATION. 



The blood which enters the right auricle of the heart by way of the superior and 

 inferior venae cavae and the coronary sinus is blood which has come from the tissues, 

 to which it has delivered the oxygen and nutritive material and from which it has re- 

 ceived carbon dioxide and other waste products. From the right auricle this blood 

 passes through the right auriculo-ventricular orifice into the right ventricle, and on 

 the contraction of this, which follows immediately upon the contraction of the auricle, 

 it is forced into the pulmonary aorta (pulmonary artery), the tricuspid valve pre- 

 venting regurgitation into the auricle. Upon the completion of the contraction of the 

 ventricle, the blood which has been forced into the pulmonary aorta and is distend- 

 ing its walls forces together the pulmonary semilunar valves and, consequently, by the 

 contraction of the walls of the vessel and by subsequent contractions of the ventricle, 

 sending new blood into the vessel, is forced onward towards the lungs. In the sub- 

 stance of these organs the pulmonary vessels divide repeatedly, and finally form a 

 dense net-work of capillaries, through the walls of which an interchange of gases be- 

 tween the blood and the air contained in the cavities of the lungs takes place. From 

 the pulmonary capillaries the pulmonary veins arise and carry the purified blood back 

 to the heart, emptying it into the left auricle. 



In this course the blood has passed from the heart through a set of capillaries 

 back to the heart, and in one sense it has completed a circuit, which is termed the minor 

 or pulmonary ciradation. In reality, however, it is not a perfect circuit, since, while 

 beginning in the right side of the heart, it terminates in the left side. In order to 

 reach again the right side, it is necessary for it to pass through the viajor or systemic 

 circulation, the general course of which is as follows. 



From the left auricle the blood passes through the left auriculo-ventricular orifice 

 into the left ventricle, and by the contraction of this is forced into the systemic aorta, 

 or, as it is more frequently termed, the aorta, the bicuspid valve preventing its pas- 

 sage back into the auricle. The aorta curves backward and to the left and passes 

 down the body lying upon the left side of the vertebral column, and in its course 

 gives of? branches which distribute the blood to all parts of the body. In the vari- 

 ous organs these branches break up into a net-work of capillaries, from which veins 

 lead the blood into either the superior or the inferior vena cava or into the coronary 

 sinus, from which it passes to the right auricle. 



In the systemic, as in the pulmonary circulation, the blood passes from the heart, 

 through one set of capillaries, and back to the heart. In the case of the blood which 

 traverses the vessels passing to the ston)ach, the intestines (with the exception of 

 the lower portion of the rectum), the pancreas, and the spleen, however, a modifi- 

 cation of this arrangement occurs, in that before returning to the heart the blood 

 is required to pass through tivo sets of capillaries. The first set is in the substance of 

 the organs named, and after passing through this the blood is collected into a vein, 

 the vena porta, which conveys it to the liver. Here the portal vein breaks up into 

 the second set of capillaries, through which the blood passes to the hepatic veins, 

 which open into the vena cava inferior, and thus return the blood to the right 

 auricle. This portion of the major circuit forms what is termed \}i\Q portal circulation. 



THE ARTERIES. 



The arteries are those vessels which conduct the blood away from the heart. 

 Since the blood is forced into the arteries under considerable pressure by the con- 

 traction of the ventricles, it is necessary that the walls of these vessels should be suffi- 

 ciently strong to withstand pressure, and at the same time elastic so as to yield to 

 each successive injection of blood from the heart and to return to the normal calibre 

 when the wave has passed. As the blood courses from the main vessels to the capil- 

 laries, it passes through channels of progressively decreasing calibre, and is, there- 

 fore, constantly encountering increased resistance, whereby the arterial pressure is 

 diminished, until finally, when the capillaries are reached, the pressure is practically 

 nothing. As the pressure is reduced, the thickness of the arterial walls diminishes. 



