142 THE CIRCULATION OF THE BLOOD 



body, and these in turn with the veins, which conduct the blood back to the 

 right side of the heart again. It will be seen, therefore, that the circulatory 

 apparatus consists of two great divisions, the pulmonary and the systemic cir- 

 culation. This arrangement is illustrated by the accompanying figure. 

 A study of this figure will show that in certain regions of the systematic circu- 

 lation there are two capillary beds between the main arteries and the main 

 veins. This subordinate stream through the liver is called the portal cir- 

 culation, and the similar arrangement existing in the kidney is called the 

 renal circulation. This, in general, is the outline of the course of the blood 

 in its circulation. 



To make a study of the various phenomena manifested in the physiology 

 of the circulatory apparatus, it is obvious that we have to do with certain 

 fundamental activities; first, the physiology of the pumping organ, the heart; 

 second, the behavior of the blood in the arteries, capillaries, and veins; third, 

 the coordination of these various divisions of the apparatus through the 

 nervous system. To understand this it will be necessary to have in mind in 

 detail the anatomical structure of the apparatus itself. 



ANATOMICAL CONSIDERATIONS. 



The Heart. The heart is contained in the chest or thorax, and 

 lies between the right and left lungs, figure 134, enclosed in a membranous 

 sac, the pericardium. The pericardium is made up of two distinct parts, 

 an external fibrous membrane, and an internal serous layer, which not only 

 lines the fibrous sac, but als6 is reflected on to the heart, which it completely 

 invests. These form a closed sac, the cavity of which contains just enough 

 fluid to lubricate the two surfaces, and thus to enable them to glide smoothly 

 over each other during the movements of the heart. The vessels passing in 

 and out of the heart receive investments from this sac to a greater or less degree. 



The heart is situated in the chest behind the sternum and costal carti- 

 lages, being placed obliquely from right to left. It is of pyramidal shape, 

 with the apex pointing downward, outward, and toward the left, and the 

 base backward, inward, and toward the right. The heart is suspended in 

 the chest by the large vessels which proceed from its base, but, excepting 

 at the base, the organ itself hangs free within the sac of the pericardium. 

 The part which rests upon the diaphragm is flattened, and is known as the 

 posterior surface, while the free upper part is called the anterior surface. 



On examination of the external surface, the division of the heart into 

 parts which correspond to the chambers inside of it may be traced, for a 

 deep transverse groove, called the auriculo-ventricular groove, divides the 

 auricles from the ventricles; and the interventricular groove runs between 

 the ventricles, both in front and in the back, separating the one from the 

 other. The anterior groove is nearer the left margin, and the posterior nearer 



