1 62 THE CIRCULATION OF THE BLOOD 



general capillaries of the body, and these in turn with the veins, which con- 

 duct 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 circulation. This arrangement is illustrated 

 by the accompanying figure. A study of this figure will show that in certain 

 regions of the systemic circulation there are two capillary beds between the 

 main arteries and the main veins. This subordinate stream through the 

 liver is called the portal circulation, 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 co-ordination 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 143, 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 also is reflected on to the heart, which it completely 

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

 pericardial 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 

 diaphragmatic surface, while the free upper part is called the sternocostal 

 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. A deep trans- 

 verse groove, called the coronary sulcus, divides the auricles from the ventri- 

 cles; and the anterior longitudinal sulcus runs between the ventricles, both in 



