CIRCULATION OF THE BLOOD. 103 



the liver, before it finally reaches the heart and completes a revolution. 

 This subordinate stream through the liver is called the Portal circulation. 

 The Forces concerned in the Circulation of the Blood. (1) 

 The principal force provided for constantly moving the blood through 

 the course of the circulation is that of the muscular substance of the 

 heart; other assistant forces are (2) those of the elastic walls of the arte- 

 ries, (3) the pressure of the muscles among which some of the veins run, 

 (4) the movements of the walls of the chest in respiration, and probably, 

 to some extent, (5) the interchange of relations between the blood and 

 the tissues which occurs in the capillary system during the nutritive 

 processes. 



THE HEART. 



The Pericardium. The heart is invested by a membranous sac 

 the pericardium, which is made up of two distinct parts, an external 

 fibrous membrane, composed of closely interlacing fibres, which has its 

 base attached to the diaphragm both to the central tendon and to the 

 adjoining muscular fibres, while the smaller and upper end is lost on the 

 large blood-vessels by mingling its fibres with that of their external coats; 

 and an internal serous layer, which not only lines the fibrous sac, but also 

 is reflected on to the heart, which it completely invests. The part which 

 lines the fibrous membrane is called the parietal layer, and that enclosing 

 the heart, the visceral layer, and these being continuous for a short distance 

 along the great vessels of the base of the heart, form a closed sac, the 

 cavity of which in health contains just enough fluid to lubricate the two 

 surfaces, and thus enable them to glide smoothly over each other during 

 the movements of the heart. Most of the vessels passing in and out of 

 the heart receive more or less investment from this sac. 



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

 tilag3s, being placed obliquely from right to left, quite two-thirds to the 

 left of the mid-sternal line. It is of pyramidal shape, with the apex 

 pointing downward, outward, and toward the left, and the base backward, 

 inward, and toward the right. It rests upon the diaphragm, and its 

 pointed apex, formed exclusively of the left side of the heart, is in con- 

 tact with the chest wall, and during life beats against it at a point called 

 the apex beat, situated in the fifth intercostal space, about two inches 

 below the left nipple, and an inch and a half to the sternal side. The 

 heart is suspended in the chest by the large vessels which proceed from 

 its base, but, excepting the base, the organ itself lies free in the sac of 

 the pericardium. The part which rests upon the diaphragm is flattened, 

 and is known as the posterior surface, whilst the free upper part is called 

 the anterior surface. The margin toward the left is thick and obtuse, 

 whilst the lower margin toward the right is thin and acute. 



