THE CIRCULATION OF THE BLOOD. 97 



named, it will be noticed also in the same figure that there is another, 

 by which a portion of the stream of blood having been diverted once 

 into the capillaries of the intestinal canal, and some other organs, and 

 gathered up again into a single stream, is a second time divided in its 

 passage through the liver, before it finally reaches the heart and com- 

 pletes a revolution. This subordinate stream through the liver is called 

 the Portal circulation. 



As a necessary step towards the consideration of the method by which 

 the circulation is maintained, it will be advisable in the first place to de- 

 vote some time to the description of various important points in the 

 anatomy and minute structure of I. The Heart; II. The Arteries; 

 III. The Capillaries; IV. The Veins. We shall then be in a better po- 

 sition to discuss the problems in the physiology of the circulation. 



(I.) The Heart. 



The heart is contained in the chest or thorax, and lies between the 

 right and left lungs (Fig. 88), enclosed in a membranous sac the peri- 

 cardium, which is made up of two distinct parts, an external fibrous 

 membrane, composed of closely interlacing fibres, which has its base at- 

 tached to the diaphragm or midriff, the great muscle which forms the 

 floor of the chest and divides it from the abdomen 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 invest- 

 ment from this sac. 



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

 tilages, 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 downwards, outwards, and towards the left, and the base back- 

 wards, inwards, and towards the right. It rests upon the diaphragm, 

 and its pointed apex, formed exclusively of the left side of the heart, is in 

 contact 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 

 7 



