172 HANDBOOK OF PHYSIOLOGY. 



through the pulmonary artery, then through the pulmonary capillaries, 

 and through the pulmonary veins to the left side of the heart (Fig. 145). 

 Thus there are two circulations through which the blood must pass; the 

 one, a shorter circuit from the right side of the heart to the lungs and 

 back again to the left side of the heart ; the other and larger circuit, 

 from the left side of the heart to all parts of the body and back again to 

 the right side; strictly speaking, however, there is but one complete 

 circulation, which may be diagrammatically represented by a double 

 loop, as in fig. 145, in which there is one continuous stream, the whole 

 of which must, at one part of its course, pass through the lungs. Sub- 

 ordinate to the circulations through the lungs and through the system 

 generally, respectively named the Pulmonary and Systemic, it will be 

 noticed also in the same figure that 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 completes a revolution. This subordinate stream 

 through the liver is called the Portal circulation. A somewhat similar 

 accessory circulation is that through the kidneys, called the Renal cir- 

 culation. Such then is the outline of the course of the circulation. 

 The problems in connection with its maintenance cannot be well under- 

 stood without a more detailed knowledge of the structure and mode of 

 action of the heart, and of the structure and properties of the blood- 

 vessels. These subjects will now be considered seriatim. 



The Heart. 



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

 right and left lungs (fig. 146), inclosed 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 

 attached 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 com- 

 pletely invests. The part which lines the fibrous membrane is called 

 the parietal layer, and that inclosing the heart, the visceral layer or epi- 

 cardium, 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 

 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 in- 

 vestments from this sac to a greater or less degree. 



