THE CIRCULATION OF THE BLOOD. 189 



tubular vessels. In smaller vessels they appear as sinous spaces lined 

 by endothelium. Sometimes, as in the arteries of the omentum, mesen- 

 tery, and membranes of the brain, in the pulmonary, hepatic, and splenic 

 arteries, the spaces are continuous with vessels which distinctly ensheath 

 themperivascular lymphatic sheaths (fig. 163). Lymph channels are 

 said to be present also in the tunica media. 



The Action of the Heart. 



The heart's action in propelling the blood consists in the successive 

 alternate contraction (systole) and relaxation (diastole) of the mus- 

 cular walls of its two auricles and two ventricles. 



Action of the Auricles. The description of the action of the 

 heart may be commenced at that period in each cycle which imme- 

 diately precedes the beat of the heart against the chest wall. The whole 

 heart is then in a passive state; the auricles are gradually filling with 

 blood flowing into them from the veins; and a portion of this blood is 

 passing at once through them into the ventricles, the opening between 

 the cavity of each auricle and that of its corresponding ventricle being, 

 during all the pause, free and patent. The auricles, however, receiving 

 more blood than at once passes through them to the ventricles, become, 

 near the end of the pause, fully distended ; and at the end of the pause, 

 they contract and expel their contents into the ventricles. 



The contraction of the auricles is sudden and very quick; it com- 

 mences at the entrance of the great veins into them, and is thence prop- 

 agated toward the auriculo-ventricular opening, forcing the contained 

 blood into the ventricle. The reflux of blood into the great veins dur- 

 ing the auricular systole is resisted not only by the mass of blood 

 within them, but also by the simultaneous contraction of the mus- 

 cular coats with which the large veins are provided near their en- 

 trance into the auricles. Any slight regurgitation from the right auri- 

 cle is limited by the valves at the junction of the subclavian and internal 

 jugular veins, beyond which the blood cannot move backward; and the 

 coronary vein is preserved from it by a valve at its mouth. 



The force of the blood propelled into the ventricle at each auricular 

 systole is transmitted in all directions, but being insufficient to open the 

 semilunar valves, it is expended in distending the ventricle. 



Action of the Ventricles. The dilatation of the ventricles which 

 proceeds during the chief part of the dilatation of the auricles is com- 

 pleted by the forcible injection of the contents of the latter. Thus 

 distended, the ventricles immediately contract : so immediately, indeed, 

 that their systole looks as if it were continuous with that of the auri- 

 cles. The ventricles contract much more slowly than the auricles, and 

 in their contraction probably always thoroughly empty themselves, 



