THE CIRCULATION OF THE BLOOD. 



115 



movement of the blood (Figs. 109 and 110). Their situation in the su- 

 perficial veins of the fore-arm is readily discovered by pressing along its 

 surface, in a direction opposite to the venous current, i.e., from the el- 

 bow towards the wrist ; when little swell- 

 ings (Fig. 109 c) appear in the position of 

 each pair of valves. These swellings at 

 once disappear when the pressure is re- 

 laxed. 



Valves are not equally numerous in all 

 veins, and in many they are absent alto- 

 gether. They are most numerous in the 

 veins of the extremities, and more so in 

 those of the leg than the arm. They are 

 commonly absent in veins of less than a 

 line in diameter, and, as a general rule, 

 there are few or none in those which are 

 not subject to muscular pressure. Among 

 those veins which have no valves may be 

 mentioned the superior and inferior vena 

 cava, the trunk and branches of the 

 portal vein, the hepatic and renal veins 

 and the pulmonary veins; those in the 

 interior of the cranium and vertebral col- 

 umn, those of the bones, and the trunk 

 and branches of the umbilical vein are 

 also destitute of valves. 



Lymphatics of Arteries and Veins. 

 Lymphatic spaces are present in the 

 coats of both arteries and veins; but in the tunica adventitia or 

 external coat of large vessels they form a distinct plexus of more or less 

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

 endothelium. Sometimes, as in the arteries of the omentum, mesentery, 

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

 arteries, the spaces are continuous with vessels which distinctly ensheath 

 them perivascular lymphatic sheaths (Fig. 111). 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 muscular 

 walls of its two auricles and two ventricles. 



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

 heart may be commenced at that period in each action which immediately 



FIG. 111. Surface view of an artery 

 from the mesentery of a frog, en- 

 sheathed in a perivascular lymphatic 

 vessel, a. The artery, with its circular 

 muscular coat (media) indicated by 

 broad transverse markings, with an 

 indication of the adventitia outside. 

 I. Lymphatic vessel ; its wall is a 

 simple endothelial membrane. (Klein 

 and Noble Smith.) 



