' 



282 THE CIRCULATION 



veins must be marked for example, because of the segmentation and 

 the peristalsis of the intestine, and in the spleen and uterus. 



THE SUCTION OF THE RELAXING AURICLES. The last of our five 

 causes of the circulation is the suction of the relaxing auricles. This is 

 certainly the least important of the forces in action, for the diastole of 

 the auricles is not a powerful movement, even the systole being rela- 

 tively weak. But the diastole recurs very frequently and in itself is 

 one of the constant causes of the circulation. Moreover, the auricular 

 expansion is greatly aided by the respiratory enlargement of the thorax 

 as described above. These agencies together are sufficient to produce 

 in the vena cava and auricles of the dog a negative pressure equal 

 to that of a few millimeters of mercury, as has often been shown by 

 manometers connected with cannulae in the vessels and auricles. Such a 

 suction must have material influence in returning the blood to the 

 heart from the flaccid veins (Fig. 65) . 



Agencies which tend to retard the circulation of the blood are easily 

 appreciated from the foregoing facts as to the forces which promote it. 

 Aside from function, these naturally are mostly pathological rather than 

 normal. Growths on the heart's valves are probably the most frequent 

 cause of disorder. The valves on this account being hindered from 

 closing tightly, allow of regurgitation of the blood in the direction opposite 

 to the proper circulation. In other cases the openings are narrowed by 

 disease causing by this stenosis, as it is called, an incompleteness in the 

 filling of the ventricles or in the distention of the great arteries. (See 

 the experiments in the Appendix.) Sometimes the heart-muscle be- 

 comes weakened by a deposit of fat, or from too long-continued overwork, 

 or from lack of normal metabolism due to worry perhaps. Often the 

 arteries become hardened (sclerosed), so that they lose much of their 

 essential elasticity. The capillaries may be far too permeable, allowing 

 too much lymph to soak outward into the tissues, so forming edema 

 (dropsy) when drainage back to the heart is poor. Sometimes by local 

 pressure (as from a tumor) on a large vein, a condition of venous stasis 

 is produced, and occasionally too much standing gives rise to a similar 

 condition of chronic venous engorgement in the legs (varicose veins), 

 man's organism not yet having become completely adapted, as it seems, 

 to his recently assumed erect posture. 



The Speed of the Blood-current has been studied mostly in the brutes, 

 and yet we probably have a fair notion of it as it is in man. It varies 

 widely in different places and at different times. The matter is more 

 complex than appears at first glance, as the principles of hydraulics 

 would suggest. All the considerations concerning speed are complicated 

 by the ever- varying caliber of all the blood and lymph vessels except pos- 

 sibly the capillaries. 



Perhaps a fair statement of the average arterial velocity in man is 

 150 mm. per second. It is much greater than this near the heart, and 

 very much less near the capillaries, for the friction in a tube increases 

 very greatly with decreasing diameter. In the aorta the speed may 



