192 THE MOTION OF THE BLOOD. 



mainly by the left ventricle, as may be seen by tying ail the 

 vessels of an extremity but the artery, and wounding the vein, 

 when the jet from the vein may be regulated by pressing the 

 artery, is perfectly uniform. By the infinite subdivisions and 

 great increase of capacity of the arterial system, the blood, 

 which is moved in jerks in the larger arteries, giving a pulse, 

 and, if the vessel is wounded, flowing more forcibly at the 

 heart's pulsation, gives no pulse in the small vessels, and, if they 

 are wounded, flows regularly ; and in the capillaries, through the 

 augmentation of space, experiences no increased momentum at 

 the heart's pulsation. When the capillaries unite into veins, 

 and the capacity of the whole vascular channel diminishes, the 

 blood moves more quickly again through the diminished space P ; 

 but, though the smaller space augments its flow again, the im- 

 pulses of the heart lost in the capillaries cannot be felt in the 

 veins, and the current in them is smooth. Neither, generally 

 speaking, is it by any means so rapid as in the arteries, because 

 much of the heart's force is expended, and the veins are gene- 

 rally so much more numerous than the arteries, and the space, 

 therefore, however less than in the capillaries, still much greater 

 than in the arteries. Nor ought the momentum to be strong 

 when the veins have all united into the cavae, because it has only 

 to reach the heart, where there is no resistance, but, on the con- 

 trary, more than one source of vacuum prepared; whereas in 

 the aorta it ought to possess a force sufficient to carry it a great 

 distance, and surmount great obstacles. 



When the veins have pulsated, the action of the heart must 

 have been very violent, or some obstruction occurred, which, in 

 Dr. Hastings's experiments, was seen to cause the heart's action 

 to be sensible in the capillaries and veins. q 



There is always a pulsation in the large veins near the heart : 

 but that arises from obstruction, as I have already mentioned. 



" These are the chief powers which move the blood, and de- 

 pend upon the structure and vitality of the sanguiferous system. 

 We say nothing of the effect of gravity, attraction, and other 

 properties, common to all matter. The more remote assistance 

 derived after birth from particular functions, v. c. respiration and 

 muscular motion, will appear in our account of those functions." 



p Dr. Hastings, when observing the circulation in the frog's foot under the 

 microscope, saw that the blood moved " faster in the arteries than in the veins, 

 and in the veins than in the capillaries." 1. c. p. 47. 



i L c. p. 47. sqq. 



