334 CIRCULATION IN THE BLOOD-VESSELS. 



rendered tense when filled, but in virtue of their elasticity, 

 their calibre is soon restored, and the muscular coat also 

 regulates the capacity of the vessels, in accordance with the 

 flow of the blood. There is no active rythraic propelling 

 force in the arterial system. The blood advances in an 

 uninterrupted column, as there is no void in the circulatory 

 apparatus, and the jerk produced by the powerful heart is 

 felt at every beat in the smallest arteries. This is the pulse. 



By the subdivision of the arteries, the capacity of the 

 arterial system increases, and this exerts a similar influence 

 on the current of blood to that observed by the widening of 

 a river on the current of water. It equalizes and retards it. 

 It has been estimated by Volkmann that the blood circulated 

 in the carotid arteries of horses, calves, and dogs, at the rate 

 of 12 inches in a second. 



The arteries usually run in protected situations, and the 

 larger ones cannot be felt in order to determine the state of 

 the pulse. Middle-sized superficial vessels running over 

 bones are therefore selected, as we shall presently see. 



Pulsation is lost in the capillary system, and here the flow 

 onwards towards the veins is due to the constant pressure 

 from behind vis a tergo which the column of blood exerts. 

 Doubtless, capillary attraction exerts some influence in these 

 hair-like tubes, to ensure an equable distribution of blood, 

 but the wisdom of creative design is admirably exemplified 

 by the vast mass of blood enclosed within the very delicate 

 capillaries being pressed forwards by liquid pressure, and 

 not subjected to the sudden jerks of a pumping heart. In 

 weak animals that have been starved for some time, pulsation 

 is perceptible in the smallest arteries and in the capillary 

 system. This may be seen in frogs, mice, and bats, whose 

 transparent structures favour microscopic examination. In 

 them we observe minute vessels, about 2-oVgth of an inch in 



