132 THE SUMMEEING OF HUNTERS. 



blood through the vessels, but this force is almost 

 spent by the time the blood gets to the fine capil- 

 laries, and, as we have said, can only meander 

 through them, so that we have to look out for 

 another force, because, when the blood or parts of it 

 have left the blood-vessels (capillaries) it has to 

 regain the blood-vessels (veins) again. As it leaves 

 the capillaries the force that sent it there is spent, or 

 nearly so, and it has yet to travel through and 

 saturate the tissues, and to travel to the veins. 

 These forces are for the most part two comparatively 

 feeble ones — namely suction and gravitation. The 

 absorbent vessels, or those channels which conve}^ 

 the used fluid from the tissues to the veins, are ex- 

 tremely delicate, and are supplied with valves at 

 very frequent intervals, and these valves only open 

 one way ; the fluid can run along the vessel through 

 and past the valves one way, but if it wants to re- 

 turn it shuts the valves down. Here, then, is a 

 decided aid to our feeble forces. There are other 

 slight aids also. Now how is suction brought about ? 

 Very simply in this way ; when a muscle is at work 

 the belly of the muscle, alternately shortens and 

 lengthens ; and, besides doing its special dut}^ — 

 moving the bones to which it is attached by its 

 tendons— in alternately shortening and lengthening, 

 it acts as a suction pump. Thus the fibres going to 

 make up the muscle run like a bundle of rods 

 parallel with one another, and the spaces or channels 

 between them are occupied by the absorbent vessels. 



