THE BLOOD AND ITS CIRCULATION. 



65. 



exercise hastens the circulation. The movement of the chest in breath- 

 ing probably aids the pulmonary circulation, the blood, as well as 

 the atmosphere, tending to till the vacuum during inspiration. 



Physical capillary force is not generally regarded as an active 

 force in the circulation. But there is an admitted force in the cajDil- 

 laries, resulting from the attraction of the tissues for the arterial 

 blood, containing the required oxygen and nutriment. " The vital 

 condition of the tissue becomes a factor in the maintenance of the 

 circulation." It is this force, primarily, which adapts the amount of 

 blood to the varying needs of any organ ; the nervous system regu- 

 lates the supply by varying the caliber of the vessels. 



The force in the capillaries, or some other force, carries the blood, 

 after death, from the arteries, where the heart leaves it, into the veins. 



Fig. 8 Cikculation ix the Web of a Frog's Foot. The black spots, some of them star- 

 ehaped, are piement, or coloring- matter, a. a venous trunk, composed of three principal 

 branches (6, 5, h\ and covered with a plexus of smaller vessels (c, c). 



Finding the arteries empty after death gave rise to the idea that they 

 conveyed only air ; whence the name. It was this belief which Harvey 

 overthrew in 1620. 



In bats the heart is aided by the rhythmic contraction of the veins 

 in the wing. Other accessory hearts of the lower animals have already 

 been mentioned. In some of the lowest creatures, the cause of the 

 circulation may be wholly the movements of the body, as in the jelly- 

 fish and anemone ; or by cilia, as in the sponge, where the sea-water 

 answers to blood. 



Under the influence of nerve-force, the walls of the arteries and 

 capillaries are usually somewhat contracted. The withdrawal of nerve 

 stimulus allows the tubes to relax, which consequently permits more 

 blood to pass through, or to accumulate, and perhaps add color to 



