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CAUSES OF THE VENOUS CIRCULATION. 319 



time its destructive assimilation is undoubtedly augmented, 

 and there is an increased demand on the blood to supply the 

 waste. It is apparently a provision of Nature that the ac- 

 tivity of a muscle, facilitating the passage of blood in its 

 veins, and consequently its flow from the capillaries, induces 

 an increased supply of the nutrient fluid. As the develop- 

 ment of tissues is generally in proportion to their vascularity, 

 this may account for the increase in the development of 

 muscles, which is the invariable result of continued exercise. 



Force of Aspiration from the Thorax. During the act 

 of inspiration, the enlargement of the thorax, by depression 

 of the diaphragm and elevation of the ribs, affects the move- 

 ments of fluids in all the tubes in its vicinity. The air rushes 

 in by the trachea and expands the lungs, so that they follow 

 the movements of the thoracic walls. The flow of blood into 

 the great arteries is somewhat retarded, as is indicated by 

 the diminution in the arterial pressure ; and finally, the blood 

 in the great veins passes to the heart with greater facility, 

 and in increased quantity. This last-mentioned phenomenon 

 can be easily observed, when the veins are prominent, in pro- 

 found or violent inspiration. The veins at the lower part of 

 the neck are then seen to empty themselves of blood during 

 the inspiration, and become distended during expiration, 

 producing a sort of -pulsation which is synchronous with res- 

 piration, This can always be observed after exposure of the 

 jugular in the lower part of the neck in an inferior animal. 

 After this operation, if we cause the animal to make violent 

 respiratory efforts, the vein will be almost emptied and col- 

 lapsed with inspiration, and turgid with expiration. The 

 movements of the veins near the thorax have long been ob- 

 served and described with tolerable accuracy. By the fol- 

 lowing simple yet conclusive experiment, the regular action 

 of the suction force was demonstrated by Magendie. Having 

 introduced a gum-elastic sound into the jugular vein of a dog, 

 and passed it down to the right auricle, he saw " that the 



