CHAPTER XXII 



THE EFFECT OF CHANGE OF POSTURE ON THE 

 CIRCULATION 



THE circulation is so contrived that it remains constant and 

 efficient, not only in the horizontal position, but also when the living 

 animal is ceaselessly shifting the position of his body. The hydro- 

 static influence of gravity must have had a most important bearing 

 on the evolution of the mechanisms which control the circulation. 



This is well demonstrated by the following 

 model ; 



Suppose a closed and rigid tube filled with water 

 and fixed to a board. When the board is placed 

 in the horizontal position, the pressure in all parts 

 of the tube will be the same. If the board be 

 turned into the vertical position, then the pressure 

 at the top end will become higher than the pressure 

 at the bottom end by the height of the column of 

 fluid. The fluid will still equally pervade the tube 

 in all its parts. This must be so, because the fluid 

 is incompressible, and the tube is rigid and unyield- 

 ing in structure. 



If the rigid tube be now replaced by an elastic 

 tube, and this at the points A and B be made to 

 expand into thin-walled elastic bags, then the con- 

 ditions become markedly different (Fig. 99). On 

 placing such a model in the vertical position, the 

 bag (B) expands under atmospheric pressure plus 

 the pressure of the column of fluid (A, B); and 

 while the water flows into B, A empties and shrinks 

 under the atmospheric pressure. 



If a pump, P, which can work with uniformity 

 and maintain a constant circulation, be placed in 

 CIRCULATION. such a model ; if the outflow tubes or arteries be made 

 of small capacity, and labile that is, possessing 

 considerable extensibility, and elasticity and the inflow tubes or 

 veins be valved, and be made of considerable capacity and slight 

 extensibility and elasticity; and if a sponge be inserted -ae a resistance 

 in the bags (A and B), then many of the conditions of the systemic 

 circulation are closely represented in the model. A is now equivalent 

 to the capillary area of the head, B to the splanchnic area of capillaries. 

 When the model is placed in the vertical position with the pump 



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