9° 



THE MECHANISM OF THE CIRCULATION. 



as long as the period arrived at by the Hering method. It is therefore 

 fallacious to use the circulation times arrived at by Hering's or 

 Stewart's method as a basis for calculating the quantity of blood in 

 the body. 



The Effect of Change of Posture on the Circulation. 



The circulation is so contrived that it remains efficient, not only 

 in the horizontal position, but also when the living animal is cease- 

 lessly shifting the position of his body. The hydrostatic influence of 

 gravity must have had a most important bearing on the evolution of 

 the mechanisms which control the circulation. Especially is this so in 

 the case of an erect animal like man, 5 to 6 ft. in stature. In a 

 man 6 ft. high the hydrostatic pressure of a column of blood reaching 

 from the vertex to the sole of the foot is equal to 140 mm. Hg, and from 

 the vertex to the middle of the abdomen about 50 mm. Hg. It is 

 obvious, then, that the influence of gravity demands careful inquiry. 



Suppose a closed and rigid tube (AB) filled with water and fixed to a board 

 (Fig. 60). When the board is placed in the horizontal position (I) the pressure 



in all parts of the tube (AB) will be 

 the same. If the board be turned 

 into the vertical position, then the 

 pressure at A will become nil, 

 while the pressure at B will be 

 equal to the height of the column 

 of fluid (AB). 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 unyielding in 

 structure. 



If the rigid tube be now re- 

 placed by an elastic tube, and this 

 at the points A and B be made to 

 expand into thin-walled elastic 

 bags, then the conditions become markedly different (II). On placing such a 

 model in the vertical position, the lower end (B) bags under the pressure of the 

 column of fluid (AB), and while the water flows into B, A empties and col- 

 lapses under the atmospheric pressure. 



Suppose a pump be placed in such a model, and that the pump work with 

 perfect uniformity and maintain a constant circulation, if the outflow tubes or 

 arteries be made of small capacity and 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 as a resist- 

 ance in the ends ( 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 (HI). 



Let the distensibility of the splanchnic capillaries and veins (B) be great, 

 and let the pump be at rest and the model be placed in the vertical position, 

 the fluid will then collect in B, and A will empty. The increase of the pressure 

 in the splanchnic area by the height of the column (AB) tends to enlarge the 

 capacity of that area while it diminishes the capacity in A. Let the pump 

 be at work, then the circulation through A will cease if the pump is unable 

 to raise the fluid from B, or to lift the fluid to A, and drive it through the 

 resistance in A. If the model be turned with B at the top, these results will 



Fig. 60. 



! 



