THE CIRCULATION OF THE BLOOD AND LYMPH 173 



carry away absorbed nutriment. Again, it is the working 

 muscles of the legs or of the arms that need the chief blood- 

 supply. But wherever the call may be, the vaso-motor mechan- 

 ism is able, in health, to answer it by bringing about a widening 

 of the small arteries of the part which needs more blood, and a 

 compensatory narrowing of the vessels of other parts whose 

 needs are not so great. 



The amount of blood flowing through an organ in a given time is 

 not, of course, proportional to the quantity contained in it at any 

 moment. For it depends also upon the velocity of the blood-stream, 

 and the blood flows at a different rate in different organs and in the 

 same organ at different times. The flow for 100 grammes of organ- 

 substance per minute under certain conditions has been determined 

 by observations with the stromuhr in dogs as follows : Posterior 

 extremity, 5 c.c. ; skeletal muscles, 12 c.c. ; head, 20 c.c. ; intestine, 

 31 c.c. ; spleen, 58 c.c. ; brain, 136 c.c. ; kidney, 150 c.c. ; thyroid 

 gland, 560 c.c. (Opitz, etc.). 



It is also through the vaso-motor system, and especially by 

 the action of that portion of it which governs the abdominal 

 vessels, and of the nerves that regulate the work of the heart, 

 that in animals to which the upright position is normal (monkey) 

 and in man the influence of changes of posture on the circu- 

 lation is almost completely compensated.* The pressure in the 

 upper part of the human brachial artery has been measured with 

 a sphygmomanometer, first in the horizontal and then im- 

 mediately afterwards in the standing posture, and in health it 

 has been found to remain practically unchanged (Hill). But if 

 the person was overworked or out of sorts, the compensation 

 was less complete. It is well known that in debilitated persons, 

 especially if long confined to bed, the sudden assumption of the 

 upright position may cause vertigo, and even syncope, the normal 

 compensatory mechanism being deranged. In such animals as 

 the rabbit this compensation is totally inefficient. When a 

 domesticated rabbit, which has been kept in a hutch, is sus- 



* Two factors may be distinguished in the blood-pressure, the hydro- 

 static and the hydrodynamic elements. The hydrostatic portion of the 

 pressure is due to the weight of the column of blood acting on the vessel ; 

 the hydrodynamic portion of the pressure is due to the work of the heart. 

 If a dog be securely fastened to a holder arranged in such a way that the 

 animal can be placed vertically, with the head up or down, and the mean 

 blood-pressure in the crural artery be measured in the two positions, there 

 will be a considerable difference. For when the legs are uppermost the 

 heart has to overcome the weight of the column of blood rising above it 

 to the crural artery ; when the head is uppermost the action of the heart 

 is reinforced by the weight of the blood. And if no change were produced 

 in the action of the heart, or in the general resistance of the vascular path, 

 by the change of position, this difference would be equal to the pressure 

 of a column of blood twice as high as the straight-line distance between 

 the cannula and the point of the arterial system at which the pressure is 

 the same with head up as with head down (indifferent point). 



