BLOOD-PRESSURE AND BLOOD-VELOCITY. 479 



a hydrostatic pressure in the opposite direction. In determinations 

 of blood-pressure in the brachial artery of man care should be taken 

 to keep the arm in the same position in a series of observations in 

 order to equalize the effect of the hydrostatic factor. The impor- 

 tance of this gravity effect is most evident in the case of the ab- 

 dominal (splanchnic) circulation. When an animal accustomed 

 to go on all fours is held in a vertical position the great vascular 

 area of the abdomen is placed under an increased pressure due to 

 gravity, and, unless there is a compensatory contraction of the 

 arterioles or of the abdominal wall, so much blood may accumulate 

 in this portion of the system that the arterial pressure in the aorta 

 will fall markedly or the circulation may stop entirely.* In most 

 cases the compensation takes place and no serious change in the 

 circulation results. In rabbits, however, which have lax abdominal 

 walls, it is said that the animal may be killed by simply holding it 

 in the erect position for some time. For the same reason an erect 

 posture in man may be dangerous when the compensatory nervous 

 reflexes controlling the arteries and the tone of the abdominal wall 

 are thrown out of action, as, for instance, in a faint or in a condition 

 of anesthesia. In such conditions the recumbent position favors 

 the maintenance of the normal circulation. Indeed, under ordinary 

 conditions some individuals are quite sensitive to the effects of a 

 vertical position, especially if unaccompanied by muscular or mental 

 activity, and may suffer from giddiness and a sense of faintness 

 in consequence of a fall in general blood-pressure. It seems prob- 

 able that in these cases the gravity effect has drafted off an undue 

 amount of blood into the splanchnic area. Individuals who have 

 been kept in bed for long periods by sickness, accident, or other 

 causes suffer from giddiness and unsteadiness when they first 

 attempt to stand or walk. It seems quite possible that in such 

 cases also the effect is due to a fall in arterial pressure brought about 

 by the dilatation in the splanchnic area. The added weight of 

 blood thrown on these vessels is not compensated by a vasocon- 

 striction of the arterioles or an increased tone in the abdominal 

 walls. 



Venous Pressures. While certain general deductions of the 

 kind given above may be made from our knowledge of the hydro- 

 dynamics and hydrostatics of the circulation, it is evident that in 

 particular cases, whether affecting special organs or the organism 

 as a whole, it is necessary to obtain directly, if possible, the facts, 

 not only for the arterial pressure and velocity but also for the 

 venous pressure and velocity, in order to draw safe conclusions 

 as to the changes in the circulation. The data and methods avail- 

 able in physiology and clinical medicine at present are more com- 

 * Hill and Barnard, " Journal of Physiology," 21, 321, 1897. 



