466 CIRCULATION OF BLOOD AND LYMPH. 



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. 



Accessory Factors Aiding the Circulation. The force of the 

 heart beat is the main factor concerned in the movement of the 

 blood, but certain other muscular movements aid more or less in 

 maintaining the circulation as it actually exists in the living animal. 

 The most important of these accessory factors are the respiratory 

 movements and the contractions of the muscles of the limbs and 

 viscera. At each inspiratory movement the pressure relations are 

 altered in the thorax and abdomen, and reverse changes occur during 

 expiration. These effects influence the flow of blood to the heart, 

 and alter the velocity and pressure of the blood in a way that is 

 described in the section on Respiration under the title of The 

 Respiratory Waves of Blood-pressure. Contractions of the skeletal 

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



