THE EFFECT OF CHANGE OF POSTURE 195 



.at work, owing to the great distensibility of the splanchnic capillaries 

 and veins (B), the fluid will collect in B, and A will empty. But if 

 B is compressed by the hand so as to raise the fluid in the vein up 

 to the pump, then the circulation will recommence. 



The blood, owing to its weight, continually presses downwards, 

 and under the influence of gravity would sink if the veins and capil- 

 laries of the lower parts were sufficiently extensile to contain it. Such 

 is actually the case in the snake or eel, for the heart empties so soon 

 as one of these animals is immobilized in the vertical posture. Thw 

 does not occur in an eel or snake immersed in w r ater, for the hydro- 

 static pressure of the column of water outside balances that of the 

 blood within. During the evolution of man there have been developed 

 special mechanisms by which the determination of the blood to the 

 lower parts is prevented, and the assumption of the erect posture 

 rendered possible. The pericardium, by its attachments, prevents 

 displacement of the heart as a whole, and also supports the right heart 

 when the weight of a long column of venous blood suddenly bears 

 upon it as, for example, when a man stands on his head. The ab- 

 dominal viscera are slung upwards to the spine; below they are sup- 

 ported by the pelvic basin and the wall of the abdomen, the muscles 

 of which are arranged so as to act as a natural waistband. If tame 

 hutch rabbits, with large patulous abdomens, be suspended and 

 immobilized in the erect posture, death may result in from fifteen to 

 thirty minutes, for the circulation through the brain ceases and the 

 heart soon becomes emptied of blood. If, however, the capacious 

 veins of the abdomen be confined by an abdominal bandage, no such 

 result occurs. 



In a man 6 feet high the hydrostatic pressure of a column of 

 blood reaching from the vertex of the head to the sole of the foot is 

 equal to 140 mm. Hg. But man is naturally provided with an efficient 

 abdominal belt, although this is often weakened by neglect of 

 exercise and by gross indolent living. The splanchnic arterioles 

 are maintained in tonic contraction by the vaso-motor centre, and 

 thus the flow of blood to the abdominal viscera is confined within 

 due limits. The veins of the limbs are broken into short segments 

 by valves, and these support the weight of the blood in the erect 

 posture. The brain is confined within the rigid wall of the skull, and 

 by this wall are the cerebral vessels supported and confined when the 

 pressure is increased by the head-down posture. Every contraction of 

 the skeletal muscles compresses the veins of the body and limbs, for 

 these are confined beneath the taut and elastic skin. The pressure 

 of the body against external objects has a like result. Guided by 

 the valves of the veins, the blood is by such means continually driven 

 upwards into the venae cavse. If the reader hangs one arm motionless 

 until the veins at the back of the hand become congested, and then 

 either elevates the limb or forcibly clenches the fist, he will recognize 

 the enormous influence which muscular exercise and continual change 

 of posture has on the return of blood to the heart. It becomes weari- 

 some and soon impossible for a man to stand motionless. When a 



