THE EFFECT OF CHANGE OF POSTURE. 91 



be exactly reversed. These hydrostatic influences are compensated fur in the 

 normal animal by vital changes in the blood vessels, in the heart, and in the 

 respiratory movements. 



In the elastic model the fluid sinks under the influence of gravity if the 

 venous and capillary tubes be made distensible by the pressure of the column of 

 fluid within them. In man the venous column is broken into short segments by 

 valves in the veins of the arms, in the root of the jugular veins, in the femoral 

 and other veins of the lower extremities. The longest column of venous blood 

 unbroken by any valve is that which extends from the intracranial sinuses, 

 down the intravertebral sinuses to the azygos veins, and thence to the end of 

 the inferior vena cava. From the innominate veins down the superior vena cava 

 to the end of the inferior vena cava there also extends an unbroken column of 

 blood. The valves are most numerous in the veins of the extremities, and are also 

 found in the portal system, in the veins to the stomach, spleen, and intestine. 

 Some of the portal valves become functionless in adult life. The suspension 

 of the heart by the deep cervical fascia, and constraint of the pericardial sac, 

 prevent the heart from rolling over and kinking the venae cavse during changes 

 of posture. The vena cava inferior passes through the central tendon of the 

 diaphragm, at a point which is immobile during the respiratory movements. 

 It is protected in its passage behind the liver from the weight of that organ, 

 for it lies in a tunnel formed by the Spigelian lobe of the liver, which, in its 

 turn, rests upon the vertebral column. 



The great and capacious abdominal veins are maintained and 

 supported under the hydrostatic stress of gravity, by means of the 

 expiratory muscles of the abdominal wall. The tone of these muscles, and 

 therefore the capacity of the veins, depends upon the integrity of the 

 respiratory centre. In consequence of the valves, the veins of the 

 limbs are constantly emptied onwards by the contraction of the skeletal 

 muscles, and by changes in posture. If a limb be kept motionless and 

 in the dependent posture, the veins quickly become congested. The 

 amount of this congestion depends on the tone of the skeletal muscles, 

 the arterioles, and the walls of the veins. By such means the disten- 

 sion of the veins and capillaries in the erect posture is largely prevented. 



The arterioles are held in a state of tone by the vasomotor centre, 

 and limit the output into the veins. When an animal passes from the 

 horizontal to the vertical feet-down position, the peripheral resistance 

 in the splanchnic area can be increased and the capacity diminished 

 by the contraction of the arterioles ; thus the circulation through the 

 head is maintained. The blood in all cases takes the pathway of least 

 resistance. The great splanchnic area forms the resistance box of the 

 circulation ; if it be contracted, the blood takes the pathway through the 

 locomotor organs ; if it be dilated, the blood passes into the capacious 

 veins of the abdomen. When the splanchnic area is fully dilated, the 

 whole of the blood can collect within the abdomen, and if the animal be 

 in the vertical feet-down position the influence of gravity will bring- 

 about this result. The splanchnic vasomotor paralysis can, however, 

 be then compensated for by the action of the respiratory pump. The 

 capacity of the abdominal vessels can be reduced by the compressive 

 action of the respiratory muscles, and while the output from the 

 arterioles is thus held in check, the blood is pressed up the veins, and can 

 be aspirated into the right heart by the action of the thoracic muscles. 



To study this question experimentally, cannula? are passed down the 

 external jugular vein and carotid artery of the clog, so that the ends of 

 the cannulas lie respectively in the vena cava superior and the aorta. 



