



THE CIRCULATION. 319 



and that toward the heart, thereby assisting venous flow in propor- 

 tion to the extent of pressure exerted. In the limbs is found this 

 aid to venous circulation. Should the muscles remain in a state of 

 tetanic contraction, the venous blood passing out collects in the sub- 

 cutaneous system, for it must be remembered that particularly numer- 

 ous anastomoses with one another, as well as the deep with the super- 

 ficial veins, are characteristic of this system. That the muscles aid 

 venous flow is nicely demonstrated by the increased flow from an 

 incised vein during contraction of its adjacent muscles when perform- 

 ing venesection. 



The action of the diaphragm and intercostals helps to render the 

 intrathoracic pressure negative during inspiration; so that the blood 

 is drawn from the peripheral portion of the venous tree toward the 

 heart; as some observer states it, the blood-column is actually lifted 

 in the ascending vena cava. 



Another, though less important, factor in venous propulsion is 

 thoracic suction. For every time that the chest expands and makes 

 in its interior an empty space, air rushes in to fill the same. The 

 venous blood, situated in the vicinity of that cavity, also is helped into 

 its intrathoracic veins. 



Pathological. By standing during the whole day barbers and 

 others have varicose veins. Walking would counteract this by mus- 

 cular contraction emptying the veins. The tone of the skin also has an 

 effect. 



Circulation in the Brain. Dr. Leonard Hill states that the brain 

 content of blood can vary suddenly only to a slight degree, and that 

 Monro's doctrine is to all intents and purposes true. When the aortic 

 pressure rises the expansion of the cerebral volume can take place only 

 to a certain limited degree, for as soon as all the cerebro-spinal fluid is 

 driven out from the cranium the brain everywhere is in contact with 

 the rigid skull. We have in the vasomotor center a protective 

 mechanism by which blood can be drawn at need from the abdomen 

 and supplied to the brain. At the moment of excitation from the 

 external world the splanchnic area contracts and more blood is driven 

 through the brain. The quantity of the blood in the brain is nearly 

 the same, but the rapidity of the circulation in the brain varies. 

 Thus, should there be any evidence of cerebral congestion, the splanch- 

 nic fibers dilate the vessels in its area and by so doing decrease the 

 amount sent to the cavity of the cranium. Should cerebral anaemia 

 occur the reverse will be the condition of affairs in the splanchnic 

 area. 



