n8 THE MECHANISM OE THE CIRCULATION. 



Experiment 5. — Injection of a Large Quantity of Normal Saline. 



Intestinal. Hepatic. Cerebral. 



Rise. Rise. Rise. 



Here, there is a general rise of venous and arterial pressure. 



Experiment 6. — Obstruction of the TJioracic Aorta. 



Intestinal. Hepatic. Cerebral. 



Fall. Rise. Rise. 



The intestinal pressure falls in consequence of the diminished input, the 

 cerebral tension rises owing to the increased pressure in the cerebral arteries. 

 The hepatic pressure rises by a back effect, for the heart, in the face of the 

 increased resistance, is unable to maintain the systolic output. 



Experiment 7. — Obstruction of the Inferior Vena Cava above the Liver. 



Intestinal. Hepatic. Cerebral. 



Doubtful. Rise. Fall. 



The blood congests in the abdominal veins, therefore the hepatic pressure 

 rises, the arterial pressure falls enormously, and thus the cerebral tension 

 decreases. On the one side of the intestinal capillaries the arterial pressure 

 falls, on the other side the venous pressure rises greatly ; which effect will 

 have the most influence is doubtful. 



By many authors it has been suggested that osmotic force, capillary 

 attraction, and the ceaseless removal of particles from the blood to supply the 

 materials of the various secretions, may be factors in the maintenance of the 

 capillary circulation. 1 Hitherto this theory has defied experimental proof. 

 We may rest assured that if the action of the heart cease, no other force 

 operates sufficiently to cause a continued and progressive .motion of the blood. 



THE CONDITIONS OF BLOOD-FLOW IN THE VEINS. 



The most striking difference between the structure of an artery and its 

 venae comites is a decrease of elastic and muscular tissue in the veins, 

 together with an increase of white connective tissue. Accompanying 

 these changes, there is, on the side of the vein, a general and considerable 

 decrease in the thickness of the wall, and increase in the capacity. In 

 consequence of these structural differences, the maximal extensibility of 

 the veins, as measured by their increase in cubic volume, is found to 

 occur at very low pressures. At higher pressures the extensibility 

 rapidly decreases. The veins are tubes with flaccid walls, which fall 

 together when empty, and under very slight pressure expand to their 

 full capacity. Beyond this point, the walls, on account of the quantity 

 of connective tissue entering into their structure, can extend but little. 

 Thus a rise of pressure from to about 40 mm. Hg doubles the cubic 

 content of a vein, and enlarges it to its utmost capacity. It follows 

 that the great variations of volume which are seen to occur in the veins 

 are occasioned less by changes of pressure than by changes in the 

 volume of blood passing into the veins. 2 In this respect an artery shows 



1 Alison, "Outlines of Physiology," 3rd edition, 1839, p. 63; Cappie, " The Intracranial 

 Circulation," Edinburgh, 1890. 



2 Roy, Journ. Physiol., Cambridge and London, 1881, vol. iii. p. 136. 



