60 CIRCULATION. 



6 to 10 inches (= 150 to 250 millimeters) of mercury 

 = J to | of an atmosphere = 3 to 5 Ibs. on the square 

 inch. It is this pressure which is the cause of the cir- 

 culation. Its maintenance is the function of the heart. 

 The sum of the lumina of the capillaries is much greater 

 than that of the aorta : the velocity of the capillary 

 blood-stream is in the same proportion less. Arterial 

 tissue recovers its length after stretching as perfectly as 

 muscle : it is however more extensible. In the living 



o 



body, its elastic properties are modified according to the 

 degree of contraction of the muscular elements it con- 

 tains. 



The arterial blood-stream can be best understood by 

 reference to the schemata described below. 



Schema I. As regards the relation of pressure to pro- 

 gressive motion, the arterial system is represented by a 

 tall cylindrical bottle from the bottom of which water 

 flows through a horizontal tube of equal width through- 

 out. In the arterial system, as in the schema, the lateral 

 pressure (supposing the velocity of the blood-stream to 

 be constant) is proportional to the sum of the resistances 

 in front. If such a bottle, having an aperture equal to the 

 lumen of the aorta, were substituted for the heart, 

 the height to which it would be necessary to fill it with 

 blood in order to carry on the circulation at the normal 

 rate, would represent the force required for that purpose. 

 That height multiplied by the weight of blood discharged 

 per second in grammes, would give in gramme-meters the 

 work done by the heart in the same time in maintaining 

 the circulation. 



Schema 2. As regards wave motion, or pulsation, it is 

 represented by an elastic tube, ab, closed at both ends 

 and moderately distended with liquid, into which water is 

 suddenly and for a short time injected. The phenomena 

 observed remain unaltered, if, instead of closing the tube 

 at its ends, we imitate the conditions of the circulation by 



