

PHYSIOLOGY. 163 



ing veins. These veins are commonly provided with valves, 

 which determine the effect of all pressure upon them, to be the 

 motion of the blood towards the heart. 



The great trunks, both of the vena cava and pulmonary 

 vein, are provided with muscular fibres, and manifestly indued 

 with muscular contractility. 



CLXVI. In the absorbent vessels, the fluids are probably 

 taken in by a capillary attraction. 



CLXVII. In the lymphatic vessels, provided with numerous 

 valves, which necessarily determine the motion of the contained 

 fluid to be towards the heart, the fluid is moved by the pressure 

 of the neighbouring muscles and arteries. But, further, as the 

 lymphatics are remarkably irritable, it is probable that the fluid 

 in them is moved by a peristaltic motion begun by the action 

 of their absorbent extremities. 



CLXVI 1 1. The motion of the blood through the vessels of 

 the lungs depends upon respiration, to be considered here- 

 after. 



CHAP III. OF THE LAWS OF THE CIRCULATION. 



CLXIX. The velocity of the blood passing out of the left 

 ventricle of the heart into the aorta, may be estimated from 

 knowing the quantity of blood passing out at each systole, the 

 area of the orifice of the aorta, and the time occupied by the 

 systole ; but none of these circumstances are exactly ascer- 

 tained. 



CLXX. As the blood moves onwards through the arteries, 

 the velocity (CLXIX.) suffers a considerable retardation from 

 several causes. 1. From the capacity of the arteries being en- 

 larged as they are more distant from the heart. 2. From the 

 frequent flexures of the arteries. 3. From the angles which 

 the branches make with the trunks from which they arise. 

 4. From anastomoses. 5. From the viscidity of the blood. 

 6. From the friction of adhesion. 7- From the weight and 

 rigidity of the parts surrounding the arteries, 



CLXXI. The velocity (CLXIX.) and the causes of re- 

 tardation (CLXX.) being given, the velocity of the blood in 

 the arteries will be as the frequency of the systole of the heart - 



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