viii BLOOD-STREAM: MOVEMENT IN VESSELS 281 



tangent of the angle which this makes with the positive direction of the 

 axis of the abscissa, and then erect upon the point of the axis of the abscissa 

 corresponding to P an ordinate proportional to the measured trigonometric 

 tangent. Fick has given a very practical method of determining these 

 tangents. 



XII. Having thus analysed the principal phenomena relating 

 to circulatory pressure and velocity, it is necessary to consider a 

 scheme for combining them synthetically. With this object Fig. 

 119 shows in a single diagram the most important facts of the 

 circulation in the arteries, veins, and capillaries. In the arteries 



Artery. 



Capillary. 



Vein. 



Fi',. IIP. Synthetic diagram of progressive variations in area of blood current, pressure, and 

 velocity, in the three main sections of the systemic circulation. (Gad and Fredericq.) The 

 schema does not show the respiratory and vasomotor oscillations of pressure and velocity. 



the circulatory area (represented by the total section of the vessels) 

 increases slowly at first from the aorta to the small arteries, and 

 then rapidly from the small arteries to the capillaries. Both the 

 pressure and the velocity curves exhibit the same general course. 

 The pulse and the cardiac oscillations of velocity in the arteries 

 diminish slowly from the aorta to the capillary threshold. 



In the capillaries, where the circulatory area becomes 

 maximal, pressure continues to diminish slowly, and velocity 

 becomes minimal and constant. 



In the veins the area again decreases, rapidly at first, then 

 more slowly to the mouth of the venae cavae, where, however, it 

 is more extensive than at the aortic orifice ; pressure decreases 

 constantly, becoming negative in the intrathoracic veins ; velocity, 

 on the other hand, increases more slowly from the farthest 



