498 



CIRCULATION OF BLOOD AND LYMPH. 



It appears from these figures that there is no proportion between 

 the size of an animal and the amount of mean arterial pressure. It 

 is probable that there may be a general relationship between the 

 size of the animal that is, the size of the heart and the amount 

 of pulse pressure or the oscillation of pressure with each heart beat, 

 but sufficient data are not at hand to determine this point. As 

 we pass from the aorta to the smaller arteries the mean pressure 

 decreases somewhat, although not very rapidly, while the pulse 

 pressure decreases also and to a more noticeable extent. 



This fact is illustrated in Fig. 199, which gives a graphic 

 representation of a number of experimental determinations* of 

 systolic and diastolic pressures in the large arteries of the dog. 



If we turn to the other end of the vascular system, the veins, 

 we find that the lowest pressure is in the venae cavse and that it 

 increases gradually as we go toward the capillary area. Accord- 

 ing to one observer,! the fall in pressure from periphery toward 

 the heart is at the rate of 1 mm. Hg for every 35 mms. of distance. 

 We have such figures as the following: 



DOG (OPITZ). 



Superior vena cava (near 



auricle) = 2.96 mms. Hg. 



Superior vena cava more 



distal = 1.38 " 



External jugular (left) . . = 0.52 mm. 



Right brachial = 3.90 mms. 



Left facial = 5.12 " 



Left femoral 5.39 



Left saphenous = 7.42 ' 



SHEEP. 



Jugular vein 0.2 mm. Hg. 



Facial vein 3.0 mms. " 



Branch of brachial . . . 9.0 



Crural . . 11.4 " " 



Fig. 200. Schematic representation of the general relations of blood-pressure (side 

 pressure) in different parts of the vascular system: a, The arteries; c, the capillaries; v, 

 the veins. The mean and diastolic pressures remain nearly constant in the arterial system, 

 as far as they can be measured accurately. The pressures in the veins are represented as 

 uniform at any one point. In the large veins near the heart there are variations of pressure 

 with each respiration and with each heart beat (Venous Pulse, p. 529). 



At the heart, therefore, the pressure of the blood upon the walls 

 of the veins is very small, and, indeed, owing to the circumstance 

 that the large veins lie in the thoracic cavity, in which the pres- 

 sure is below that of the atmosphere, the pressure of the blood in 



* Dawson, "American Journal of Physiology," 15, 244, 1906. 



t Burton-Opitz,, "American Journal of Physiology," 9, 198, 1903. 



