CIRCULATION. 85 



meters high ; or, in the parlance of the laboratory, has been found equal to 

 from 24 to 54 millimeters of mercury. 1 



Summary of the Capillary Flow. — Whether in the Lungs or in the rest 

 of the body, the general characters of the capillary How, as learned from direct 

 inspection and from experiment, may be summed up as follows: The blood 

 moves through the capillaries toward the veins with much friction, contin- 

 uously, slowly, without pulse, and under low pressure. To account for these 

 facts is to deal systematically with the mechanics of the circulation ; and to 

 that task we must now address ourselves. 



C. The Pressure of the Blood in the Arteries, Capillaries, and 



Veins. 



Why does the blood move continuously out of the arteries through the 

 capillaries into the veins? Because there is continuously a high pressure of 

 blood in the arteries and a low pressure in the veins, and from the seat of high 

 to that of low pressure the blood must continuously flow through the capillaries, 

 where pressure is intermediate, as already stated. 



Method of Studying- Arterial and Venous Pressure, and General 

 Results. — Before stating quantitatively the differences of pressure, we must 

 see how they are ascertained for the arteries and veins. The method of obtain- 

 ing the capillary pressure has been referred to already. If, in the neck of a 

 mammal, the left common carotid artery be clamped in two places, it can, 

 without loss of blood, be divided between the clamps, and a long straight glass 

 tube, open at both ends, and of small calibre, can be tied into that stump of 

 the artery which is still connected with the aorta, and which is called the 

 "proximal" stump. If now the glass tube be held upright, and the clamp 

 be taken off which has hitherto closed the artery between the tube and the 

 aorta, the blood will mount in the tube, which is open at the top, to a consid- 

 erable height, and will remain there. The external jugular vein of the other 

 side should have been treated in the same way, but its tube should have been 

 inserted into the "distal" stump — that is, the stump connected with the veins 

 of the head, and not with the subclavian veins. If the clamp between the tube 

 and the head have been removed at nearly the same time with that upon the 

 artery, the blood may have mounted in the upright venous tube also, but only 

 to a small distance. To cite an actual case in illustration, in a small etherized 

 dog the arterial blood-column has been seen to stand at a height of about loo 

 centimeters above the level of the aorta, the height of the venous column 

 about 18 centimeters above the same level. The heights of the arterial and 

 venous columns of blood measure the pressures obtaining within the aorta and 

 the veins of the head respectively, while at the same time the circulation con- 

 tinues to be free through both the aorta and the venous net work. Therefore, 

 in the dog above referred to, the aortic pressure was between eight and nine 



1 N. v. Kries : "Ueberden Druckinden Blutcapillaren der menschlichen Haul." BerichL 

 iiber die Verhnndlungen der k. sachsischen Qeselkchaft der Wwsenschaften zu Leipzig, math.-physische 

 Classe, 1875, S. 149. 



