CIRCULATION. 91 



up to, or above, 200 millimeters. In man it i> probably higher than in the 

 dog. The pressure in the other arteries derived from the aorta which have 

 been studied manoinetricalry is not very greatly lower than in that vessel. In 

 the pulmonary arteries the pressure is probably much lower than in the aortic 

 system. The pressure in the small veins of the head of the dog, the cannula 

 being in the distal stump of the external jugular vein, we have found already 

 in one case to equal about 14 millimeters of mercury. In such a case the 

 presence of valves in the veins and other elements of difficulty make the 

 mean pressure hard to obtain as opposed to the maximum pressure during 

 the period of observation. 



If a cannula be so inserted as to transmit the pressure obtaining within 

 the great veins of the neck just at the entrance of the chest, without interfer- 

 ing with the movement of the blood through them, and if a manometer be 

 connected with this cannula, the fluid will fall below the zero-point in the 

 distal limb, indicating a slight suction from within the vein, and thus a 

 slightly " negative " pressure. 1 This negative pressure may sometimes become 

 more pronounced during inspiration and regain its former value during ex- 

 piration. Sometimes, again, the pressure during expiration may become posi- 

 tive. The continuous flow from the great arteries through the capillaries to 

 the veins, and through these to the auricle, is therefore shown by careful 

 quantitative methods, no less than by the tube of Hales, to be simply a 

 case of movement of a fluid from seats of high to seats of lower pressure. 



The Symptoms of Bleeding- in Relation to Blood-pressure. — The dif- 

 ferences of pressure revealed scientifically by the manometer exhibit them- 

 selves in a very important practical way when blood-vessels are wounded and 

 bleeding occurs. If an artery be cleanly cut, the high pressure within drives 

 out the blood in a long jet, the length of which varies rhythmically with the 

 cardiac pulse, but varies only to a moderate degree. From wounded capil- 

 laries, or from a wounded vein, owing to the low pressure, the blood does not 

 spring in a jet, but simply flows out over the surface and drips away without 

 pulsation. At the root of the neck, where the venous pressure may rhythmi- 

 cally fall below and rise above the atmospheric pressure, the bleeding from 

 a wounded vein may be intermittent. 



D. The Causes of the Pressure in the Arteries, Capillaries, 



and Veins. 



The causes of the continuous high pressure in the arteries musl first engage 

 our attention. 



Resistance. — The great ramification of the arterial system at a distance 

 from the heart culminates in the formation of the countless arterioles mi the 

 confines of the capillary system. We have already seen direct evidence of the 

 friction in the minute vessels which results from this enormous subdivision of 

 the blood-path. The force resulting from this friction i- propagated back- 



1 II. Jacobson : " Ueber die Blutbewegung in den Venen," Reicherts •"<</ </" Boie-iJey- 

 mond's Arckiv fur Anatomic, Phyziologie, etc., 1867, 8. 224. 



