94 CIRCULATION OF THE BLOOD IN THE VESSELS. 



The intermittent action of the heart, which pervades the whole arterial 

 system, is generally absorbed, as it were, in the passage of the blood through 

 the capillaries ; but when the arterioles of any part are very much relaxed, 

 the cardiac impulse may extend to the veins. When the glands are pouring 

 out their secretions, the quantity of blood which they receive is very much 

 increased. It is then furnished to supply material for the secretion, and not 

 exclusively for nutrition. If the vein be opened at such a time,, it is found 

 that the blood has not lost its arterial character, that the quantity which 

 escapes is increased, and that the flow is in an intermittent jet, as from a divided 

 artery (Bernard). This is due to the relaxed condition of the arterioles of 

 the part, and the phenomenon thus observed constitutes the true venous 

 pulse. What thus occurs in a restricted portion of the circulatory system 

 may take place in all the veins, though in a less marked degree. Physicians 

 have frequently noticed, after the blood has been flowing for some time in 

 the operation of venesection, that the color changes from black to red and 

 the stream becomes intermittent, often leading the operator to fear that he 

 has pricked the artery. In all probability this is due to the relaxation of 

 the arterioles as one of the effects of abstraction of blood, producing the 

 same condition that has been noted in some of the glands during their 

 activity. 



Pressure of Blood in the Veins. The pressure in the veins is always 

 much less than in the arteries. It is very variable in different parts of the 

 venous system and in the same part at different times. As a rule, it is in 

 an inverse ratio to the arterial pressure. Whatever favors the passage of blood 

 from the arteries into the capillaries has a tendency to diminish the arterial 

 pressure, and as it increases the quantity of blood which passes into the veins, 

 it must increase the venous pressure. The great capacity of the venous sys- 

 tem, its frequent anastomoses and the presence of valves which may shut off a 

 portion from the rest, are conditions which involve considerable variations 

 in pressure in different vessels. It has been ascertained that as a rule, the 

 pressure is greatest at the periphery and progressively diminishes in the direc- 

 tion of the heart. In an observation on the calf, Volkmann found that with 

 a pressure of about 6'5 inches (165-1 mm.) of mercury in the carotid, the 

 pressure in the metatarsal vein was 1-1 inch (28 mm.), and but 0-36 (9-1 mm.) 

 in the jugular. Analogous results were obtained in the more recent experi- 

 ments by Jacobson. * Muscular effort has a marked influence on the force of 

 the circulation in certain veins and produces an elevation in the pressure. 

 As the reduced pressure in the veins is due in a measure to the great rela- 

 tive capacity of the venous system and the free communications between the 

 vessels, it would seem that if it were possible to reduce the capacity of the 

 veins in a part and force all the blood to pass to the heart by a single vessel 

 corresponding to the artery, the pressure in this vessel would be greatly 

 increased. Poiseuille has shown this to be the fact by the experiment of 

 tying all the veins coming from a part, except one which had the vol- 

 ume of the artery by which the blood was supplied, forcing all the blood 

 to return by this single channel. This being done, he found the press- 



