376 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



On the whole we are disposed to the following conclusion respect- 

 ing the capillary circulation, namely, that it is maintained by the 

 vis a tergo of the heart ; but it is regulated and modified partly 

 by the elasticity of the walls of the capillary blood-vessels, partly 

 Iso by their contractility, which is greatly influenced by changes 

 in the nervous system, but chiefly by the operation of a force, 

 developed by those chemical and physical changes, which take place 

 between the blood and the tissues, and in which the phenomena of 

 nutrition essentially consist. For the due exercise of this force a 

 normal constitution of both the blood and the tissues is the most 

 important condition. 



Of the Circulation in the Veins. The blood moves in the veins 

 in a continuous stream, a fact sufficiently apparent to all who have 

 watched its escape from a vein after venesection, and as is likewise 

 apparent in examining the circulation in minute veins under the 

 microscope. The velocity of the venous current is considerably 

 less than that of the arterial, but greater than that of the capillaries, 

 and, as Volkmann has shown, it increases in the veins which are 

 nearest to the heart. 



That the vis a tergo of the heart is sufficient to maintain the 

 circulation in the veins is abundantly proved by all those facts 

 which have been already recited with reference to its influence on 

 the capillary circulation. Of these the most important are the 

 experiments of Majendie, already referred to (p. 369), and the fact 

 that in states of debility a distinct venous pulse is formed synchronous 

 with the heart, and evidently resulting from the extension of the 

 heart's impulse through the capillaries. Such a pulse (which may 

 be called the systolic venous pulse) is in rare instances observable 

 in the human subject in states of great prostration of strength, 



tudinal fibres, and a thickened and opaque state of ths walls of the Malpighian 

 capillaries ; while the intertubular capillaries and veins remain unaltered, save 

 in a diminution of their number, some having become wasted and obliterated 

 in consequence of the arrested action of the secreting cells. These observa- 

 tions show an obvious connexion between the activity of the organic changes 

 connected with the act of secretion and that of the capillary circulation, and, 

 indeed, an interdependence between the movement of the blood and the 

 secretory function of the gland ; but the hypertrophy of the muscular fibres 

 of the capillary arteries seems rather to indicate that in these extreme capil- 

 lary arteries, some propulsive power (vis a tergo) may be exercised by their 

 muscular fibres in promoting the flow through the capillary system. See 

 Dr. Johnson's Paper on Albuminous Urine and Dropsy in the 33rd vol. of the 

 Med. Chir. Trans. 





