CHAP. XXVIII.] THE CAPILLARY CIRCULATION. 3C9 



pressure of the blood into them, or to their distension under the 

 same pressure in consequence of diminished tone of their wall. 

 Their contraction is caused either by an inherent contractile power 

 in them, or by the diminution of their contents in consequence of 

 the contraction of the capillary arteries, in which latter case 

 diminished pressure permits them to contract, in virtue of the 

 elasticity of their walls. This latter would be the more probable 

 view, in default of any proved existence of a muscular structure 

 in the walls of the true capillaries, but there is no good reason 

 why the nuclei observed in them should not be regarded as belong- 

 ing to muscular tissue here in a membranous rather than a fibrous 

 form. 



The rate at which the blood moves in the capillary circulation 

 has been made the subject of direct observation by various phy- 

 siologists. It is slower than in the smallest veins, and still more 

 so than in the smallest arteries. Hales had stated the rate of 

 the circulation in the capillaries of the muscles of a frog to be an 

 inch in a minute and a half, and in the pulmonary capillaries five 

 times that velocity. Subsequent observers, Weber, Valentin, and 

 Volkmann, give a somewhat greater velocity : Weber and Valen- 

 tin make it about an inch and three-quarters in a minute, and 

 Volkmann found it about the same in cold-blooded animals, but 

 twice as much in the capillaries of the mesentery of a young dog. 

 These estimates are probably rather below the real rate of motion 

 of the blood in the capillaries, if we allow for the degree of pressure 

 and constraint to which they must be subjected in making the 

 observations. 



Of the Forces which maintain the Capillary Circulation. 

 The principal force by which the circulation is supported in the 

 capillary system, is the vis a tergo of the heart. We have already 

 adduced sufficient evidence to prove that that force is capable of 

 driving the blood throughout the whole circulating system. The 

 following facts may be stated in proof of this doctrine. 



1 . The pressure of the blood may be measured in the veins, in 

 the same way as in the arteries, and this varies with the force of 

 the heart. If, then, the heart's force extends to the veins, it must 

 do so through the capillaries. 



2. The capillary and venous circulation in any segment of the 

 body, is greatly influenced by the circulation in the main artery of 

 that segment. Thus, Majendie found the circulation much retarded 

 in the femoral vein by stoppage of that in the corresponding 

 artery : and by the hsemadynamometer it may be shown that the 



