MOVEMENT OF THE BLOOD IN THE ARTERIES. 325 



the complete division of the tube is often the readiest means of checking the 

 flow of blood. Secondly, under the influence of the nervous system it seems 

 to regulate the supply of blood to the different tissues and organs of the body 

 in accordance with the variations of their functional activity. And lastly, 

 it enables the whole vascular system to accommodate itself to the very vary- 

 ing amount of fluid it contains at different times, so that the pressure under 

 which it is distributed through the body is approximately the same. It may 

 be readily demonstrated by experiment. Thus, when a ligature is placed 

 upon an artery in a living animal, the part of the artery beyond the ligature 

 becomes gradually smaller, and is emptied to a certain degree, if not com- 

 pletely, of the blood it contained. Again, when part of an artery in a living 

 animal is isolated by means of two ligatures, and is punctured, the blood issues 

 from the orifice, and the inclosed portion of the artery is almost completely 

 emptied of its contents. This contraction is. much greater than could be ac- 

 counted for by the simple elasticity of the tissue ; and is more decided in small 

 than in large vessels. The empty condition of the arteries, generally found 

 within a short time after death, seems to be in part due to the same cause; 

 since their calibre is usually much diminished, and is sometimes completely 

 obliterated. A remarkable example of the same slow contraction, is that 

 which takes place in the end of the upper portion of an arterial trunk, when 

 the passage of blood through it is interrupted by a ligature; for the current 

 of blood then passes off by the nearest lateral branch, and the tube of the 

 artery shrivels and soon becomes impervious from the point at which the 

 ligature is applied back to the origin of that branch. This last fact is im- 

 portant, as proving how little influence the vis a tergo possesses over the 

 calibre of arterial tubes, since without any interruption to the pressure of 

 blood occasioned by it the tube becomes impervious. Other evidence of the 

 contractility of the arteries may be readily obtained by observing the effects 

 of various stimuli, mechanical, chemical, or electrical, upon the vessels of a 

 transparent membrane, such as the bat's wing or the frog's foot. Thus if, 

 whilst we watch the movement of blood in a companion artery and vein, we 

 draw the point of a fine needle across them three or four times, without ap- 

 parently injuring them or the membrane over them, they will both presently 

 contract and close ; then, after remaining for a few minutes in the contracted 

 state, they will begin again to dilate, and will gradually increase in diameter 

 until they acquire a larger size than before the stimulus was applied. When 

 in this condition, they will not again contract on the same stimulus as before ; 

 the needle may now be drawn across them much oftener and more forcibly, 

 but no contraction ensues, or only a trivial one which is quickly followed by 

 dilatation ; with a stronger stimulus, however, such as that of great heat, they 

 will again contract and close, and such contraction may last more than a day, 

 before the vessels again open and permit the flow of blood through them. 1 - 

 The comparative effects of chemical and other stimuli have been especially 

 studied by Mr. Wharton Jones, 2 by whom they are thus classified. (1.) Con- 

 striction may slowly take place, and be slowly succeeded by the normal width ; 

 this is the action of the sulphate of atropia. (2.) Constriction may quickly 

 take place, and be soon succeeded by the normal width, or a width not much 



1 See Mr. Facet's Lectures on Surgical Pathology, vol. i, pp. 302, 803. As Mr. 

 Faget justly remarks, it is from the mechanical stimulus of the knife, that small 

 divided vessels contract and close, so as speedily to cease bleeding; but this contrac- 

 tion lasts only for a time; and lisemorrhage would commence on their dilatation, 

 if their mouths were not sealed by coagula of blood or lymph. When secondary 

 haemorrhage does occur from want of such coagulation, it is most effectually con- 

 trolled by the application of such stimuli as, like the actual cautery, induce a more 

 prolonged contraction of the vessels. 



2 Guy's Hospital Reports for 1850, pp. 8, 9. 



