126 PHYSICAL PROPERTIES OF THE BLOOD-VESSELS. 



the parts which underwent change. Tarchanoff found that mechanical or electrical 

 stimulation caused a change in the shape and size of the nuclei, so that he regards 

 these as the actively contractile parts. [Severini also attaches great importance 

 to the contractility of the capillaries.] Strieker's observations were made on the 

 capillaries of tadpoles. These phenomena became less marked as the animal 

 became older. Rouget observed the same result in the capillaries of new-born 

 mammals. As the capillaries are excessively thin and delicate, and as they are 

 soft structures, it is obvious that the form of the individual cells must depend to a 

 considerable extent upon the degree to which the vessels are filled with blood. In 

 vessels which are distended with blood the endothelial cells are flattened, but when 

 the capillaries are collapsed, they project more or less into the lumen of the vessel 

 (Renaut). 



[It is a well-known fact that the capillaries present great variations in their 

 diameter at different times. As these variations are usually accompanied by a 

 corresponding contraction or dilatation of the arterioles, it is usually assumed that 

 the variations in the diameter of the capillaries are due to differences of the pres- 

 sure within the capillaries themselves viz., to the elasticity of their walls. Every 

 one is agreed that the capillaries are very elastic, but the experiments of Roy and 

 Graham Brown show that they are contractile as well as elastic, and these 

 observers conclude that under normal conditions, it is by the contractility of the 

 capillary wall as a whole that the diameter of these vessels is changed, and to all 

 appearance their contractility is constantly in action. ' ' The individual capillaries 

 (in all probability) contract or expand in accordance with the requirements of the 

 tissues through which they pass. The regulation of the vascular blood-flow is thus 

 more complete than is usually imagined " (Roy and Graham Brown).] 



Physical Properties. Amongst the physical properties of the blood- 

 vessels, ELASTICITY is the most important ; their elasticity is small in 

 amount, i.e., they offer little resistance to any force applied to them so 

 as to distend or elongate them, but it is perfect in quality, i.e., the 

 blood-vessels rapidly regain their original size and form after the force 

 distending them is removed. 



[The elasticity of the arteries is of the utmost importance in aiding the conversion 

 of the interrupted flow of the blood in the large arteries into a uniform flow in 

 the capillaries. E. H. Weber compared the elastic wall of the arteries with the 

 air in the air-chamber of a fire-engine. In both cases an elastic medium is acted 

 upon the air in the one case and the elastic tissue in the other which in turn 

 presses upon the fluid, propelling it onwards continually, while the action of the 

 pump or the heart, as the case may be, is intermittent.] 



According to E. H. Weber, Volkmann, and Wertheim, the elongation of a blood- 

 vessel (and most moist tissues) is not proportional to the weight used to extend it, 

 the elongation being relatively less with a large weight than with a small one, so 

 that the curve of extension is nearly [or, at least, bears a certain relation to] a 

 hyperbola. 



According to Wundt, we have not only to consider the extension produced at 

 first by the weight, but also the subsequent "elastic after-effect" which occurs 

 gradually. The elongation which occurs during the last few moments occurs so 

 slowly and so gradually that it is well to observe the effect by means of a magni- 

 fying lens. Variations from the general law occur to this extent, that if a certain 

 weight is exceeded, less extension, and, it may be, permanent elongation of the 

 artery not unfrequently occur. K. Bardeleben found, especially in veins 

 elongated to 40 or 50 per cent, of their original length, that when the weight 

 employed increased by an equal amount each time, the elongation was proportional 



