g6 PHYSICAL PROPERTIES OF THE BLOOD-VESSELS. 



chloral differ in their action according to the dose. In these experiments the effect was ascer- 

 tained by the amount of fluid which flowed out of the vessels in a given time.] If blood 

 containing certain drugs be perfused through the blood-vessels of a freshly excised organ, the 

 blood-vessels are dilated; e.g., by amyl nitrite, chloral hydrate, morphia, CO, paraldehyde, 

 kairin, quinine, atropin, ferricyanide of potassium, (urea andsodic chloride in the renal vessels), 

 they are contracted by digitalin, veratria, helleborin {Kobcrt). Heat causes contraction of the 

 blood- vessels of the frog's mesentery {Gartner). According to Roy the blood-vessels shorten 

 when heated. 



That the capillaries undergo dilatation and contraction, owing to variations in 

 the size of the protoplasmic elements of their walls, must be admitted. 



Strieker has described capillaries as " protoplasm in tubes," and observed that in the tad- 

 pole they exhibited movements when stimulated. Golubew described an active state of contrac- 

 tion of the capillary wall, but he regarded the nuclei as the parts which underwent change. 

 Rouget observed the same result in the capillaries of new-born mammals. 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 and especially of their nuclei as influencing the blood-stream. 

 Oxygen acts on the nuclei of the capillary wall (membrana nictitans of frog) and causes them 

 to swell, while C0 2 has an opposite effect. The circulation through a lung suddenly rilled with 

 O or atmospheric air is at first very rapid, but it soon diminishes, while with C0 2 the circula- 

 tion remains constant.] As the capillaries are excessively thin, soft, and delicate, 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 en- 

 dothelial cells are flattened, but when the capillaries are collapsed they project more or less 

 into the lumen of the vessel (Renaut). 



[It is well known 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 pressure 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." 



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. 



[Uses of Elasticity. The elasticity of the arteries is of the utmost importance in aiding the 

 conversion of the unequal movement 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 inter- 

 mittent. The ordinary spray-producer acts on this principle. A uniform spray or jet is 

 obtained by pumping intermittently, but only when the resistance is such as to bring into action 

 the elasticity of the bag between the pump and the spray-orifice.] 



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

 iiwist tissues generally 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 takes place during the last few 

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

 magnifying 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 to the square-root of the weight. This is apart from any elastic after-effect. 

 Veins may be extended to at least 50 per cent, of their length without passing the limit of their 

 elasticity. 



