PHYSICAL PROPERTIES OF THE BLOOD-VESSELS. 125 



the same vessel, or another at a lower level. They break up chiefly in the outer 

 coat, and none enter the inner coat.] In structure they resemble other small 

 blood-vessels, and the blood circulating in the arterial or venous wall is returned 

 by small veins. 



Intercellular blood-channels. Intercellular blood-channels of narrow calibre 

 and without walls occur in the granulation tissue of healing wounds. At first 

 blood-plasma alone is found between the formative cells, but afterwards the blood- 

 current forces blood-corpuscles through the channels. The first blood-vessels in 

 the developing chick are formed in a similar way from the formative cells of the 

 mesoblast. 



Properties of the Blood-Vessels. The larger blood-vessels are 

 cylindrical tubes composed of several layers of various tissues, more 

 especially elastic tissue and plain muscular fibres, and the whole is lined 

 by a smooth layer of epithelium. One of the most important properties 

 is the CONTRACTILITY of the vascular wall, in virtue of which the 

 blood-vessel becomes contracted, so that the calibre of the vessel and, 

 therefore, the supply of blood to a part are altered. The contractility 

 is due to the plain muscular fibres which are, for the most part, 

 arranged circularly. It is most marked in the small arteries, and of 

 course is absent where no muscular tissue occurs. The amount and 

 intensity of the contraction depend upon the development of the 

 muscular tissue ; in fact, the two go hand-in-hand. [If an artery be 

 exposed in the living body it soon contracts under the stimulus of the 

 atmosphere (J. Hunter) acting upon the muscular fibres.] 



[Action of Alkalies and Acids on the Vascular System. Gaskell finds 



that very dilute alkalies and acids have a remarkable effect on the blood-vessels 

 and also upon the heart. A very dilute solution of lactic acid (1 part to 10,000 

 parts of saline solution), passed through the blood-vessels of a frog, always enlarges 

 the calibre of the blood-vessels, while an alkaline solution (1 part sodium hydrate 

 to 10,000 or 20,000 parts saline solution) always diminishes their size, usually to 

 absolute closure, and indeed the artificial constriction of the blood-vessels may be 

 almost complete. These fluids are antagonistic to each other as far as regards 

 their action on the calibre of the arteries. Microscopic observations which con- 

 firmed these results, were also made on the blood-vessels of the mylohyoid muscle 

 of the frog. Dilute alkaline solutions act on the heart in the same way. After 

 a series of beats, the ventricle stops beating, the stand-still being in a state of con- 

 traction. Very dilute lactic acid causes the ventricle to stand still in the position 

 of complete relaxation. The action of the acid and alkali solutions are antagonistic 

 in their action on the ventricle. Gaskell attaches considerable importance to the 

 " tonic " and " atonic " conditions of the whole vascular system produced by very 

 dilute solutions of alkalies and acids respectively.] 



That the capillaries undergo dilatation and contraction, owing to 

 variations in size of the protoplasmic elements of their walls, must be 

 admitted. 



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

 they exhibited movements when stimulated in living animals. Golubew described 

 an active state of contraction of the capillary wall, but he regarded the nuclei as 



