CIRCULATION. 



73 



experiment indicates the existence of that power of slow contraction 

 in the arteries to which the name of Ihnicity has been given ; this, 

 however, does not seem to be anything else than a particular mani- 

 festation of the general property of vital contractility, and is quite 

 distinct from ordinary elasticity. The manifestation of this property 

 is seen when a ligature is applied to an artery, the part beyond 

 the ligature becoming gradually smaller, and emptied to a greater or 

 less degree of the blood which it contained. The empty condition of 

 the arteries after death is due to the same cause. It is to the moderate 

 action of the Tonicity of arteries, that these contractions upon the 

 stream of blood passing through them, is due. If the tonicity be 

 excessive, the pulse is hard and wiry ; but if it be deficient, the pulse 

 is very compressible, though bounding, and the flow of blood through 

 the arteries is retarded. 



The smaller arteries in every organ of the body, before they be- 

 come capillary, are connected by repeated anastomoses with each 

 other, the object of which arrangement is, to supply the circulation 

 to any part, when an important vessel leading to that part is either 

 compressed or obliterated. The capillary system of all connected 

 parts being continuous, all the vessels, whether arteries or veins, are 

 united through its medium. 



Capillaries. — It has been shown that in all organic textures the 

 transmission of the blood from the minute branches of the arteries to 

 the minute veins is effected by a network of microscopic vessels, in 



Fig. 15.* 



* Shows the blood-vessels and their anastomoses. 1, l. Veins. 2, 2, 2, Arteries. 



7 



