I 4 2 THE CIRCULATION. 



the smaller arteries with muscular tissue add to these two 

 purposes, that of regulating and determining, according to 

 its requirements, the proportion of the whole quantity of 

 blood which shall be distributed to each part. 



It must be remembered, however, that this regulating 

 function of the arteries is itself governed and directed by 

 the nervous system. 



The muscular tissue of arteries is supplied with nerves 

 chiefly, if not entirely, by branches from the sympathetic 

 system. These so-called vaso-motor nerves are again con- 

 nected, through the medium of ganglia, with the fibres 

 from the sympathetic system supplied to the organs 

 nourished by these same arteries. Thus, any condition in 

 these organs which causes them to need a different amount 

 of blood, whether more or less, produces a certain im- 

 pression on their nerves, and by these the impression is 

 carried to the ganglia, and thence reflected along the 

 nerves which supply the arteries. The muscular element 

 of these vessels responds in obedience to the impression 

 conveyed to it by the nerves ; and, according to its contrac- 

 tion or dilatation, is a larger or smaller quantity of blood 

 allowed to pass. 



Another function of the muscular element of the middle 

 coat of arteries is, doubtless, to co-operate with the elastic 

 in adapting the calibre of the vessels to the quantity of 

 blood which they contain. For the amount of fluid in the 

 blood-vessels varies very considerably even from hour to 

 hour, and can never be quite constant, and were the elastic 

 tissue only present, the pressure exercised by the walls of 

 the containing vessels on the contained blood would be 

 sometimes very small, and sometimes inordinately great. 

 The presence of a muscular element, however, provides 

 for a certain uniformity in the amount of pressure exer- 

 cised ; and it is by this adaptive, uniform, gentle, muscular, 

 contraction, that the tone of the blood-vessels is maintained. 

 Deficiency of this tone is the cause of the soft and yield- 



