CHAPTEE IV. 



THE VASCULAR MECHANISM. 



IN order that the blood may be a satisfactory medium of com- 

 munication between all the tissues of the body, two things are 

 necessary. In the first place, there must be through all parts of 

 the body a flow of blood, of a certain rapidity and general con- 

 stancy. In the second place, this flow must be susceptible of both 

 general and local modifications. In order that any tissue or organ 

 may readily adapt itself to changes of circumstances (action, 

 repose, &c.), it is of advantage that the quantity of blood passing 

 to it should be not absolutely constant, but capable of variation. 

 In order that the material equilibrium of the body may be main- 

 tained as exactly as possible, it is desirable that the loading of the 

 blood with substances proceeding from the unwonted activity of 

 any one tissue, should be accompanied by a greater flow of blood 

 through some excretory or metabolic tissue by which these 

 substances may be removed. Similarly it is of advantage to 

 the body that the general flow of blood should in some circum- 

 stances be more energetic, and in others less so, than normal. 



The first of these conditions is dependent on the mechanical 

 and physical properties of the vascular mechanism ; and the 

 problems connected with it are almost exclusively mechanical or 

 physical problems. The second of these conditions depends on 

 the intervention of the nervous system ; and the problems con- 

 nected with it are essentially physiological problems. 



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