466 MATERIA MEDIC A AND THERAPEUTICS. 



for reducing the intercardiac pressure is called the depressor 

 mechanism of the circulation. 



The capillaries effect the final distribution of blood to the 

 tissues. Their soft protoplasmic walls, through which the 

 plasma, the oxygen, and the corpuscles pass into the tissues, 

 have irritability of their own, and they are subject to many 

 other influences, viz. those of the nervous system, of the blood 

 which they contain, of the arteries and the veins at either 

 extremity, and of the activity of nutrition. In the capillaries 

 we discover the other element of the peripheral resistance. 



The veins convey the blood back to the heart as com- 

 paratively passive tubes. They are probably subject to special 

 nervous influences, but they are chiefly influenced physically 

 by the volume of blood passing through them, that is, by the 

 condition of the heart in front and of the arteries and capillaries 

 behind. Thus, shortness of diastole, i.e. frequency of the heart, 

 diminishes the time of emptying the veins, and raises the 

 pressure within them. A low arterial pressure and a free flow 

 through the capillaries have the same effect. Conversely, the 

 veins react physically on the heart and capillaries ; if they 

 are dilated and full, the return of the blood to the auricle is 

 delayed, and the force of systole weakened from lowness of the 

 charge, whilst the capillaries are obstructed, and the flow of 

 the plasma and metabolic products between the vessels and the 

 tissues disturbed. 



"We can now understand the meaning of the expression, the 

 general blood-pressure. The elasticity of the arteries being taken 

 as constant, the pressure of blood within the arterial system at 

 any given moment will depend upon (1) the total quantity of 

 blood in circulation; (2) the action of the heart; (3) the 

 freedom of the flow into the veins, i.e. the peripheral resistance, 

 due to vasor constriction and capillary obstruction. The 

 arterial pressure is so far self -regulated, through the quantity 

 of blood in circulation, by means of the Malpighian bodies of 

 the kidney. In this mechanism, the general arterial pressure 

 is brought to bear upon a length of unsupported arteriole, so as 

 to press or excrete the water of the blood through the vascular 

 wall into the uriniferous tubule. By the muscular and nervous 

 Structures in the walls of the afferent and efferent arterioles, 

 the pressure upon the glomerulus may be cut off, or thrown on, 

 as the system requires, the result being less or more watery 

 excretion, and corresponding rise or fall of the blood pressure. 

 The perspiratory excretion, and, indeed, all exudations, probably 

 act in the same way as the urinary, only less powerfully. 



Another powerful influence on the circulation as a whole 

 is muscular activity, exertion being attended by cardiac excite- 



