ARTERIES. 57 



The blood pressure is increased or diminished by influences acting upon 

 the heart or upon the peripheral resistance of the capillaries, viz. : 



If, while the force of the heart remains the same, the number of pulsa- 

 tions per minute increases, thus increasing the volume of blood in the 

 arteries, the pressure rises. If the rate remains the same, but the force 

 increases, the pressure again rises. Causes that increase the peripheral 

 resistance by contracting the arterioles, e. g., vasomotor nerves, cold, etc., 

 produce an increase of the pressure. 



On the other hand, influences which diminish either the volume of the 

 blood, or the number of pulsations, or the force of the heart, or the peri- 

 pheral resistance, lower the pressure. 



The Pulse is the sudden distention of the artery in a transverse and 

 longitudinal direction, due to the injection of a volume of blood into the 

 arteries at the time of the ventricular systole. As the vessels are already 

 full of blood, they must expand in order to accommodate themselves to the 

 incoming volume of blood. The blood pressure is thus increased, and the 

 pressure originating at the ventricle excites a pulse wave, which passes 

 from the heart toward the capillaries at the rate of about twenty-nine feet 

 per second. It is this wave that is appreciated by the finger. 



The Velocity with which the blood flows in the arteries diminishes from 

 the heart to the capillaries, owing to an increase of the united sectional area 

 of the vessels, and increases in rapidity from the capillaries toward the 

 heart. It moves most rapidly in the large vessels, and especially under 

 the influence of the ventricular systole. From experiments on animals, 

 it has been estimated to move in the carotid of man at the rate of sixteen 

 inches per second, and in the large veins at the rate of four inches per 

 second. 



The Calibre of the blood vessels is regulated by the vasomotor 

 nerves, which have their origin in the gray matter of the medulla oblongata. 

 They issue from the spinal cord through the anterior roots of spinal nerves, 

 pass through the sympathetic ganglia, and ultimately are distributed to the 

 coats of the blood vessels. They exert, at different times, a constricting and 

 dilating action upon the vessels, thus keeping up the arterial tonus. 



Capillaries. The capillaries constitute a network of vessels of micro 

 scopic size, which distribute the blood to the inmost recesses of the tissues, 

 inosculating with the arteries on the one hand and the veins on the other ; 

 they branch and communicate in every possible direction. 



The diameter of a capillary vessel varies from the ^Vtf to ^ e 3uW f 

 an inch; their walls consist of a delicate homogeneous membrane, the 

 E 



