52 HUMAN PHYSIOLOGY. 



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 oblon- 

 gata. 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 -^^ to the -g^j of 

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

 TffffTnr f an i nc h * n thickness, lined by flattened, elongated, endothelial 

 cells, between which, here and there, are observed stomata. 



It is through the agency of the capillary- vessels that the phenomena of 

 nutrition and secretion takes place, for here the blood flows in an equable 

 and continuous current, and is brought into intimate relationship with the 

 tissues, two of the essential conditions for proper nutrition. 



The rate of movement in the capillary vessels is estimated at one inch 

 in thirty seconds. 



In the capillary current the red corpuscles may be seen hurrying down 

 the centre of the stream, while the white corpuscles in the still layer ad- 

 here to the walls of the vessel, and at times can be seen to pass through the 

 walls of the vessel by amoeboid movements. 



