140 HUMAN PHYSIOLOGY. 



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 a dilating action upon 

 the vessels, thus keeping up the arterial tonus and the average blood 

 pressure. 



Capillaries. The capillaries constitute a network of vessels of 

 microscopic 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 -Q-^Q-Q to joVo" ^ an 

 inch ; the walls of these consist of a delicate, homogeneous mem- 

 brane, ^o'ff^o ^ an i ncn i n thickness, lined by flattened, elongated, 

 endothelial cells, between which, here and there, are observed 

 stomata. 



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 center of the stream, while the white corpuscles in the 

 still layer adhere to the walls of the vessel, and at times can be 

 seen to pass through the walls of the vessel by ameboid movements. 



The function of the capillary blood-vessel is to permit of the 

 passage of the nutritive materials of the blood out into the tissue- 

 spaces and the passage of waste products from the tissue-spaces into 

 the blood. 



The passage of the blood through the capillaries is mainly due 

 to the force of the ventricular systole and the elasticity of the 

 arteries; but it is probably also aided by a power resident in the 

 capillaries themselves, the result of a vital relation between the blood 

 and the tissues. 



The veins are the vessels which return the blood to the heart ; 

 they have their origin in the venous radicles, and as they approach 

 the heart converge to form larger trunks, and terminate finally in 

 the venae cavse. 



They possess three coats 



1. External, made up of areolar tissue. 



2. Middle, composed of non-striated muscle-fibers ; yellow, elastic, 

 and fibrous tissue. 



