126 HUMAN PHYSIOLOGY. 



The diameter of a capillary vessel varies from ^Vo to s^Vu of an mcn 5 

 the walls of these vessels consist of a delicate, homogeneous membrane, 

 zokisTS f an inch in thickness, lined by flattened, elongated, and endothelial 

 cells, between which, here and there, are observed stomata. In many capil- 

 laries the wall consists only of the endothelial cells. 



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 

 possibly also aided by a power resident in the capillaries themselves, the result 

 of a vital relation between the blood and the (issues. 



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 venag cavae. 



They possess threecoats 



1. External, made up of areolar tissue. 



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

 fibrous tissue. 



3. Internal, an endothelial membrane similar to that of the arteries. 



Veins are distinguished by the possession of valves throughout their 

 course, which are arranged in pairs, and formed by a reflection of the in- 

 ternal coat, strengthened by fibrous tissues; they always look toward the 

 heart, and when closed prevent a reflux of blood in the veins. Valves are 

 most numerous in the veins of the extremities, but are entirely absent in 

 many others. 



The onward flow of blood in the veins is mainly due to the action of 

 the heart, but is assisted by the contraction of the voluntary muscles and the 

 force of respiration. 



Muscular contraction, which is intermittent, aids the flow of blood in the 

 veins by compressing them. As regurgitation is prevented by the closure 

 of the valves, the blood is forced onward toward the heart. 



During the movement of inspiration the thorax is enlarged in all its diam- 

 eters, and the pressure on its contents at once diminishes. Under these 



