58 HUMAN PHYSIOLOGY. 



2(jtf(HF f an * 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 

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

 the walls of the vessel by amoeboid movements. 



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 cavae. 



They possess three coats 



I . External^ made up of areolar tissue. 



2.. Middle, composed of non-striated muscular fibres, 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 inter- 

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

 and when closed prevent a return 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. 



Rhythmical movements of veins have been observed in some of the lower 

 animals, aiding the onward current of blood. 



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



