CHAPTER XL 



THE BLOOD-VESSELS. 



BY EDMUND C. WENDT, M.D., 



Curator of St. Francis' Hospital, New York City, etc. 



IN man, a closed circuit of branching tubes, which proceed 

 from a central organ, the heart, and, ramifying throughout the 

 body, return the blood to this central organ, constitutes the 

 blood- vascular system, as it has been named. 



Of these vessels we recognize three different kinds : arteries, 

 capillaries, and veins. The arteries convey the blood to the 

 various capillary districts, whence it is again collected and car- 

 ried back to the heart by the veins. 



The arteries, highly elastic throughout, are composed of 

 three superimposed layers or tunics. The veins, less elastic, 

 and consequently more flaccid and compressible^ likewise con- 

 sist of three coats or tunics. In both sets of vessels these 

 coats have received the names of intima for the inner, media 

 for the middle, and adventitia for the external layer. The 

 capillaries, intervening between the two, form minute branch- 

 ing tubules, which generally have but a single exceedingly 

 thin and permeable membrane as the sole constituent of their 

 walls. 



Of course, all these vessels merge into one another, so that 

 a sharp line of demarcation can nowhere be drawn ; but in 

 their typical forms they present clearly defined structural dif- 

 ferences, necessitating a separate description of them. We 

 begin with the simplest and yet most important class : 



The capillary blood-vessels. They are composed, as we 

 have already said, of a single layer of cells, arranged in tubu- 

 lar form, and containing nuclei. These corpuscles are di- 

 rectly continuous, on the one hand, with the inner coat of 



