OP THE ARTERIES. 277 



that they present to the circulation. The same is the case 

 with their flexuosities. 



The termination of the arteries, when they become capillary 

 and microscopic, occurs by their being continued into veins, 

 either by red capillary communications, or by communications 

 of colourless vessels in consequence of their extreme minute- 

 ness. 



408. The arteries are cylindrical when examined inter- 

 nally, their section is circular, excepting in the largest arte- 

 ries, which, when empty, are slightly depressed, and present 

 an elliptic section. 



Each of the two arterial trunks is furnished with three valves 

 at its origin in the heart. These semi-lunar valves are attached 

 by their convex edge to the contour of the festoons of the ar- 

 tery; their free margin is straight and somewhat thick, espe- 

 cially at the middle, which present a small enlargement. One 

 face is turned toward the parietes of the artery, and the other 

 towards the axis of the vessel. These valves are formed by the 

 inner membrane of the arteries, doubled on itself, and contain- 

 ing in its substance a thin layerof ligamcntous or fibrous tissue; 

 their free margin contains a small cord of this tissue, and its 

 middle a fibro-cartilaginous point. When these valves fall, the 

 face, which corresponds to the ventricle, becomes convex, and 

 the other, which corresponds to the canal, becomes concave; 

 their free edges meet, touch each other, and they exactly close 

 the vessel. In all the rest of their extent the arteries are de- 

 prived of valves. 



The internal surface is smooth, polished, and moistened. 

 The external surface corresponds to the common and particular 

 cellular tissue in which the arteries ramify. The cellular tissue 

 moulded around them, or separated by their presence, forms a 

 cellular sheath for them. This sheath is confounded externally 

 with the rest of the cellular tissue, or with the substance of the 

 organs; internally it is united to the artery in so loose a man- 

 ner, as to permit the latter to slide easily in its interior during 

 the different motions, and to retire within it by contracting in 

 the longitudinal direction when it is divided. This sheath is 

 pretty firm around the arteries of the limbs; in the thorax and 



