CAPILLARIES. 



129 



clavian, splenic, renal, and dorsalis penis arteries, and in the umbilical 

 arteries they form a considerable inner layer. They are said to occur 

 especially near the places of branching. Between the circular muscles 

 there is a varying amount of connective tissue with wide meshed nets of 

 elastic fibers. The proportion between the muscle and elastic substance 

 varies greatly. In the aorta and pulmonary arteries the elastic tissue 

 far surpasses the muscular, and it predominates also in the carotid, axillary 

 and common iliac arteries. Muscular tissue is ascendant in the distal 

 arteries. The former group of vessels contains the conducting arteries, 



Endothelium. 

 Internal 

 elastic 

 membrane. 





Media. 



FIG. 154. PORTION OF A CROSS SECTION OF THE BRACHIAL ARTERY OF MAN. x 100. 



which always remain freely open; the latter are distributing arteries 

 which by changing their caliber control the blood supply in their areas 

 of distribution. After death these vessels contract, the muscle nuclei 

 becoming spirally twisted, and the intima thrown into longitudinal folds. 

 The blood is forced on into the capillaries and veins. Then as the rigidity 

 of the muscles passes off, the elastic tissue distends the vessel which remains 

 comparatively empty of blood; for this reason the ancients supposed that 

 arteries contained air. The umbilical arteries are exceptionally deficient 

 in elastic tissue and remain contracted, which aids in preventing haemor- 

 rhage when the umbilical cord is ruptured at birth. 



