174 HUMAN ANATOMY. 



in layers forms nets, constituting so-called perforated or fenes- 

 trated membranes. 



The tunica intima, or internal coat, thinnest, most trans- 

 parent, and elastic, is composed of lining endothelium, basement 

 membrane, and layers of elastic tissue. It becomes continuous 

 with capillaries. The walls of larger arteries are supplied by 

 blood-vessels from neighboring arteries. Nutrient arteries form 

 a net-work of capillaries, and returning veins empty into con- 

 tiguous veins. The nerves are both medullated and non- 

 medullated and chiefly pass to the muscle-fibres. 



Arteries dilate and contract with the action of the heart, con- 

 stituting the pulse. 



THE VEINS. Larger, more numerous, and more capacious 

 than the arteries. With one exception (the portal vein) they 

 convey blood to the heart. The large veins emptying into the 

 heart are: 



Four pulmonary; 



Superior and inferior venae cavaa; 



Coronary veins from walls of the heart through the coronary 

 sinus. 



The larger arteries have each one companion vein, the 

 medium arteries two. In some positions they form venous plex- 

 uses, corresponding to arterial anastomoses. 



Many large veins have valves, usually in pairs, attached by 

 convex border. 



There are, however, no valves in the pulmonary, superior 

 and inferior cavaa, azygos, portal vein and branches, hepatic, 

 renal, uterine, and spinal, and most of those of head or neck. 



In certain membranes and organs channels exist, lined by 

 internal coat of blood-vessel, termed venous sinuses, as in the 

 dura mater, bones, and uterus. 



The coats of the veins are similar to those of the arteries, 

 and differ chiefly in the weakness of the middle or muscular coat, 

 which allows them to remain open. 



The external fibrous coat has also longitudinal, unstriated 

 muscular tissue in veins of abdominal cavity : 



The middle, unstriated muscular and fibrous tissue; 



The internal, no fenestrated membrane. 



The walls have nutritive vessels^ and are well supplied with 

 non-medullated and medullated nerves, which pass to the muscu- 

 lar coat. 



Both arteries and veins are insensitive in health. 



THE CAPILLARIES communicate with the terminations of the 

 arteries and veins, and do not communicate with tissue proper, 

 which is supplied by imbibition. They vary in size in different 



