THE VASCULAR SYSTEM. 



THE vascular or circulatory system includes the organs immediately 

 concerned in conveying throughout the body the fluids which bring to the 

 tissues the nutritive substances and oxygen necessary for their metabolism 

 and carry away from them to the excretory organs the waste products formed 

 during metabolism. 



The system is composed of two parts, the one consisting of organs in 

 which circulates the blood, while the organs of the other contain a colorless 

 or white fluid known as lymph or chyle. The former of these subsystems 

 is the blood-vascular system and the latter is the lymphatic system. Since, 

 however, the two systems communicate and the lymphatic system develops 

 in close relations with the blood-vascular, it is evident that they are 

 intimately associated both anatomically and embryologically. 



THE BLOOD-VASCULAR SYSTEM. 



The blood-vascular system consists of (i) a system of canals known 

 as the blood-vessels, traversing practically all parts of the body, and 

 (2) of a contractile organ, the heart, by whose contractions the blood 

 is forced through the vessels. The latter, in turn, are divisible into: (a) 

 the arteries, which carry the blood from the heart to the tissues; (<) the 

 capillaries, exceedingly fine vessels which form a network in the tissues ; and 

 (c) the veins, which return the blood from the tissues to the heart. 



General Structure of Blood-Vessels. Although passing into one 

 another insensibly and without sharp demarcation, typical arteries, capillaries 

 and veins present such characteristic histological 

 pictures that they are readily distinguished. All 

 blood-vessels, including the heart, possess an endo- 

 thelial lining which may constitute a distinct inner 



FIG 



'Jfj^f '/' 



a. 124. A, etidothelial lining of small artery after silver-staining. X . B, endothelial cells more 



highly magnified. X 300. 



coat, the tunica intima, or, as in the capillaries, even the entire wall of the 

 vessel. Usually, however, the intima consists of the endothelium reinforced 

 by a variable amount of fibro-elastic tissue, in which the elastica predominates. 

 Except within the walls of capillaries, external to the intima lies a thick middle 

 coat, the tunica media, composed of intermingled lamellae of involuntary mus- 

 cle and elastic tissue and fine fibrous fibrillae. Outside the media follows the 

 tunica externa or adventitia, which, although usually thinner than the middle 



(87) 



