THE VASCULAR SYSTEM. 



The vascular system is composed of the organs immediately concerned in the 

 circulation throughout the body of the^fiuids which convey to the tissues the nutritive 

 substances and oxygen necessary for their metabolism and carry from them to the 

 excretory organs the waste products formed during metabolism. 



The system is usually regarded as being composed of two portions, the one con- 

 sisting of organs in which circulates the red fluid which we term 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-vasadar system and the latter is the lymphatic 

 system. It must be recognized, however, that the two systems communicate, and that 

 the lymphatic system develops as an outgrowth from the blood-vascular system ; so 

 that while it proves convenient for descriptive purposes to regard the two systems as 

 distinct, nevertheless, they are intimately associated both anatomically and embryo- 

 logically. 



THE BLOOD-VASCULAR SYSTEM. 



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

 vessels, traversing practically all parts of the body, and (2) of a contractile organ, the 

 heart, by whose pulsations the blood is forced through the vessels. The vessels are 

 again divisible into ( i ) vessels which carry the blood from the heart to the tissues 

 and are known as arteries, (2) exceedingly fine vessels which form a net-work in the 

 tissues and are termed capillaries, and (3) vessels which return the blood from the 

 tissues to the heart and are known as veins. 



THE STRUCTURE OF BLOOD-VESSELS. 



Although passing into one another insensibly and without sharp demarcation, 

 where typically represented the arteries, capillaries, and veins present such character- 

 istic histological pictures that they are readily distinguished from one another. 



All blood-vessels, including the heart, possess an endothelial lining which may 

 constitute a distinct inner coat, the tiaiica inti?na, or, as in the capillaries, even the 

 entire wall of the vessel. l5sually, however, the intima consists of the endothelium 

 reinforced by a variable amount of fibro-elastic tissue in which the elastica predomi- 

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

 middle coat, the tunica media, which typically is composed of intermingled lamellae 

 of involuntary muscle and elastica and line fibrillae of fibrous tissue. Outside the media 

 follows the tiinica externa or adventitia, which, although usually thinner than the 

 middle coat, is of exceptional strength and toughness — characteristics conferred by 

 its fibro-elastic tissue and upon which the integrity of a ligature often depends. 



It should be noted that the endothelial tube is the fundamental and primary 

 structure in all cases, the other coats being secondary and variable according to the 

 size and character that the vessel attains. The customary division into the three 

 coats is more or less artificial and in the larger vessels is often uncertain. The 

 recognition of an inner endothelial and an outer musculo-elastic coat often more closely 

 corresponds to the actual arrangement of the tissues than the conventional subdivision 

 into three tunics. 



The endothelial lining of the arteries consists of elongated spindle-shaped 

 plates united by narrow sinuous lines of cement substance which, after silver-staining, 

 map out the irregular contours of the cells with diagrammatic clearness (Fig. 634). 

 At the junction of the plates, occasional accumulations of the cement substance mark 

 minute intercellular areas, the stigmata, that indicate points of less accurate apposi- 

 tion. Within the veins, the endothelial plates are shorter and broader than in the 

 arteries, approaching somewhat irregular polygons in outline. The demarcation of 



43 673 



