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STRUCTURE OF VEINS. : (oe 
tissue, and are more or less connected with it by fine strands of fibrillated con- 
nective tissue. 
Structure of veins.——The walls of veins are similar in structure to those 
of arteries; they are, however, thinner, so much so that, although veins are 
cylindrical tubes when full of blood, they collapse when empty and their lumina 
almost disappear. The structural details of the three coats vary somewhat in 
different veins; in most the inner coat is marked by folds which constitute valves. 
Like the arteries, the veins are enclosed in connective tissue sheaths. 
The inner coat (tunica intima).—In the majority of the veins the inner coat 
includes an internal endothelial layer, a middle layer of sub-endothelial connective 
tissue, and an outer layer of elastic tissue. The 
inner coat of a vein is less brittle than the inner 
coat of an artery, and is more easily peeled off from 
the middle coat. The sub-endothehal tissue is a 
fine fibrillated connective tissue, less abundant than 
in the arteries, and indeed in many cases it 1s absent. 
The elastic layer consists of lamelle of elastic fibres 
which are arranged longitudinally, and it rarely 
forms a fenestrated membrane. 
One of the chief peculiarities of the inner coat 
is the presence of folds of its substance which con- 
stitute valves. The valves are of semilunar shape, 
and they are usually arranged in pairs. Their convex 
borders are continuous with the vessel wall, and their 
free borders are turned towards the heart; whilst, 
therefore, they do not interfere with the free flow 
of blood onwards, they prevent any backward flow Fic. 544.—Transverse Section oF 
towards the periphery, and they help to sustain the BEE ON AN EN 
column of blood in all vessels in which there is an —  Te™eantima. |B, Tunica media. 
upward flow. Each valve consists of a fold of the 
inner or endothelial layer, strengthened by a little connective tissue. As a general 
rule, the wall of the vein is dilated above each valve into a shallow pouch or sinus ; 
consequently, when the veins are distended they assume a nodulated appearance. 
The middle coat (tunica media) is much thinner than the corresponding coat 
of an artery, and it contains a smaller amount of muscular and a larger amount 
of ordinary connective tissue; indeed, so much does the latter preponderate that 1t 
separates the muscular fibres into a number of bands isolated from each other by 
strands of connective tissue, and the muscle fibres do not form a continuous layer. 
In some of the veins the more internal muscular fibres do not retain the transverse 
direction which is usually met with both in arteries and veins; on the contrary, 
they run longitudinally. This condition is met with in the branches of the 
mesenteric veins, in the femoral and iliac veins, and in the umbilical veins. The 
middle coat is absent in the thoracic part of the inferior vena cava; it is but 
slightly developed in many of the larger veins, whilst in the jugular veins its 
muscular tissue is very small in amount. 
The external coat (tunica externa).—This coat consists of white fibrous and 
elastic tissue. In many of the larger veins a considerable amount of muscular 
tissue is also present; this is the case in the iliac and axillary veins, the’ 
abdominal part of the inferior vena cava, the azygos veins, and in the renal, 
spermatic, splenic, superior mesenteric, portal, and hepatic veins. The external 
coat is frequently thicker than the middle coat, and the two are not easily separ- 
able from one another. 
Vascular and Nervous Supply of arteries and veins.—Blood-vessels.—The 
walls of the blood-vessels are supplied by numerous small arteries, called vasa 
vasorum, which are distributed to the outer and middle coats. They arise either 
from the vessels they supply or from adjacent trunks, and after a short course enter 
the walls of the vessels in which they end. The blood is returned by corre- 
~ spondingly small veins. 
Lymphatics— Although the cell spaces in the middle and inner coats may be 
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