STEUCTUKE OF AETEEIES, 



869 



FIG. 749. TRANSVERSE SECTION OP 

 THE WALL OF A VEIN. 



A, Tunica intima. B, Tunica media. 

 C, Tunica externa. 



In all but the smallest arteries numerous elastic fibres are also present. The 

 elastic element is specially strong near the middle coat in small and medium 

 sized vessels, and is sometimes described as an external elastic membrane. In 

 some arteries longitudinally arranged unstriped muscular fibres are also found in 

 the external coat. 



Vagina Vasis. In addition to the three tunics above described, arteries 

 are enclosed in a sheath of the surrounding connective tissue, and are more 

 or less connected with it by fine strands of fibrillated connective 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 luniina almost disappear. 

 The structural details of the three tunics vary 

 somewhat in different veins ; in most the innermost 

 tunic is marked by folds which constitute valves. 

 Like, the arteries, the veins are enclosed in connec- 

 tive tissue sheaths. 



Tunica Intima. In the majority of the veins 

 the tunica intima includes an internal endothelial 

 layer, a middle layer of subendothelial connective 

 tissue, and an outer layer of elastic tissue. The 

 innermost tunic of a vein is less brittle than the 

 inner coat of an artery, and is more easily peeled off 

 from the middle coat. The subendothelial tissue 

 is a fine fibrillated connective tissue, less abundant 

 than in the arteries, and in many cases it is absent. 

 The elastic layer consists of lamellae of elastic 

 fibres which are arranged longitudinally ; it rarely 

 has the appearance of a fenestrated membrane. 



One of the chief peculiarities of the tunica 

 intima is the presence of folds of its substance which constitute valves. The cusps 

 of 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 from the periphery, they prevent any backward flow towards it, and 

 they help to sustain the column of blood in all vessels in which there is an upward 

 flow. Each valve cusp consists of a fold of the endothelial layer, strengthened 

 by a little connective tissue. As a general rule, the wall of the vein is dilated on 

 the central side of each valve into a shallow pouch or sinus ; consequently, when 

 the veins are distended they assume a nodulated appearance. The valves are more 

 numerous in the deep than in the superficial veins, and in the veins of children 

 than in the veins of adults. 



Tunica Media. The middle tunic is much thinner than the corresponding 

 tunic 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 it separates the muscular fibres into a number of bands, which are isolated from 

 one another by strands of connective tissue ; therefore 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 tunic 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. 



Tunica Externa. This tunic 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 



nferior vena cava, the azygos and hemiazygos veins, and in the renal, spermatic, 



splenic, superior mesenteric, portal, and hepatic veins. The striped muscle fibres of 





