456 VEINS. 



consisting of fibrous tissue, their inner of a serous membrane continuous with the 

 serous membrane of the veins. 



Yeins have thinner walls than the arteries, which is due to the small amount 

 of elastic and muscular tissues which they contain. The superficial veins usually 

 have thicker coats than the deep veins, and the veins of the lower limbs are 

 thicker than those of the upper. 



Yeins are composed of three coats ; internal, middle, and external. 



The internal coat is similar in structure to that of the arteries. In the smallest 

 veins, it consists of epithelium and nucleated connective tissue, arranged so as to 

 form an outer and an inner layer ; the latter, which is the thinnest, representing 

 the middle coat. As these vessels approach the capillaries, the epithelium and 

 outer layer of connective tissue become gradually lost. On the contrary, in those 

 of rather larger size, there is superadded a layer of muscular fibre-cells, a circu- 

 lar fibrous coat, with areolar elastic tissue beneath the epithelium, and in the 

 muscular and external coats. In medium-sized veins, the internal coat consists of 

 epithelium supported on one or more striped nucleated lamellae, external to which 

 is a layer of elastic fibrous tissue. In the veins of the gravid uterus, and in the 

 long saphenous and popliteal veins, muscular tissue is one of the component parts 

 of the inner coat. In the largest veins, as the inferior vena cava, the trunks of 

 the hepatic, and in the innominate veins, the internal coat has a structure similar 

 to that already mentioned ; but is somewhat thicker, owing to the increase in the 

 number of the striped lamellae, and the greater thickness of the elastic fibrous coat. 



The middle coat is thin, and differs in structure from the middle coat of arteries 

 in containing a smaller amount of elastic and muscular tissues, and more con- 

 nective tissue. In the smallest veins, as already mentioned, it consists merely of 

 a thin layer of nucleated connective tissue, the fibres of which run in a longitu- 

 dinal direction ; to which is added, in those of rather larger size, a layer of mus- 

 cular tissue, the cells of which are disposed transversely. In medium-sized veins, 

 such as the cutaneous and deep veins of the limbs, as far as the brachial and pop- 

 liteal, and the visceral veins, the middle coat is of a reddish-yellow color, remark- 

 able for its great thickness, being more developed than the same coat in the large 

 veins. It consists of a thick inner layer of connective tissue with elastic fibres, 

 having intermixed in some veins a transverse layer of muscular fibres ; and an 

 outer layer consisting of longitudinal elastic lamellee, varying from five to ten in 

 number, alternating with layers of transverse muscular fibres and connective 

 tissue, which resembles somewhat in structure the middle coat of large arteries. 

 In the large veins, as in the commencement of the vena portas, in the upper part 

 of the abdominal portion of the inferior vena cava, and in the large hepatic 

 trunks within the liver, the middle coat is thick, and its structure similar to that 

 of the middle coat in medium-sized veins ; but its muscular tissue is scanty, and 

 the longitudinal elastic networks less distinctly lamellated. The muscular tissue 

 of this coat is best marked in the splenic and portal veins, it is absent in certain 

 parts of the vena cava below the liver, and wanting in the subclavian vein and 

 terminal parts of the two cavaa. 



The external coat is usually the thickest, increasing in thickness with the size 

 of the vessel; it is similar in structure to the external coat of arteries, but its 

 chief peculiarity is that in some veins it contains a longitudinal network of mus- 

 cular fibres. In the smallest veins, it consists of a thick layer of nucleated con,- 

 nective tissue. In medium-sized veins, it is much thicker than the middle^ coat, 

 and consists of elastic and connective tissues, the fibres of which are longitudinally 

 arranged. In the largest veins, this coat is from two to five times thicker than 

 the middle coat, and contains a large number of longitudinal muscular fibres. 

 This is most distinct in the hepatic part of the inferior vena cava, and at the 

 termination of this vein in the heart ; in the trunks of the hepatic veins ; in all 

 the large trunks of the vena portas ; in the splenic, superior mesenteric, external 

 iliac, renal, and azygos veins. Where the middle coat is absent, this_ muscular 

 layer extends as far as the inner coat. In the renal and portal veins, it extends 



