76 GENERAL ANATOMY. 



composed 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 lamella, 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. 

 The outer coat is similar in all essential respects to that of the arteries. In the 

 large veins, as in the commencement of the vena portae, 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 subcla- 

 vian vein and terminal parts of the two cava3. 



In the largest veins, the outer coat is from two to five times thicker than the 

 middle coat, and contains a larger 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 portae; 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 through the whole thickness of the outer coat: but in the other veins 

 mentioned, a layer of connective and elastic tissues is found external to the 

 muscular fibres. All the large veins which open into the heart are covered for 

 a short distance by a layer of muscular tissue continued on to them from the 

 heart. 



Muscular tissue is wanting in the veins : 1. Of the maternal part of the 

 placenta. 2. In most of the cerebral veins and sinuses of the dura mater. 3. 

 In the veins of the retina. 4. In the veins of the cancellous tissue of bones. 

 5. In the venous spaces of the corpora cavernosa. The veins of the above- 

 mentioned parts consist of an internal epithelial lining, supported on one or 

 more layers of areolar tissue. 



Most veins are provided with valves, which serve to prevent the reflux of 

 the blood. They are formed by a reduplication of the middle and inner coats, 

 and consist of connective tissue and elastic fibres, covered on both surfaces by 

 epithelium; their form is semilunar. They are attached by their convex edge 

 to the wall of the vein; the concave margin is free, directed in the course of 

 the venous current, and lies in close apposition with the wall of the vein as 

 long as the current of blood takes its natural course ; if, however, any regurgi- 

 tation takes place, the valves become distended, their opposed edges are 

 brought into contact, and the current is intercepted. Most commonly two such 

 valves are found, placed opposite one another, more especially in the smaller 

 veins, or in the larger trunks at the point where they are joined by small 

 branches; occasionally there are three, and sometimes only one. The wall of 

 the vein immediately above the point of attachment of each segment of the 

 valve, is expanded into a pouch or sinus, which gives to the vessel, when in- 

 jected or distended with blood, a knotted appearance. The valves are very 

 numerous in the veins of the extremities, especially of the lower extremities, 

 these vessels having to conduct the blood against the force of gravity. They 

 are absent in the very small veins, also in the venae cavae, the hepatic vein, 

 portal vein and its branches, the renal, uterine, and ovarian veins. A few 

 valves are found in the spermatic veins, and one also at their point of junction 

 with the renal vein and inferior cava in both sexes. The cerebral and spinal 

 veins, the veins of the cancellated tissue of bone, the pulmonary veins, and 

 the umbilical vein and its branches, are also destitute of valves. They are 

 occasionally found, few in number, in the venae azygos and intercostal veins. 



The veins are supplied with nutrient vessels, vasa vasorum, like the arteries ; 



