84 GENERAL ANATOMY. 



tive-tissue cells is found around the simple capillary tube, particularly in the 

 larger ones, and in places such as the lymphatic glands where the capillaries 

 are supported by a retiform connective tissue. 



In the largest capillaries (which ought, perhaps, to be described rather as the 

 smallest arteries) there is, outside the endothelial layer, a muscular layer, consisting 

 of contractile fibre-cells, arranged transversely, as in the tunica media of the larger 

 arteries (Fig. 60). 



The veins, like the arteries, are composed of three coats internal, middle, and 

 external ; and these coats are, with the necessary modifications, analogous to the 

 coats of the arteries ; the internal being the endothelial, the middle the muscular, 

 and the external the connective or areolar. The main difference between the 

 veins and the arteries is the comparative weakness of the middle coat of the 

 former, and to this it is due that the veins do not stand open when divided, as 

 the arteries do, and that they are passive rather than active organs of the 

 circulation. 



In the veins immediately above the capillaries the three coats are hardly to be 

 distinguished. The endothelium is supported on an outer membrane separable into 

 two layers, the outer of which is the thicker, and consists of a delicate, nucleated 

 membrane (adventitia), while the inner is composed of a network of longitudinal 

 elastic fibres (media). In the veins next above these in size (one-fifth of a line, 

 according to Kolliker) a muscular layer and a layer of circular fibres can be traced, 

 forming the middle coat, while the elastic and connective elements of the outer 

 coat become more distinctly perceptible. In the middle-sized veins the typical 

 structure of these vessels becomes clear. The endothelium is of the same character 

 as in the arteries, but its cells are more oval, less fusiform. It is supported by a 

 connective-tissue layer, consisting of a delicate network of branched cells, and 

 external to this is a layer of longitudinal elastic fibres, but seldom any appearance 

 of a fenestrated membrane. This constitutes the internal coat. The middle coat is 

 composed of a thick layer of connective tissue w r ith elastic fibres, intermixed, in some 

 veins, with a transverse layer of muscular fibres. The white fibrous element is 

 in considerable excess, and the elastic fibres are in much smaller proportion in the 

 veins than in the arteries. The outer coat consists of areolar tissue, as in the 

 arteries, with longitudinal elastic fibres. In the largest veins the outer 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 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. 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 tissue is found external to the muscular fibres. All the large 

 veins Avhich open into the heart are covered for a short distance with u layer of 

 striped 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 the venous 

 sinuses of the dura mater and the veins of the pia mater of the brain and spinal 

 cord ; (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 endothelial lining supported on one or more 

 layers of areolar tissue. The internal and external jugular veins and the subclavian 

 vein are said to contain either no muscular fibres at all, or at all events only a 

 slight amount in their middle coat. 



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

 blood. They are formed by a reduplication of the inner coat, strengthened by 

 connective tissue and elastic fibres, and are covered on both surfaces with endo- 

 thelium, the arrangement of which differs on the two surfaces. On the surface 

 of the valve next the wall of the vein the cells are arranged transversely ; whilst 

 on the other surface, over which the current of blood flows, the cells are arranged 

 vertically in the direction of the current. Their form is semilunar. They are 



