326 THE CIRCULATION OF THE BLOOD. [CHAP, xxvin. 



the various tissues and organs. As the arteries divide and sub- 

 divide, the veins follow a contrary course. They commence from 

 the capillary plexuses of the tissues and organs by minute radicle 

 vessels, which by their junction form larger ones, and these again 

 unite to form still larger ones ; and so, by the fusion of the smaller 

 veins, larger trunks are produced, until, at length, the venous blood 

 from all parts of the body, is returned to the heart by two great 

 venous trunks, the superior and the inferior vence cavce. 



Veins are much more numerous, and for the most part more 

 capacious than arteries. In the extremities and the trunk they are 

 arranged upon two planes, a superficial plane and a deep-seated 

 one : the latter accompanying the deep-seated arteries ; the former 

 being immediately adjacent to the skin. The superficial veins are 

 more numerous, and present greater variety both as to number 

 and arrangement, than the deep veins. Their smaller radicles 

 anastomose in the same manner as has been just described in the 

 arteries. 



A distended vein has a cylindrical form, which, however, in some 

 is interrupted here and there by a knotted appearance, caused by 

 its enlargement at the situation of its various sets of valves. The 

 coats of veins are essentially the same as those of arteries, but are 

 less developed. Proceeding from without inwards, we find, first, 

 an external tunic, composed of a thin layer of areolar tissue, answer- 

 ing in structure, position, and function to the external coat of 

 arteries. Secondly, we find a fibrous tunic of which the outer 

 portion consists of circular fibres; the inner portion of longitudinal 

 fibres both coarse and fine. The circular fibres are but slightly 

 developed ; they are of the same nature as those in arteries and in 

 the larger veins, and exhibit somewhat of the penniform disposition, 

 which we have described in the fibres of the arterial circular tunic. 

 With them are mingled unstriped muscular fibres in less quantity 

 but of precisely the same form and character as those in arteries. 

 In the veins, near the heart, these circular fibres are replaced by 

 similarly disposed muscular fibres of the striped kind continuous 

 with and resembling those of the auricles.* 



The longitudinal fibres are well developed, consisting of the outer 

 coarse layer which, in the large veins, as the cava ascendens, are 

 arranged in the form of large bundles, parallel to the long axis of 



* Rauschel states, that these fibres can be traced in the superior cava, as 

 far as the clavicle, and in the inferior as far as the diaphragm, and in the 

 pulmonary veins as far as the first subdivision of each. 



