366 BLOOD-VESSELS. 



Minute ganglia are found in various parts connected with the arteries, but their 

 existence does not appear to be by any means universal. 



VEINS. 



Mode of distribution. The veins are ramified throughout the body, like the 

 arteries, but in most regions and organs of the body they are more numerous and 

 larger, so that the venous system is altogether more capacious than the arterial. 

 The pulmonary veins form an exception to this rule, for they do not exceed in 

 capacity the pulmonary arteries. 



The veins are arranged in a superficial and a deep set, the former running imme- 

 diately beneath the skin, and thence named subcutaneous, the latter usually accom- 

 panying the arteries, and named vence comites vel satellites arteriarum. The large 

 arteries have usually one accompanying vein, and the medium-sized and smaller 

 arteries two, but there are exceptions to this rule. The veins within the skull and 

 spinal canal, the hepatic veins, and the most considerable of those belonging to the 

 bones, run apart from the arteries. 



The communications or anastomoses between veins of considerable size, are more 

 frequent than those of arteries of equal magnitude. 



Structure. The veins have much thinner coats than the arteries, and collapse 

 when cut across or emptied ; whereas a cut artery presents a patent orifice. But, 

 notwithstanding their comparative thinness, the veins possess considerable strength, 

 more even, according to some authorities, than arteries of the same calibre. The 

 number of their coats has been differently reckoned, and the tissues composing them 

 differently described by different writers, and this discrepancy of statement is 

 perhaps partly due to the circumstance that all veins are not perfectly alike in 

 structure. In most veins of moderate size, three coats may be distinguished, which, 

 as in the arteries, have been named external, middle, and internal. 



Internal coat. This is less brittle than that of the arteries, and therefore 

 admits of being more readily peeled off without tearing ; but, in other respects, the 



Fig. 425. TRANSVERSE SECTION 0;? PART 



OP THE WALL OF ONE OF THE POSTERIOR 

 TIBIAL VEINS (MAN). (E. A. S. ) 



o, epithelial and subepithelial layers of 

 inner coat ; b, elastic layers of inner coat ; 

 c, middle coat consisting of irregular layers 

 of muscular tissue, alternating with connec- 

 tive tissue, and passing somewhat gradually 

 into the outer connective tissue and elastic 

 coat, d. 



two are much alike. It consists of an 

 endothelium, a sulendothelial connective tissue layer, and an elastic layer (fig. 425, a, V). 



The endothelium of the veins is similar in character to that of the arteries, but 

 the cells are shorter and broader. The subendothelial layer is less developed in most 

 veins than in the arteries, and indeed is absent altogether in many. It is better 

 marked in some of the medium-sized veins than in the larger trunks. The elastic 

 tissue of the inner coat occurs as dense lamelliform networks of longitudinal elastic 

 fibres, and but seldom as fenestrated membranes. Longitudinal muscular bundles, 

 as well as isolated contractile cells, are found in the inner coat of some veins. 1 



Middle coat. This tunic is thinner than that of the arteries, and has a much 

 larger mixture of white connective tissue. It is pervaded by an elastic network, 

 but this is less conspicuous in the veins than in the arteries. In the veins of the 

 limbs (especially the upper limb) and in those of some other parts, the muscular 



1 Veins which are apparently healthy, sometimes exhibit here and there well-marked thickenings of 

 the inner coat ; these thickenings may represent rudimentary valves (Bardeleben). 



