STRUCTURE OF VEINS. clxxiii 



immediately beneath the skin, and thence named subcutaneous, the latter 

 commonly accompanying 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 ; thus, 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. Notwithstanding their comparative thinness, however, 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 tolerable size, three coats may be distinguished, which, as in the 

 arteries, have been named external, middle, and internal 



The internal coat is less brittle than that of the arteries, and therefore 

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

 respects, the two are much alike. It consists of an epithelium, a striated 

 lamella containing nuclei, and the usual elastic layers ; these occur as dense 

 lamelliform networks of longitudinal elastic fibres, and but seldom as fene- 

 strated membranes. 



The middle coat is much thinner than that of the arteries, and its muscu- 

 lar tissue has a much larger admixture of white connective tissue. Its 

 fibres are both longitudinal and circular, the one set alternating with the 

 other in layers. The former are well developed elastic fibres, longitudinally 

 reticulating ; the circular layers consist of bundles of muscular fibre-cells 

 and white connective tissue, mixed with a smaller proportion of fine 

 elastic fibres. In medium-sized veins the middle coat contains several suc- 

 cessions of the circular and longitudinal layers, but the latter are all more 

 or less connected together by elastic fibres passing through the intervening 

 circular layers. In the larger veins the middle coat is less developed, 

 especially as regards its muscular fibres, but in such cases the deficiency may 

 be supplied by muscularity of the outer coat. Kolliker states that the 

 middle coat is wanting altogether in most of the hepatic part of the vena 

 cava, and in the great hepatic veins ; and even where its thickness is con- 

 siderable, it is less regularly or not at all disposed in layers, and its muscular 

 fibres are more scanty. The muscularity of the middle coat is best marked 

 in the splenic and portal veins ; it is apparently wanting in certain parts of 

 the abdominal cava and in the subclavian veins. 



The external coat is usually thicker than the middle coat ; it consists of 

 connective tissue and longitudinal elastic fibres. In certain large veins, as 

 pointed out by Remak, this coat contains a considerable amount of plain or 

 non-striated muscular tissue. The muscular elements are well marked in 

 the whole extent of the abdominal cava, in which they form a longitudinal 

 network, occupying the inner part of the external coat ; and they may be 

 traced into the renal, azygos, and external iliac veins. The muscular tissue 

 of the external coat is also well developed in the trunks of the hepatic veins 

 and in that of the vena portse, whence it extends into the splenic and 

 superior mesenteric. 



