VEINS. 173 



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 state- 

 ment 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 ex- 

 ternal, 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 two are much alike. It consists of an epiihelmil 

 layer, a siibeptthelial connective tissue taijer, said to be the most marked 

 in the smaller veins, and the usual clastic layers ; these occur as dense 

 lamelliform networks of longitudinal elastic fibres, and but seldom as 

 fenestrated membranes. 



Middle coat. — This coat is much thinner than that of the arteries, 

 and its muscular tissue has a much larger admixture of white connec- 

 tive 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 successions 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 mus- 

 cular fibres, but in such cases the deficiency may be supplied by muscu- 

 larity of the outer coat. The middle coat is wanting altogether in most 

 of the hepatic part of the vena cava, and in the great hepatic veins 

 (Kolliker) ; its muscularity is best marked in the splenic and portal veins. 



External coat. — This is usually thicker than the middle coat; it 

 consists of dense areolar tissue and longitudinal elastic fibres. In cer- 

 tain large veins, as pointed out by Remak, this coat contains a consi- 

 derable 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 

 portge, whence it extends into the splenic and superior mesenteric. 



Other veins present iDeculiarities of structiire, especially in respect of muscu- 

 larity. 1. The striated muscular tissue of the auiicles of the heart is prolonged 

 for some way on the adjoining- jjart of the vense cavce and pulmonary veins. 

 2. The plain muscular tissue is largely developed in the veins of the gravid uterus, 

 in which, as well as in some other veins, it is described as being present in all 

 three coats. 3. On the other hand, muscular tissue is wanting in the following 

 veins, viz., a, those of the maternal part of the placenta ; /;, most of the veins 

 of the brain and pia mater ; c. the veins of the retina ; d, the venous sinuses of 

 the dura mater ; e, the cancellar veins of the bones ; /, the venous spaces of the 

 coi"pora cavernosa. In most of these cases the veins consist merely of an epithe- 

 lioid layer and a layer or layers of connective tissue more or less developed ; in 

 the coiTDora cavernosa the epithelium is applied to the trabecular tissue. It may 

 be added that in the thickness of their coats the superficial veins surpass the 

 deep, and the veins of the lower limbs those of the upper. 



