INTRODUCTORY. 



21 



external or fihro -areolar coat consists of more or less con- 

 densed areolar tissue, with a slight predominance of elastic 

 fibres ; it serves to protect the vessels and to form the 

 nidus in which the small nutrient vessels of the arterial 

 coats (vasa vasorum) break up into capillaries. It is 

 directly continuous with the areolar tissue of the channel 

 through which the artery runs, which sometimes becomes 

 developed into a marked sheath enclosing other structures 

 in addition to the artery ; the carotid sheath is an example. 

 The middle or musculo -elastic coat consists of unstriated 

 muscular fibre in union with yellow elastic tissue. The 

 muscular fibres assume a direction transverse to the longi- 

 tudinal axis of the vessel ; they predominate in the smallest 

 arteries, those most distant from the heart. The elastic 

 fibres mainly take a longitudinal direction, but are 

 arranged in the form of a network ; in the largest arteries, 

 as the aorta, they form the major portion of the arterial 

 wall. The internal coat is of a serous nature, presenting 

 an internal layer of tessellated epithelium and a peculiarly 

 modified basement membrane. The latter, termed the fenes- 

 trated membrane of Henle, shows a peculiar tendency to 

 curl up when removed from its situation, and it has 

 numerous perforations. These characters seem to indi- 

 cate its intermediate nature between common basement 

 membrane and ordinary yellow elastic tissue with 

 which in some vessels its external surface intimately blends. 

 This coat of the artery is continuous centrally with the 

 endocardium, peripherally with the constituent membrane 

 of the capillaries. The course of arteries is in some cases 

 irregular, either as a result of disease or of original 

 formation. Descriptions of these cannot be accurate /or 

 all cases. When arteries run near one another they 

 almost always anastomose or inosculate, communicate 

 more or less closely, either directly or by branches. 

 Hence provision is made for continuance of supply of 

 blood after obliteration of one source ; the collateral sources 

 after injury resulting in obliteration of an important 

 artery generally prove equal to the emergency— a point of 

 great value in surgery. 



The veins carry blood from the capillaries to the heart 

 with exception of the portal veins, which convey it from 

 the alimentary organs of the abdomen to the liver, where it 

 is purified prior to admission into the general round of 



