] 10 THE ESSENTIALS OF HISTOLOGY. 



vessel. Its outer limit is not sharply marked, for it tends to blend 

 with the surrounding connective tissue (hence it has been termed 

 tunica adventitia). 



Variations in structure. The aorta (fig. 135) differs in some respects in 

 structure from an ordinary artery. Its inner coat contains a considerable 

 thickness of sub-epithelial connective-tissue, but the elastic layers of this coat 

 are chiefly composed of fine fibres, and are not especially marked off from 

 those of the middle coat, so that the inner and middle coats appear almost 

 blended with one another. On the other hand, there is a very great develop- 

 ment of elastic tissue in the middle coat, forming membranous layers which 

 alternate with layers of the muscular tissue. A good deal of connective 

 tissue also takes part in the formation of the middle coat, so that the wall is 

 unusually strong. The inner and middle coats constitute almost the entire 

 thickness of the wall, the outer coat being relatively thin. 



The other variations which occur in the arterial system chiefly have refer- 

 ence to the development and arrangement of the muscular tissue. Thus in 

 many of the larger arteries there are longitudinal muscular fibres at the 

 inner boundary of the middle coat, and in some arteries amongst the circular 

 fibres of the middle coat. This is the case in the aorta. Iri some parts of 

 the umbilical arteries there is a complete layer of longitudinal fibres internal 

 to the circular fibres and another external to them, whilst the amount of 

 elastic tissue is very small. Longitudinal fibres are also present in some 

 other arteries (iliac, superior meseiiteric, splenic, renal, etc.), external to the 

 circular fibres, and therefore in the outer coat of the artery. 



The veins (fig. 136) on the whole resemble the arteries in structure, 

 but they present certain differences. In the internal coat the same 



FIG. 136. TRANSVERSE SECTION OF PART OF THE WALL OF ONE OF THE POSTERIOR 

 TIBIAL VEINS (MAN). 



a,, epithelial and sub-epithelial layers of inner coat ; b, elastic layers of inner coat; c, middle 

 coat consisting of irregular layers of muscular tissue, alternating with connective tissue, 

 and passing somewhat gradually into the outer connective tissue and elastic coat, d. 



layers may be present, but the elastic tissue is less developed and 

 seldom takes the form of a complete membrane. The endothelium-cells 

 are less elongated than those of the arteries. The middle coat (c) 

 contains less elastic tissue and also less muscular tissue, being partly 

 occupied by bundles of white connective-tissue fibres. These are 

 derived from the external coat, which is relatively better developed 

 in the veins than in the arteries, so that, although thinner, their walls 

 are often stronger. 



Many of the veins are provided with valves, which are semilunar folds 



