90 THE ESSENTIALS OF HISTOLOGY 



respects in structure from an ordinary artery. Its inner coat contains 

 a considerable thickness of sub-epithelial connective tissue, but its 

 elastic layers 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 development of elastic tissue in the middle coat, this tissue 

 forming membranous layers which alternate with layers of the mus- 

 cular 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 reference 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. In some parts of the umbilical arteries there is a com- 

 plete 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 mesenteric, splenic, renal, &c.), external to the circular 

 fibres, and therefore in the outer coat of the artery. The larger 

 arteries themselves receive blood-vessels, vasa vasorum, which ramify 

 chiefly in the external coat. Nerves, derived for the most part from the 

 sympathetic system, are distributed to the muscular tissue of the 

 middle coat. 



The veins (fig. 115) on the whole resemble the arteries in structure, 

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



FIG. 115. TRANSVERSE SECTION OF PART OF THE WALL OF ONE OF THE POSTERIOR 



TIBIAL VEINS (MAN). 



a, epithelial and sub-epithelial layers of inner coat : 6, 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 epithelium -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 



