n STRUCTURE OF ARTERIES 33 



more complex in structm-e. The inner coat now consists 

 of thin flattened cells lying on a distinct and special layer 

 of elastic tissue of variable thickness. The middle coat, 

 to which the thickness of the arterial wall is chiefly due, 

 consists of alternating layers of plain muscle fibres, 

 lying transversely to the axis of the vessel, and of elastic 

 fibres which as a rule run lengthwise. The outer coat, 

 also of considerable thickness, is made up of fibi'ous con- 

 nective tissue, mixed with fibres of elastic tissue. 



h-a.-s 





Pig. 8. — Transverse Section of part of the Wall of a mepifm-sized 

 Artery, magnified V5 diameters. (Schaj-er.) 



a, epithelial (endothelial) layer of inner coat ; 6, elastic layer (fenes- 

 trated membrane) of inner coat, appearing in section as a bright line ; 

 c, muscular layers (middle coat) ; </, outer coat, consisting of connective 

 tissue bundles, interspersed with connective tissue nuclei, and, es- 

 pecially near the muscular coat, with elastic fibres cut across. 



From the above description of the structure of an artery 

 we see at once that arteries are strong, muscular and 

 elastic. The largest arteries are, as a rule, characteris- 

 tically more elastic than the smaller, while in the latter 

 the muscular tissue is present in large amount relatively 

 to the elastic tissue. The significance of this difference 

 will become apparent later on (see pp. 62 and 65). 



The plain muscular fibres in the arterial wall possess 

 that same power of contraction, or shortening in the long, 

 and broadening in the narrow, dii-ections, which, as was 

 stated in the preceding Lesson, is the special property of 

 muscular tissue. And when they exercise this power, they, 



