STRUCTURE OF THE LARGER BLOOD-VESSELS 



89 



The outer coat is formed of connective tissue with a good many 

 elastic fibres, especially next the middle coat. The strength of an 

 artery depends largely upon this coat ; it is far less easily cut or torn 

 than the other coats, and it serves to resist undue expansion of the 



FIG. 112. ELASTIC NET- 

 WORK OF ARTERY. 



(Toldt.) 



FIG. 113. MUSCULAR FIBRE-CELLS FROM 

 SUPERIOR THYROID ARTERY. (340 

 diameters.) 



FIG. 114. SECTION OF THORACIC AORTA AS SEEN UNDER A LOW POWER. (Toldt.) 



a, the inner coat consisting of three layers, viz. : 1. Epithelium seen as a fine line. 2. Sub- 

 epithelial. 3. Elastic layers. In the part of the inner coat, at its junction with the 

 middle, a layer of longitudinal muscular fibres is represented as cut across, b, middle coat 

 with its elastic membranes ; c, outer coat with two vasa vasorum. 



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. 114) differs in some 



