STRUCTURE OF THE LARGER BLOOD-VESSELS. 10 



as the carotid and its immediate branches, but in the smaller arteries of 

 the limbs the middle coat is almost purely composed of muscular tissue. 

 The muscular fibres are comparatively short, with long rod-shaped 

 nuclei, and are often irregular in shape (as in fig. 134). 



FIG. 133. ELASTIC NET- 

 WORK OF ARTERY. 

 (Toldt.) 



FIG. 134. MUSCULAR FIBRE-CELLS FROM 



SUPERIOR THYROID ARTERY. (340 dia 



meters. ) 





FIG. 135. 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. 



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

 elastic fibres, especially next to 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 



