The Circulatory System 169 



others, which in turn join to form the large portal vein. This latter 



vein enters the liver, and breaks into capillaries. The hepatic capil- 

 laries unite to form the hepatic vein which enters the postcaval. 



ANTCRIOB JUGULAR. 



INTERNAL JUGULAR 



EXTERNAL JUGULAR. 



INNOMINATE 



RIGHT LYMPHATIC 

 OUCT 



PRECAVML 

 AZYGOUS 



LEFT THORACIC 

 DUCT 



PULMONARY 



POST Cm/AL 



GASTROEPIPLOIC 



INFERIOR MESENTERIC 



ADRENAL 

 DESCENDING COLON 



.GENITAL 



ASCENDING COLON. 



RIGHT COMMON ILIAC 



LEFT COMMON ILIAC 



Fig. 49. — Diagram of the venous circulation of a mammal. 



The Histology of the Blood Vessels. — The wall of a large artery 

 (Fig. 50) is composed of three distinct layers. The innermost of these, 

 the tunica intima, is formed of several different tissues. That portion 

 of the intima which lines the vessel is a special type of epithelium termed 

 endothelium. It is but a single layer of thin cells which are continu- 

 ous with one another, forming a smooth lining. Beneath the en- 

 dothelium is a mass of fibrous connective tissue which merges into the 

 second layer, the tunica media. This middle layer is formed chiefly 

 of elastic connective tissue and muscular fibers. In the larger vessels, 



