THK CIRCULATION. 159 



this blood to the liver, where it passes through the 

 capillaries of that organ before reaching the hepatic veins 

 and so returns to the heart. 



It will be observed that the blood passing this way 

 passes in one complete circulation through three sets of 

 capillaries instead of two. This circulation of blood through 

 the stomach, spleen, pancreas, intestine and liver is called 

 the portal circulation. The physiological reasons why the 

 blood from the spleen, stomach, and intestines should first 

 be sent to the liver before being returned to the heart, will 

 be discussed in detail in the chapter on nutrition. In an- 

 ticipation it may be said here that some of the deepest phy- 

 siological reasons exist for such a course. 



THE HISTOLOGY OF ARTERIES, VEINS AND CAPILLARIES. 



Examining with the microscope, an artery is seen to 

 consist of three coats, called the inner, the middle and the 

 outer coat. The inner coat consists of a delicate membrane 

 of closely packed elastic tissue, on the outside of which 

 there is a still more delicate serous membrane of flattened 

 cells bound to the elastic membrane by a finely fibrillated 

 connective tissue. The middle coat consists of alternating 

 layers of plain muscular tissue, which is usually arranged 

 circularly, and layers of yellow elastic fibers running mainly 

 longitudinally. The outer coat is the toughest, and is 

 made up mainly of closely felted bundles of white fibrous 



Fig. 78. CROSS-SECTION OF PORTION OF THE WALL OF HUMAN TIBIAL ARTERY. (E. A. S.) 

 a, inner epithelial coat; 6, the elastic fenestra ted membrane ; c, muscular layer; d, 

 outer coat of connective tissue fibres mainly. 



tissue. This coat then gradually shades off into the looser 

 connective tissue in which the artery is packed. These 



