THE CIRCULATION OF THE BLOOD. 



297 



THE ARTERIAL CIRCULATION. 



Structure and Properties of the Arteries. The arteries serve 

 to transmit the blood ejected from the heart to the capillaries; that 

 this may be accomplished they divide and subdivide and ultimately 

 penetrate each and every area of the body. Their repeated division 

 is attended by a diminution in size, a decrease in the thickness and a 

 change in the structure of their walls. 



A typical artery consists of three coats: an internal, the tunica 

 intima; a middle, the tunica media; 

 an external, the tunica adventitia. 



The internal coat consists of a 

 structureless elastic basement mem- 

 brane, on the inner surface of which 

 rests a layer of elongated spindle- 

 shaped epithelial cells. The middle 

 coat consists of several layers of cir- 

 cularly disposed, non-striated muscle- 

 fibers, between which are networks 

 of elastic fibers. The external coat 

 consists of bundles of connective tis- 

 sue of the white fibrous and yellow 

 elastic varieties. Between the exter- 

 nal and middle coats there is an ad- 

 ditional elastic membrane. In the 

 small arteries there is but a single 

 layer of muscle-fibers. In the large 

 arteries the elastic tissue is very 

 abundant, exceeding largely in 

 amount the muscle-tissue. It is also 



more closely and compactly arranged. 



mi i , n i 11 FIG. 134. COATS OF A SMALL 



The external coat is well developed ARTERY, a. Endothelium. b. 



in the large arteries (FigS. 135 and Internal elastic lamina, c. Cir- 



j ., AN cular muscular fibers of the 



T .i" . , . middle coat. d. The outer 



In virtue of the presence in their C oa.t.(Landois and Stirling.) 



walls of both elastic and contractile 



elements, the arteries possess the two properties of elasticity and con- 

 tractility. 



The elasticity is especially well developed in the large arteries, 

 which are capable, therefore, of both distention and elongation, and, 

 when the distending force is withdrawn, of returning to their previous 

 condition. The elasticity permits of a wide variation in the amount 

 of blood the arterial system can hold between its minimum and 

 maximum distention. Thus the capacity of the aorta and carotid 

 artery of the rabbit can be increased four times and six times respec- 



