THE ARTERIES 115 



are enclosed in delicate connective tissne in which lies a network of 

 capillaries. The richness of the ca])illary network is related to the 

 functional activity of the organ supplied. Where great functional 

 activity of the organ is constant, the meshwork is close and the 

 capillaries are large. A good illustration of this is seen in the 

 capillary network about the alveoli of the lung and the tubules of 

 the kidney. The capillary system of the liver and spleen is atypical 

 and will be described when considering these organs. 



Healing of tissue lesions may involve development of new capillar- 

 ies and of small arterioles and venules. In case such structures form, 

 they first appear as buds from existing capillaries. 



An examination of the circulation in the web between the toes 

 of the frog's foot reveals a rapidly flowing stream of blood cells in 

 the small arterial branches and a much slower streaming in the 

 network of capillaries. Erythrocytes are carried along like lea\'es 

 in a stream, some having to bend in making a turn from one channel 

 into another. The slow flow through capillaries permits diffusion 

 of food and oxygen out into the tissue spaces and a return of organic 

 wastes to the blood stream from the tissues. As the capillaries 

 unite to form small veins, the speed of the current increases again, 

 but is not as rapid as in the corresponding arteries. 



THE ARTERIES. 



The transition from capillaries to arteries is marked by the 

 gradual appearance of smooth muscle and fibroelastic connective 

 tissue. (Fig. (iG.) In general, three coats are usually indicated in 

 a study of the wall of an arter}.- an inner, tunica intima; a middle, 

 tunica media; and an external, tunica externa. The intima is com- 

 posed of the lining endothelium with a slight amount of fibroelastic 

 connecti\'e tissue. The media is characterized b}' smooth muscle 

 associated with varying amounts of fibroelastic connective tissue. 

 The adventitia is chiefly fibroelastic connective tissue. Arteries are 

 usually divided into three groups on the basis of size and composition 

 of the media. The large or elastic arteries include the aorta, pul- 

 monary arteries, carotids, and a few others with a similar structure. 

 The medium-sized, or muscular, arteries include other arteries 

 named by anatomists; these vessels agree in structure, although they 

 vary in size. To the small arteries belong a great number that 

 have no special names. Two more groups might be added, namely, 

 arterioles and precapillary arterioles. Although the composition of 



