THE ANATOMY OF THE HEART AND BLOOD-VESSELS 337 



known as the intima, enveloped by a variable number of layers 

 composed of membranes or networks of elastic tissue. The middle 

 coat is made up of alternating layers of elastic fibers and plain 

 muscular tissue; the former running for the most part longitu- 

 dinally and the latter across the long axis of the vessel. The outer 

 coat is the toughest and strongest because it is mainly made up of 

 white fibrous connective tissue; it contains a considerable amount 

 of elastic tissue also, and gradually shades off into a loose areolar 

 tissue which forms the sheath of the artery, or the tunica adventitia, 

 and packs it between surrounding parts. The smaller arteries 

 have all the elastic elements less developed. The internal coat is 

 consequently thinner, and the middle coat is made up mainly of 

 smooth muscular fibers. As a result the large arteries are highly 

 elastic, the aorta being physically much like a piece of india- 

 rubber tubing, while the smaller arteries are highly contractile, in 

 the physiological sense of the word. 



Structure of the Capillaries. In the smaller arteries the outer 

 and middle coats gradually disappear, and the elastic layers of 

 the inner coat also go. Finally in the capillaries the intima alone 

 is left, with a more or less developed layer of connective-tissue 

 corpuscles around it, representing the remnant of the tunica 

 adventitia. These vessels are thus extremely well adapted to 

 allow of filtration or diffusion taking place through their thin 

 walls. 



Structure of the Veins. In these the same three primary coats 

 as in the arteries are found; the inner and middle coats are less de- 

 veloped, while the outer one remains thick, and is made up almost 

 entirely of white fibrous tissue. Hence the venous walls are much 

 thinner than those of the corresponding arteries, and the veins 

 collapse when empty while the stouter arteries remain open. 

 But the toughness of their outer coats gives the veins great 

 strength. 



Except the pulmonary artery and the aorta, which possess the 

 semilunar valves at their cardiac orifices, the arteries possess no 

 valves. Many veins, on the contrary, have such, formed by semi- 

 lunar pouches of the inner coat, attached by one margin and hav- 

 ing the edge turned towards the heart free. These valves, some- 

 times single, oftener in pairs, and rarely three at one level, per- 

 mit blood to flow only towards the heart, for a current in that 



