THE BLOODVESSELS. 



55 



muscular fibres in the outer coat ; and all the coats of the veins in the 

 limbs, especially in the lower limbs, are thicker than elsewhere. Upon 

 the great veins as they are entering the auricles of the heart, even a 

 few striated muscular fibres may be found. The veins have their vasa 

 vasorum, and a few nerves. Within many of the veins, at certain 

 intervals, and also at the mouths of their branches, Fig. 31, are found 

 little projecting folds or flaps, called valves, formed by the internal 



Fig. 31. 



Fig. 31. (The Author) a, a portion of a medium-sized artery laid open, showing its three coats, viz., the 

 external areolar coat, the middle muscular coat, and the internal elastic and epithelial coat. 6, a piece 

 of a medium-sized vein laid open, exhibiting its three coats. Besides this, it shows the valves in the inte- 

 rior of the vein. 



coat, strengthened by a few fibrous bands. These are either single, 

 double, or even three in number ; and are always so attached that their 

 free edge is towards the heart. They are most numerous in the veins 

 of the limbs, especially of the lower limbs. Valves are not found in 

 the smallest veins, nor in the largest, as the venae cavse ; nor are they 

 found in the pulmonary veins or hepatic veins, which return the blood 



Fig. 32. 



Fig. 32. (Allen Thomson.) Outline of the under surface of a frog's hind foot, to show the general branch- 

 ing of its small arteries and veins, in the web between the toes. 



from the lungs and liver. They are also absent in the cranial, spinal, 

 renal, portal, and a few other veins. 



The capillaries are the intermediate vessels which connect the finest 



