224 PRACTICAL HISTOLOGY. [XIX. 



of endothelium resting on an elastic lamina composed of elastic 

 networks, or an elastic membrane. In many arteries, however, 

 there is a layer of connective tissue between the epithelium and the 

 elastic lamina the sub-epithelial layer. 



(2.) Tunica media, or middle coat, consists of a varying number 

 of layers of circularly-disposed, short, smooth, muscular fibres ; 

 but in most arteries, and chiefly in the large ones, it is intermixed 

 with elastic fibres or laminae. 



(3.) Tunica adventitia, composed of fibrous tissue with elastic 

 fibres, the latter especially numerous near the middle coat. 



There are, however, great variations in structure in arteries, 

 according to their size and other conditions. 



Veins. Speaking broadly, the veins have the same general 

 structure as the arteries. They are, however, much thinner, and 

 some of them have valves. They consist of three coats ; the inner 

 coat is thinner than in arteries, and the elastic lamina thinner 



FIG. 207. A. Fibres of the heart cut longitudinally ; B. Transverse sections of the 

 heart-fibres ; c. Cell ; n. Nucleus ; a. Connective tissue ; v Vein. 



and often incomplete. The shape of the endothelial cells is different 

 (fig. 213, V). The middle coat is also thinner, and has less 

 muscular and elastic tissue, and relatively more connective tissue. 

 The outer coat is relatively very strong, and is composed of fibrous 

 tissue, which sends processes into the middle coat. There are, 

 however, great variations in the structure of veins. 



Capillaries. They form networks of fine tubes of uniform 

 diameter, sufficient to allow blood-corpuscles to pass along them 

 freely in single file. The arrangement of the network varies in 

 different tissues. The walls, when examined fresh, appear to be 

 homogeneous, but they are composed of flattened epithelial cells 

 or endothelial cells united to each other by their edges by cement 

 substance, which is blackened by silver nitrate. 



1. Heart. Harden small pieces of the heart (human) in alcohol. 

 Stain a small piece in bulk in borax-carmine, and then place it for 

 twenty-four hours in acid alcohol. It is best to cut sections by the 

 paraffin method and mount them in balsam. A very instructive 



