66 PRACTICAL MICROSCOPY. 



Fig. 50 has been drawn from fresh cardiac muscle, teased in nor- 

 mal salt solution, and tinted with eosin. 



BLOOD-VESSELS. 



Blood-vessels include arteries, arterioles, capillaries, venules, 

 and veins. They are all lined with flattened endothelial cells 

 cemented by their edges; and their walls are constructed from non- 

 striated muscular, yellow elastic and fibrous connective tissues, in 

 proportions varying according to the size and function of the vessel. 

 Arteries are the active, while the veins are comparatively passive 

 agents in the circulation of the blood. 



The large arteries are eminently elastic, from preponderance of yel- 

 low elastic tissue; while the arcerioles are eminently contractile, from 

 excess of muscular fibre. 



FIG. 51. TRANSVERSE SECTION OF A MEDIUM-SIZED ARTERY, PARTLY DIAGRAMMATIC. 



A. The endothelial cells in profile. 



B. Elastic and connective tissue supporting the endothelium. 



C. The internal elastic lamina or fenestrated membrane. A, B, and C constitute the INTIMA 

 of the artery. 



D. The MEDIA. It consists of muscular and elastic tissues in alternating layers. 



E. Points to one of the elastic layers. 



F. The ADVENTITIA. Loose, connective tissue, with few elastic fibres. 



Arteries possess three coats: the intima (internal), media (middle), 

 and adventilia (external). 



Fig. 51 represents a medium-sized typical artery. The intima, or 

 internal coat (1), consists of a layer of flattened endothelial cells, 

 which rest upon fibrous connective tissue, with a few elastic fibres. 

 The last is surrounded by a layer of elastic tissue, the elastic lamina 

 or fenestrated membrane, which is the external limit of the intima. 

 It presents in a transverse section as a wavy (from contraction of 



