BLOOD-VESSELS. O7 



the media) shining line; and is an important element, from its rela- 

 tion to certain abnormalities of the blood-vessels. The media (2) 

 consists of alternate layers of elastic and muscular tissue. The ad- 

 ventitia (3) is composed of fibrous connective tissue, containing some 

 elastic elements. 



As we approach the larger arteries, the muscular tissue diminishes 

 in quantity and the elastic tissue is increased. On the other hand, 

 the elastic element diminishes with preponderance of muscle as we 

 approach the smaller arteries, until we meet the arterioles, the walls 

 of which are made almost exclusively of involuntary muscular fibre. 



The walls of capillaries consist of a single layer of flattened endo- 

 thelial cells cemented by their edges. The union is not quite con- 



FIG. 52. ISOLATED BLOOD CAPILLARIES. 



A. Plexus from a pulmonary alveolus, stained with ssilver. X 350. 



B. Capillary from omentum, stained with silver and hsema. X 700. 



In A, the cells are outlined by the silver; while in B the nuclei in addition are brought out by 

 the haema. 



tinuous, as minute openings (stomata) are to be seen at irregular in- 

 tervals. 



The walls of veins are much thinner than those of arteries. The 

 intima presents an endothelial lining, but no fenestrated membrane; 

 and the line of demarcation between this coat and the media is often 

 indistinct. The media contains muscular, but little elastic tissue; 

 and the adventitia, usually the most prominent of the three coats, is 

 composed largely of fibrous connective tissue. 



I shall defer the microscopical examination of blood-vessels until 

 we meet them in future sections of organs, as they are best studied 

 in such connection. 



