XIX.] 



THE HEART AND BLOOD-VESSELS. 



227 



Fio. 211. L.S. Human 

 Thoracic Aorta. A. In- 

 ternal, E. Middle, and 

 .C. External coat, x 20. 

 Drying, picro-carmiue, 

 and acid glycerine. 



(a.) (L) Observe the subdivision into three coats (fig. 211), the 

 tunica intima (inner), media (middle), and adventitia (outer). 



(b.) (H) The inner coat is lined by a layer of squames, whose 

 nuclei may be detected as slight oval swellings (not seen in the 

 dried specimen). Under this several layers 

 of yellow elastic membrane, with a small 

 amount of pink-stained connective tissue 

 between them. The outermost layer of 

 elastic membrane is generally thicker than 

 the others, and marks the outer limit of this c 

 coat. 



(c.) The middle coat, composed also of 

 numerous elastic laminae, stained in this case 

 bright yellow, and between them patches of 

 smooth muscle with a somewhat brownish 

 tint, and some connective tissue stained 

 pink. The colour of these two tissues is quite distinct. 



(d.) The outer coat, composed of white fibrous tissue (pink), 

 some elastic laminae, and a few smooth muscular fibres. 



It is obvious, therefore, that elastic tissue enters largely into the 

 structure of the larger arteries. 



6. Fenestrated Membrane of Henle. Tear off a thin lamella 

 from the inner surface of a large artery, e.g., the aorta of a sheep. 

 Irrigate it with acetic acid, or place it 



in 35 per cent, caustic potash, and 

 mount it in Farrant's solution. 



(H) Observe the elastic laminae, 

 some of them with holes in them 

 (fig. 123). These laminae tend to 

 curl up at their edges. (See also 

 Lesson X. 7.) 



7. T.S. Medium Sized Artery, e.g., 

 the femoral artery of a child, prepared 

 and stained as the aorta. 



(a.) (L) No te the three coats. 



(b.) (H) The inner coat, with its 

 endothelium and internal elastic 

 lamina. In many arteries a layer of 

 sub-epithelial connective tissue lies between the endothelium and the 

 elastic lamina. The elastic lamina is thrown into folds in an empty 

 artery owing to the contraction of the middle coat (fig. 212). 



(c.) The middle coat is composed of several layers of smooth 

 muscle arranged circularly. In a Canada balsam preparation the 

 nuclei stand out distinctly. Amongst the muscle-fibres are twisted 

 elastic fibres (fig. 212). 



M. 



FIG. 212. T.S. Artery. 7. Tunica 

 intima ; M. Media ; E. Externa. 



