228 



PRACTICAL HISTOLOGY. 



[XIX. 



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

 intermingled with elastic fibres, some of which are cut trans- 

 versely. The elastic fibres are more numerous towards the inner 

 part of this coat. Here and there a section of a blood-vessel may 

 be seen. 



8. Endothelial Lining of Veins and Arteries. Cut open the 

 external jugular vein of a rabbit just killed. Pin it to a piece of 

 cork inner surface uppermost by means of hedgehog-spines. 

 Wash the internal surface with distilled water, and then apply 

 to it for five minutes J per cent, solution of silver nitrate. "Wash 

 off the silver, place the vein in water or alcohol and water, and 

 expose it to light. Do the same with any large artery. 



(A.) Snip out a small portion of the vein, and mount it in 

 balsam, inner surface uppermost. 



(H) Observe the "silver lines," indicating the existence of a 

 layer of polygonal squames composing part of the inner coat. The 



FIG. 213. A. Epithelial lining of an artery of a calf, and V. of the jugular vein of a 

 rabbit. The arrows show the direction of the blood-stream. Silver nitrate. 



long axis of the squames lies across the long axis of the vein itself 

 (%. 213, V). 



By focussing through the thickness of the wall, narrow fusiform 

 areas, bounded by black lines, may be seen, indicating the existence 

 of the smooth muscular fibres in the middle coat. 



(B.) With a razor shave off a thin layer from the brown inner 

 coat of the artery. Mount it in balsam. 



(H) Observe the elongated lancet-shaped endothelial cells (fig. 

 213, A); the long axis of each cell in the long axis of the tube. 

 The variation in the shape of the epithelial lining of vessels seems 

 to have relation to the velocity of the blood-stream in these 

 vessels. 



9. Pia Mater, Capillaries, Small Arteries, and Veins (H). 

 Carefully remove a small piece of the pia mater from the brain of 

 a sheep recently killed. Lay it on a glass plate, outer surface 



