126 HOW TO WORK 



tion with a sharp knife. The specimen is then moistened with water. 

 It may be treated with different reagents, and afterwards preserved in 

 glycerine, glycerine jelly, or other preservative fluid. 



22S. Examination of Arteries and Veins. The Structure of 

 arteries and veins may be well studied in any of the smaller vertebrate 

 animals, especially in the frogs. In mammalia beautiful specimens 

 may be obtained from the mouse. Those in the mesentery, the pleura, 

 and pericardium may be subjected to examination without difficulty, 

 but the smaller arteries and veins of the pia mater, or vascular mem- 

 brane of the brain, and those of the folds (choroid plexuses) of the 

 same membrane in the cavities (ventricles) of the brain are more free 

 from connective tissue and can be easily isolated. 



The yellow elastic tissue of the arterial coats of the larger arteries 

 may be demonstrated in any artery of a quarter of an inch in dia- 

 meter or more. The fibres vary much in character, sometimes 

 appearing rather as an expanded elastic membrane perforated here 

 and there, than as separate fibres. In the smallest arteries and veins 

 there is very little elastic tissue, but this is represented by muscular 

 fibres. On the other hand in the largest vessels, the muscular fibres 

 appear to have almost given place to yellow elastic tissue. 



I have obtained beautiful specimens of the muscular fibre cells 

 arranged circularly round the arteries by injecting the vessels with 

 plain size, and gradually increasing the force so as to distend them 

 as much as possible without rupture. In this manner the cells are 

 as it were, gradually unravelled. When cold, thin sections may be 

 very easily made in various directions, and even isolated fibre cells 

 can be obtained. The arrangement of the muscular fibre cells in the 

 smaller vessels, is well seen in the small arteries from the frog and 

 newt. Seepl. XXIX, fig. 189. 



The arrangement of the numerous nerve fibres distributed to the 

 small arteries and veins may be demonstrated in the frog with the 

 greatest distinctness, and in connection with the small vessels which 

 supply the viscera numerous ganglia will be found from which 

 bundles of nerve fibres may be traced in different directions. 

 These often form plexuses around the vessels and give off finer 

 bundles, and fibres may be followed even to the capillary vessels. 



22G. Examination of the Capillaries. Capillary vessels may be 

 obtained from any of the tissues and organs of the lower animals. 

 The mode of displaying their general arrangement by injection has 

 been already described, p. 87. The masses of germinal matter in 

 connection with their walls vary in number greatly in different parts. 

 In some textures the capillary appears to be almost entirely sur- 

 rounded with them, pi. XXIX. figs. 187, 188, also fig. 163, pi. XXV, 



