XIX.] 



THE HEART AND BLOOD-VESSELS. 



Arterioles and Small Arteries. (a.) (H) Select a small artery 

 or arteriole. Note the layers already described. If the circular 

 muscular fibres be arranged in one layer, note that the fibres are 

 arranged in alternate groups on opposite sides of the vessel (fig. 

 215, A). 



(b.) Trace an arteriole into a capillary, and note the change in 

 structure, especially how the muscle disappears (fig. 215, B). 



(c.) Select a larger vessel, and in it observe the structure already 

 described and shown in fig. 218. 



(d.) Find capillaries, and note the endothelium of which they 



FlQ. 218. Middle-sized Ar- 

 tery of Brain, a. Endo- 

 thelium ; b. Fenestrated 

 membrane; c. Middle 

 or muscular coat ; d. 

 Adventitia; e. Pigment, 

 x 300. 



FlQ. 219. Arteriole of the Rabbit's Small 

 Intestine. E. Endothelial cells of the 

 intima; in, Circular muscular fibres ; 

 indicated by silver lines, x 200. 

 Nitrate of silver. 



are composed (fig. 220)0 In order to see the nuclei of these cells, 

 UK; preparation should be stained with logwood. 



(ii.) Instead of a rabbit, the blood-vessels of a frog may be 

 injected in the same way from the aorta or ventricle. Use a glass 

 syringe. Wash out the vessels first with normal saline, then with 

 distilled water, and, finally, inject the silver nitrate solution. .The 

 bladder, intestine, and mesentery are particularly serviceable for 

 obtaining small vessels and capillaries. 



12. Circulation of Blood. A day before the. frog is required, 



