XII.] STRUCTURE OF BLOOD-VESSELS. 1^1 



Note with a low power the small arteries branch- 

 ing into capillaries and the uniting of the capil- 

 laries to form veins which again unite to form 

 larger veins. 



Note under a high power the outlines of the flat 

 elongated epithelium cells of the small arteries, 

 of the capillaries and of the small veins; the 

 outlines in the capillaries are more irregular than 

 in the arteries or veins ; in the veins the cells are 

 rather broader than in the arteries. 

 In the small arteries the cementing substance 

 between the cells of the muscular coat (here 

 usually a single layer) will also shew as black 

 transverse lines, the small veins are without 

 these transverse markings. 



D. Circulation of Blood. Inflammation. 



Take a frog with a pale web which a day or two 

 previously has been deprived oi its brain by the 

 Demonstrator (cp. p. 116, footnote) and kept in 

 a moist place. "With scissors make a small cut 

 through the skin of the back, with a fine pipette 

 introduce underneath the skin one drop of a ]. p.c. 

 solution of urari, leave the frog under a glass 

 jar until it does not move on being pinched; 

 this should be the case in about three-quarters 

 of an hour, if it occurs earlier too much urari has 

 been given and the blood-vessels will be abnor- 

 mally dilated. Lay the frog on its belly on the 

 frog-board, and tie, not too tightly, a piece of soft 

 cotton round the end of the (e. g.) 3rd and 4th 



9-2 



