ARTIFICIAL CIRCULATION. 



319 



Fig. 141. 



JwH 



be kept at a constant temperature more easily than when a 

 syringe is employed. The pressure may be alternately increased 

 and diminished so as to imitate the beats of the heart by raising 

 and depressing the flask A. This may be done by passing a 

 string over a pulley, and attaching one end to the flask and the 

 other to a treadle worked by the foot. Warm blood has the 

 disadvantage, that it undergoes change and becomes decomposed 

 quickly : and cold blood may, therefore, be sometimes preferred. 

 When cold blood is employed, only the flask which contains the 

 blood is necessary ; and it may be raised or lowered in the same 

 way as the other. 



Artificial Circulation in Frogs. — Artificial circulation may be 

 kept up in frogs by simply inserting a cannula into the aorta* 

 and allowing blood to flow into it from a raised reservoir, as 

 done by Eollett. By using two, as in the experiments on 

 the frog's heart, normal blood may be allowed first to circu- 

 late through the vessels ; and, the web being put under the 

 microscope, their diameter may be measured ; and then 

 poisoned blood may be allowed to flow through them, and any 

 change in their diameter noticed. 



Observation of Vessels. — The parts best adapted for observing 

 changes in the size of vessels in mammals are the ear in rabbits 

 and the mesentery. When the mesentery is chosen for observa- 

 tion, the abdominal parietes should be divided ; but the peri- 

 toneum should not be opened, as changes in the diameter of the 

 mesenteric vessels may be observed through it, and they are 

 thus protected from the disturbing element which the irritation 



