CHAPTER XXI 

 The Capillary Circulation 



THE flow of blood in the smallest arteries and veins and in the 

 capillaries is observed with the microscope in transparent parts of 

 animals, such as the web and mesentery of the frog, the tail of the 

 tadpole, the lung and tongue of the toad, and the mesentery of small 

 mammals. 



1. In the frog's web. The frog, the brain of which may be 

 destroyed, is immobilised with curari or by immersion in water satur- 

 ated with chloroform or ether. The animal is laid on a flat cork with 



FIG. 75. Frog-cork for displaying mesentery, lung, tongue, 

 or web of foot. 



a slot at one end (Fig. 75, a) : the margins of this are raised in the way 

 shown in the figure, and the web of one foot is spread out over the slot 

 with the aid of pins and threads, but not stretched so tightly as to 

 obstruct the circulation. A low power of the microscope is used ; the 

 web need not be covered with a cover-glass. 



2. In the frog's mesentery. For this a hole is made at one side of 

 the cork (Fig. 75, 6) with its margins raised somewhat higher than for 

 the web. The hole is covered by a circular disc of glass, not too thick, 

 fixed to the cork with sealing wax. A loop of intestine is drawn out 

 through an aperture in the abdominal wall, and arranged round the 



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