202 THE BIOLOGY OF THE ANIMAL 



your ribs move ? Can you inspire deeply without 

 moving the ribs ? Has the frog ribs, or anything 

 corresponding to them? If so, are these parts 

 movable ? Can they assist in inspiration ? Does 

 the frog breathe as you do? Why can a frog 

 stay under water for several days without coming 

 to the surface to breathe? Why do frogs die 

 when their skin becomes dry ? 



e. Circulation. 



1. The teat of the heart. — Chloroform a frog and ex- 



pose its heart. Note the beat of the heart, con- 

 sisting of a contraction or systole, followed 

 by an expansion or diastole. How rapid are 

 the pulsations ? How do the ventricle and the 

 auricles behave ? Do any of the parts change 

 color during the beat ? Shape ? Size ? With 

 the finger lightly touch the various parts of the 

 heart while it is beating, and note the varia- 

 tions in tension of the walls. Remove a heart 

 from the body by cutting the blood-vessels, and 

 immerse it in a salt solution in a watch-glass. 

 Does the heart still beat, though it contain no 

 blood? If so, how long does its activity last? 



2. The circulation through the web. — Provide a piece 



of thin board — e. g., such as is used for the backs 

 of pictures — about four or five inches long and 

 two or three inches wide. In the middle of one 

 end bore a hole or cut a notch, over which the 

 web of the frog's foot maj'' be stretched. Ren- 

 der the frog insensible with chloroform, but do 

 not kill it. Lay it on the board, ventral side 

 downward, and fasten it with a tape passed 

 firmly, but not tightly, around its body a few 



