THE CIRCULATION OF THE BLOOD. 41 



cut any large blood-vessels. Cut open, carefully, the peri- 

 cardium and watch the action of the beating of the heart. 

 Observe : 



a. The alternate beats of the two auricles and the ven- 

 tricle. 



b. The synchronous beats of the two auricles. 



c. The ventricle during contraction is pale, and its apex 

 is thrown forward and upward. 



d. The pause or systole before the auricle contracts. 



e. Count the beats in a minute. 



f. Remove the heart from the body and place it in 

 normal salt solution in a watch crystal, and see if it still 

 beats. 



g. Apply a gentle heat and watch the effect. 

 h. Count the number of beats. 



i. Apply ice and watch the effect. 

 j. Count the number of beats. 



G. The Circulation in the Web of the Foot of a Frog. 



Material. A live frog or tadpole. 



Apparatus. Glass slides. Cover-glasses. Piece of cork two inches 

 square. Cloth. Pins. Compound microscope. 



To show the circulation in the web of the foot of the 

 frog, the foot should be spread out perfectly smooth and 

 held in place. Take a very thin piece of cork about two 

 inches square, and cut a hole about half an inch in diameter 

 in the centre of it. Lay a glass slide over the opening. 

 Spread out the foot on the slide over the opening, and 

 secure it there by pinning narrow strips of cloth across 

 the ends of the toes. The frog may be kept quiet by 



