LABORATORY MANUAL OF PHYSIOLOGY. 



II. DIRECT OBSERVATION OF THE ACTION or THE FROG'S 

 HEART. 



Pith the frog used in the previous experiment or, if necessary, 

 take a fresh frog. Lay the frog on its back upon the cork frog- 

 board, spreading the fore legs out and pinning them to the board. 

 Do the same to the hind legs. Make a median incision through the 

 skin of the thorax, crossing this transversely with an incision at the 

 level of the fore limbs. Lay back the flaps of skin and, with small 



v. D. 



FIG. 27. Frog's Heart. V, Ventral view; D, dorsal view. B.A, Bulbus arteriosus ; 

 S.V, sinus venosus. 



but strong scissors, continue the median incision through the tho- 

 racic muscles and sternum. Be careful to keep the lower blade of 

 the scissors snugly against the inner surface of the sternum in order 

 to avoid injury to the pericardium and heart. Hook back the di- 

 vided sternum so as to expose the heart in the pericardium. 



Note the relations of the pericardium to the heart and great ves- 

 sels and to the surrounding viscera. Keep the preparation moist 

 with o.6-per-cent NaCl solution. 



Before opening the pericardium, note the rate of the heart beat, 

 counting the number of beats in a minute. 



Now open the pericardium so as to obtain a better view of the 

 different pulsating portions of the heart and make the following 

 observations : 



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