100 PEACTICAL PHYSIOLOGY 



fluid which has been kept in broken ice, and take another record. 

 The cooled heart gives slow and forcible beats. The periods of con- 

 traction and relaxation are prolonged, the frequency greatly diminished. 

 Now fill the beaker with Ringer's fluid at 25 C. The frequency 

 becomes greatly increased, and the period of contraction and relaxation 



Fio. 95. Contraction of the frog's heart recorded by the suspension method 15 C. 

 and then immersed in saline at 25 C. The curve should be read from left to right. 

 The time is marked in seconds. (L.H.) 



greatly shortened. A temperature of about 35 C. causes diminution 

 of the tone of the heart. The ventricle ceases to follow the auricular 

 rhythm, although it still responds to excitation. At 38 to 43 C. the 

 whole heart ceases to beat, and gradually passes into the condition of 

 heat rigor. The heat contraction, when once fully established, is not 

 set aside by cooling. 



CHAPTER XXVII. 

 THE STANNIUS HEART. 



The Stannius Heart. The heart of a pithed frog is exposed and a 

 thread is tied to the fraenum which is then cut away from the posterior 

 surface of the pericardium. Pass a ligature under the two aortae and 

 then by means of the thread attached to the fraenum gently pull the 

 heart towards the mouth of the frog. The dorsal aspect of the heart is 

 now readily seen. Draw the ligature round the white crescentic line 



