92 



PRACTICAL PHYSIOLOGY 



the investigation of the electromotive properties of the frog's heart ; 

 it responds to very rapid changes of electrical potential. It consists 

 (Fig. 88) of a glass tube C drawn out at one end to a fine capillary 

 tube ; this is filled with mercury and is connected with a pressure 

 apparatus by the rubber tubing RT. The capillary tube dips into a 

 small trough filled with 10 per cent, sulphuric acid ; the bottom of this 

 vessel is covered with mercury M in order to provide good electrical 

 conduction with the platinum wire. The movements of the column of 

 mercury in the capillary tube are observed by means of a microscope 

 fitted with a micrometer scale. 



The passage of an electrical current through the 

 capillary tube alters the surface tension, and this 

 alteration causes a movement of the mercury in 

 the capillary tube. The movement of the column 

 of mercury is from positive to negative, and the 

 extent of the movement is roughly proportional to 

 the difference in electrical potential. Based upon 

 these facts are the determination of the direction 

 of, and the measurement of the electromotive force 

 of, the current which is under investigation. 



With the capillary electrometer the electromotive 

 properties of the frog's heart are demonstrated. The 

 base and the apex of the ventricle are led off by 



FIG. SS. Diagram 



of the capillary eiec- unpolarisable electrodes to the electrometer : each 



ti ometer. -,,-, -, , 



time the heart contracts there will be a diaphasic 

 variation, the contracted portion at first becomes negative and then 

 positive to the uncontracted part. 



CIRCULATION. 



CHAPTER XXV. 



THE ANATOMY OF THE FROG'S HEART AND ITS CONTRACTION. 



Anatomy of the Frog's Heart. The cerebrum can be destroyed by 

 the application of a strong pair of Spencer-Wells forceps to the skull. 

 The medulla oblongata and spinal cord are left intact, so that the vaso- 

 motor control continues and the circulation is unimpaired. The frog is 

 pinned on the corkplate belly uppermost. The skin over the abdomen 

 is pinched up and slit up to the mouth. The abdominal wall is then 

 divided slightly to one side of the mid line to avoid cutting the anterior 



