ELEMENTAKY EXPERIMENTAL PHYSIOLOGY 53 



used to excite the nerve of preparation B. Further, ligature of the 

 nerve B with a moist thread will show that there is no escape with 

 a weak induction-shock ; the ligature destroys the physiological 

 continuity and prevents the passage of the excitatory state but not 

 that of an electrical current. 



FIG. 53. Diagram of the experiment on secondary twitch. 



Secondary Tetanus. If the nerve be stimulated with a rapid series 

 of induction-shocks the muscle B goes into tetanus and its ' currents of 

 action ' stimulate the nerve A, with the result that the tetanus is also 

 observed in the muscle A. This ' secondary tetanus ' can be produced 

 by rapid mechanical stimuli. 



Further experiments upon the electromotive properties of muscle 

 and nerve are given in the Advanced Course (page 331). 



CIRCULATION, RESPIRATION, AND ANIMAL HEAT. 

 CHAPTER XII. 



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



Anatomy of the Frog's Heart. At the mid-point of a horizontal 

 line drawn through the posterior margin of the eyes a blanket pin is 

 passed through the skull of a frog, and the cerebrum destroyed. To 

 prevent bleeding the hole is plugged with a piece of lucifer match cut 

 to a blunt point. The cerebrum can be destroyed equally well 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 vasomotor 

 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 



