making a transverse slit through the heavy muscle^ on the hack 

 of the neck. Continue the slit through a joint between the verte- 

 brae and destroy the brain with a wire. I 'lug the cavity with 

 cotton. Sever the union between plastron and carapace at the sides 

 with bone shears and cut the skin and muscles as near the plastron 

 as possible so as to remove the plastron from the body. ( )n pulling 

 the forelimbs straight the heart will be seen beating and by a 

 little careful dissection can be freed from the pericardium. The 

 latter is attached to the tip of the ventricle, and this strand should 

 be used to take hold of in handling the organ. 



264. Conduction of impulse in the heart. In the frog and 

 turtle the impulse originates in the sinus and spreads to the auricle 

 and ventricle; in the mammal the impulse starts in the right auricle 

 near the venae cavae and spreads to auricles and ventricles and 

 also to a certain distance over the veins opening into the auricle. 

 On reaching the auricle-ventricular junction there is a distinct 

 pause termed the auriculo-ventricular interval ; finally, the excita- 

 tion reaches the ventricle, and the contraction wave is seen to tra- 

 verse, the ventricular muscle. The auricuLo-veaitricular interval 

 may be lengthened by any natural or artificial hindrance to the 

 excitation wave. 



a. Place the Gaskell clamp about the auriculo-ventricular junc- 

 tion. Very carefully turn the screw until the rubber edge makes 

 a gentle pressure on the cardiac tissues at that point. With care- 

 ful work a degree of pressure will be reached that diminishes the 

 conductivity of the muscle fibres joining the auricle and ventricle 

 so far as to allow only every second or third excitation to pass. 

 The auricle will beat without change of frequency, but the ventricle 

 will beat only when the excitation succeeds in passing the block. 



b. Repeat experiment a, but place the screw clamp across the 

 middle of the ventricle. The passage of the excitation from one 

 part of the ventricle to another will be delayed or interrupted by 

 the lowering of the conductivity in the compressed portion. Many 

 irregularities in the frequency and force of the heart can be ex- 

 plained by variation in conductivity of its several parts. 



265. Automaticity of different chambers of the heart, a. Care- 

 ful observation is required to detect contractions in small pieces 

 of the heart. Determine the rate of the whole heart. Cut off 

 the sinus venosus. Does it beat? Rate? Does the remainder of 

 the heart beat? Rate? Cut the sinus into small pieces. Rate 

 of each piece? It is best to tie a ligature (of Stannius) between 



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