130 THE FROG HEART. 



the neck on one side, avoiding injury to the cutaneous vessels. Then expose 

 the scapula, and sever with the scissors the cartilaginous from the bony 

 scapula ; remove the former, dividing the muscles attached to it, then expose 

 the sterno-mastoid muscle which connects the outer part of the petrous bone 

 and the posterior border of the cartilaginous ring of the membrana tympani 

 with the concave anterior border of the scapula. Remove or draw aside the 

 sterno-mastoid so as to expose the slender muscles (petrohyodei) which run from 

 the petrous bone to the posterior horn of the hyoid bone, embracing the 

 cavity of the pharynx. Parallel with these muscles, and in close relation with 

 them, are seen the carotid artery and several nerves, of which the two nearest 

 the cranium are the glosso-pharyngeal and the vagus. 



b. Expose the vagus in a pithed preparation. Expose the heart as in III. I, 

 and introduce a small test tube into the gullet. Fix the preparation in such a 

 position on a cork, that the electrodes can be conveniently applied to the nerve, 

 at the same time that the motion of the heart can be observed. 



5- The Stannius Heart. Prepare a frog heart with fraenum ligature 

 as before. Then pass a thick ligature under the bifurcation of the aorta 

 between it and the venae cavae superiores. Then, seizing the fraenum ligature 

 with the forceps, turn the heart up. Carefully observe the position of the 

 "crescent," and loop the ends of the ligature so that when it is tightened it 

 may embrace the crescent. On tightening, the heart will stop in diastole. 



In the heart so prepared, sever the ligatured parts from the rest of the pre- 

 paration with sharp scissors. The auricles and ventricle resume their normal 

 rhythmical action. 



Cut off in a preparation which has been so treated, the remainder of the 

 auricles and the bulb, leaving the ventricle and auriculo-ventricular septum. 

 The heart continues to beat normally, or, if the beats cease, they are renewed 

 by a pinch, by an induction shock, or by bringing a hot wire into the neigh- 

 bourhood of the cut surface. 



6. Localization Of the Motor Centres. In one of two such pre- 

 parations (called ventricle preparations) which beat rhythmically, cut off the 

 whole of the auriculo-ventricular furrow with sharp scissors. The preparation so 

 obtained (the ventricle apex) does not contract spontaneously, but responds to a 

 single excitation, whether mechanical or electrical, by a single contraction, the 

 duration of which is dependent on the temperature. In the other preparation, 

 divide the ventricle by two parallel cuts into a middle and two lateral thirds. The 

 middle third includes the ventricular border of the interauricular septum, the 

 right lateral third contains the root of the bulb. The middle third beats 

 rhythmically, the lateral thirds respond to excitations by single contractions, 

 but do not beat of themselves. 



7- Action Of MllSCarin and Atropin. In an entire heart (a 

 heart removed by severing the vessels, for which purpose the organ should be 

 lifted out of the pericardium by a ligature tied to the fraenum), stop rhythmical 

 action by applying to it a drop of serum containing a trace of inuscarin. 

 Observe the relaxed and motionless condition of the ventricle. After a few 

 minutes apply (in serum) a drop of 0*2 percent, solution of atropin. Observe 

 the gradual restoration of rhythmical action in the atropinized heart. Observe 

 that faradization of the inhibitory centre is without effect. 



