PRACTICAL EXERCISES 185 



has cooled, a narrow groove, just sufficient to lay bare the wires on 

 the upper side, is made in it, and the nerve is laid in this groove.) 



Experiments 7, n (i) and n (2) will be rendered more exact 

 by connecting a second electro-magnetic signal with a Pohl's com- 

 mutator without cross-wires (Fig. 82), in such a way that the circuit 

 is interrupted at the instant when stimulation begins. 



12. The Action of Inorganic Salts on Heart-muscle. Expose and 

 remove the heart of a tortoise or turtle (p. 180) . Cut off the apical two- 

 thirds of the ventricle by an incision parallel to the auriculo- ventricular 

 groove. By a second parallel cut remove a ring of tissue 2 or 3 milli- 

 metres wide from the upper end of this portion of the ventricle. 

 Divide the ring at opposite ends of a diameter, so as to form two 

 strips. Tie a fine silk thread to each end of one strip. Attach one 

 of the threads to the short limb of a glass rod bent at right angles, 

 so that it can be immersed at will in a beaker. The other end of the 

 rod is fixed in a holder sliding on a stand. Attach the second thread 

 to the short arm of a counterpoised lever arranged to write on a 



FIG. 82. ARRANGEMENT FOR RECORDING THE BEGINNING AND END OF 



STIMULATION. 



C, Pohrs commutator without cross -wires ; B, battery in circuit of primary 

 coil P ; B', battery in circuit of ^electro -magnetic signal T ; K, simple key in 

 primary circuit ; S, secondary coil. When the bridge of the commutator is 

 tilted into the position shown in the figure, the primary circuit is closed and the 

 circuit of the signal broken. 



slowly-moving drum. If the strip is still beating, wait till the 

 contractions have ceased ; then 



(1) Immerse the strip in a beaker filled with 0*7 per cent, solution 

 of sodium chloride. After a time it begins to beat rhythmically. 

 The contractions become rapidly stronger, and then after a while 

 diminish, and gradually cease. The tone or tonus of the strip is 

 diminished by the solution. 



(2) Arrange the other strip in the same way, and immerse it in a 

 solution of calcium chloride (about i per cent.) isotonic with the 

 sodium chloride solution used in (i). If the strip is contracting, 

 the contractions will cease. Rhythmical contractions will not 

 appear as in the sodium chloride solution. The tone of the strip 

 may be increased. 



(3) Remove most of the calcium chloride solution from the beaker, 

 and fill it up with 0*7 per cent, sodium chloride solution. The 

 rhythmical contractions will appear after a longer or shorter latent 



