HEART-MUSCLE 501 



In other words, the heart contracts on the principle of "all or none." 

 The aluminum heart-lever may be used instead of the older heart-pan; 

 the platinum electrode is then applied directly to the suspended heart. 



As has been learned in the work on cross-striated muscle, the force of 

 its contraction varies somewhat with the stimulus; but the heart, as we 

 see, does not beat on this principle. It is impossible to be sure as yet 

 why this is so. Gaskell, one of the leading authorities on cardiac contrac- 

 tion, thinks this phenomenon (and also that of the refractory period and 

 the fact that the heart cannot easily be tetanized) may be explained by 

 supposing that in case of the heart all the heart-cells are stimulated 

 together by even a weak stimulus; or (on the supposition that the 

 metabolism, both up-and-down, of the heart is slower than that of 

 cross-striated muscle and the energy-material more stable), that they 

 are less easily katabolized in contraction. (See also notes at end of the 

 directions for Expt. 67.) 



Expt. 63. Muscular "Automaticity." (" Engelmann's Incisions.") 

 (Apparatus: Fine scissors and a frog.) Expose the heart of the pithed 

 frog; lift it gently by the lower end of the apex, and with a pair of small, 

 sharp-pointed scissors make transverse incisions into the ventricles from 

 both sides, so that those from each side extend beyond the middle line 

 but do not connect. By these means any nerves which might extend 

 from base to apex might be cut. Observe that the contraction-wave 

 passes over the slashed ventricle down the zigzag strip of muscle from 

 base to apex. 



This " automaticity" of certain muscular tissue is a large question, 

 which becomes more complicated rather than simpler. It is essentially 

 the problem whether muscle can act normally without any influence 

 from nerves so long as it is supplied with nutriment and kept in physio- 

 logical condition. It involves, too, the whole matter of the general func- 

 tion of the nervous system. Lately the ions and the theory of the electro- 

 lytic dissociation of inorganic salines have come into the problem. 

 Discussion of the matter is out of the question here, but see the theo- 

 retical notes of Expts. 67 and 68, and the discussion in the body of 

 the book, page 293, etc. 



Expt. 64. The Inherent Beat-rhythm. (Apparatus: Dry-cell, rheo- 

 cord, stand-rod, femur-clamp, key, wires, dissecting-plate.) Connect 

 the rheocord to a cell through a key. From the block and anodal post 

 of the rheocord run wires to the jaws of the clamp so that their ends 

 may be applied to the heart in a fixed position, the hands not being steady 

 enough (do not twist the wires together). Sever the ventricles of a frog's 

 heart carefully at the auriculo-ventricular groove, lay them on the glass 

 plate, and keep them only barely moist with saline. This isolated 

 "apex" does not beat. Now stimulate it (one electrode on each side) 

 with the constant galvanic current, gradually increasing its strength 

 until the make just causes a contraction. At this strength let the current 

 pass continuously through the apex. It will beat rhythmically, such 

 being its life-habit and that of its cardiac ancestors. 



