1 6 COMPARATIVE PHYSIOLOGY 



by its sustained tonus. A double neuromuscular mechanism 

 probably of the same type exists in the adductor of Astacus, 

 Homarus, Carcinus and other decapod Crustacea (Lapicque, 

 Keith Lucas). The economy of such an arrangement, which 

 combines the rapidity of action of striped muscle with the low 

 energy output of the tonus mechanism, is evident. Tonus is 

 not associated with increased metabolism, and cannot there- 

 fore be of the same nature as a low-grade tetanus. 



The tonus of a catch muscle may be looked upon as a natural 

 form of isometric response. We are probably dealii(ig here not 

 with a change in the intensity factor but with the capacity factor 

 of the surface energy of the cell. It is possible to think of a 

 mechanism of isometric response by which a mechanical stress 

 sets up in some part of the protoplasmic system a change in 

 phase relations of the colloidal constituents such as to oppose it 

 by a virtual tension co- extensive with the maintenance of the 

 external force. The coagulation of liquid silk in stretching is 

 possibly analogous. 



(g) Relation of the Muscle Cell to Electrolytes.— Certain 

 phenomena, notably those associated with the maintenance of 

 tone which has just been discussed, raise difficulty in the way 

 of any attempt to extend to plain muscle the conclusions 

 respecting the contractile mechanism in striped muscle. This 

 is also the case in considering the role of the hydrogen and 

 calcium ions respectively in the contractile process. The 

 effect of adding acid to a saline medium in which a preparation 

 of striped muscle is immersed is to produce contraction ; on 

 the other hand, it is found both for the cardiac muscle of the 

 vertebrate heart (frog, dogfish, skate), and the unstriped muscle 

 of the molluscan heart (Pecten) (Mines, 19 13), as well as for 

 various forms of mammalian plain muscle which have been 

 recently studied with great care by Lovatt Evans (1923), that 

 increased hydrogen ion concentration produces arrest, at first 

 reversibly and beyond a certain point irreversibly, in the relaxed 

 condition. The action of calcium on striped muscle has been 

 carefully studied by Overton (1904) and Mines (1912) ; in a 

 medium containing sodium ions but no calcium the muscle 

 displays rhythmical spontaneous twitching ; the addition of a 



