68 ELECTRO-PHYSIOLOGY CHAP. 



The histological differences between quick and sh 



^ 



muscles are much less in the majority of cases. As a rule it 

 may be said that the latter, at least in vertebrates, are richer in 

 sarcoplasm, dark, and often smaller ; while the quid; excitall; 

 more easily fatigued muscle-fibres contain less sarcoplasm, and are 

 therefore clearer and usually broader. As shown above, the dark 

 fibres are often, though not invariably, stained by haemoglobin 

 and other colouring matters. This is very conspicuous in the 

 Pecten family, where the greater (yellowish-gray) portion of the 

 adductor muscle consists of striated, the (whitish) remainder of 

 smooth, muscle -cells. As Coutance and Thoring showed, the 

 former serves only the rapid closing of the shell, while the 

 smooth muscle closes it dowly, but permanently and forcibly. 

 This becomes impossible, and the shell gapes open, if the smooth 

 part of the muscle is cut through; the striated part can still 

 effect rapid closure on excitation, but never of long duration. 

 The nicety of such an arrangement is obvious. Coutance (18), 

 and later Knoll (17), showed the marked difference in the 

 contraction process between the smooth and striated parts of the 

 adductor muscle when directly excited. Lima inflata (according 

 to Knoll) behaves like Pecten ; its adductor muscle does not, 

 indeed, consist of two macroscopically distinct portions, but it 

 contains smooth elements in many layers at the periphery, and 

 singly in the interior, while the bulk of the muscle is composed 

 of cross- (or obliquely-) striated cells. When undisturbed in sea- 

 water, the shells of these muscles usually gape open pretty widely, 

 but from time to time they are sharply closed, and this move- 

 ment, as in Pecten, jerks the animal a step farther. In the 

 Oyster also the adductor muscle consists of two parts, one trans- 

 parently gray, the other white and tendon-like ; in Mytilus only 

 the white, in Solen only the gray are present. Schwalbe, who 

 was first to recognise that the gray part consisted of " bi- 

 obliquely striated " muscle-cells, also pointed out the functional 

 differences between the two portions. If the act of adduction is 

 compared in the shell of Ostrea and Mytilus the first is seen to 

 close sharply and suddenly with external excitation, the second 

 very slowly and gradually, so that the adductor muscle can be 

 divided while the shell is open, and the knife is not wedged 

 in, as would happen in the oyster. Schwalbe therefore thinks 

 that the bi-obliquely striated fibres of Ostrea serve for sharp, 



