250 



SCIENCE 



[N. S. Vol. XXXVI. No. 921 



appearances. I am, therefore, inclined to favor 

 tins interpretation. According to this con- 

 ception these structures are of no essential 

 importance in contraction except in so far 

 as they prevent relative displacement of ad- 

 jacent fibrils. But such displacement, if per- 

 mitted, would interfere with the contractile 

 properties of the system, as I shall show 

 later, so that the physiological role thus as- 

 signed to these structures may after all be in- 

 dispensable. This view is however quite op- 

 posed to any hypothesis — like that of Piitter 

 — ^which regards the intermediate lines as cor- 

 responding to membranes whose state of 

 permeability determines the degree of osmotic 

 distension of the muscle segments, and so the 

 state of contraction. The evidence of their 

 membrane character seems insufficient to 

 serve as support for such a speculative view 

 as this. On the other hand, the lateral coher- 

 ence of the fibrils is an undoubted fact which 

 must have a structural basis, and direct ob- 

 servations exist which indicate that at least 

 the Z lines — which appear to be the more 

 constant of the two — do in fact cross the 

 intervals between the fibrils. Structures thus 

 continuous from fibril to fibril are most nat- 

 urally interpreted as connectives. 



We may now consider the physiological 

 problem as to the nature of the essential phys- 

 ical changes which form the direct condition 

 of the characteristic mechanical deformation 

 of active muscle. The precise nature of the 

 energy-yielding metabolic process need not 

 concern us here. As well known, various more 

 or less seriously conflicting hypotheses exist. 

 There is considerable evidence of a displace- 

 ment of fluid within the contractile elements 

 during activity, and this fact has led to the de- 

 velopment of theories which assign to this 

 transfer of fluid the essential role in the proc- 

 ess of contraction. Thus Engelmann and 

 many others have referred the process to a 

 water-absorption or swelling of certain struc- 

 tural elements — just as gelatine sheets swell 

 in water, and more rapidly when the water 

 is warm or acidulated than when it is cold or 

 neutral. The general structural conception 

 put forward some time ago by McDougall, 



and recently favored by Meigs and others, has 

 been that of a tissue composed of elements 

 comparable to somewhat elongated or spindle- 

 shaped fluid-containing sacs with inexten- 

 sible walls; these elements on distension ap- 

 proach a spherical form, with consequent ap- 

 proximation of their opposite ends. A cer- 

 tain degree of shortening (about 37 per cent.) 

 may thus theoretically be accounted for. 

 That the actual shortening often greatly ex- 

 ceeds this proportion is unfavorable to such 

 hypotheses, though perhaps not necessarily 

 incompatible with them. The structural con- 

 ditions might conceivably be such that the 

 swelling elements at their maximal normal 

 distension adopt a form in which the ratio 

 of transverse to longitudinal diameters — as- 

 suming these to correspond with those of the 

 muscle-cell — is increased sufficiently to ac- 

 count for a much greater degree of shorten- 

 ing than the above. All that would be neces- 

 sary to render such a hypothesis adequate 

 in this respect would be to assign a certain 

 definite structure and form to the elements. 

 Take for instance the case of a tissue com- 

 posed of structural units resembling the ex- 

 tended elaters of the spores of Equisetum; as 

 is well known, these structures when slightly 

 moistened wind themselves closely around the 

 spore; the total diameter of the system, spore 

 plus elaters, may thus be reduced to a small 

 fraction of the original diameter as meas- 

 ured between the tips of the extended elaters. 

 One might conceive of muscle as composed of 

 elements similar in contractile properties to 

 elaters, united in a definite manner to form 

 a contractile system; and it would no doubt 

 be possible, by exercising some ingenuity, to 

 reconcile such a conception with the main 

 histological appearances. Hypotheses that 

 refer the contraction to swelling of the con- 

 tractile elements are thus not to be dismissed 

 by advocates of other views as inadmissible 

 on the ground of purely geometrical considera- 

 tions. The objections to this type of explana- 

 tion are of quite another nature; and since 

 they are in my opinion sufficiently weighty 

 to render it extremely doubtful that this kind 

 of physical charge could ever form the basis 



