248 



SCIENCE 



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



a certain change in the procedure of inves- 

 tigators has become apparent. Experience of 

 the readiness with which the structure or " ag- 

 gregation-state " of a colloidal system may he 

 altered has made it clear that such structures 

 are not always to be regarded as preexistent in 

 the living tissue. They are rather to be re- 

 garded as appearances from which, on the 

 basis of an adequate knowledge of the be- 

 havior of colloidal systems under similar treat- 

 ment, inferences may be drawn as to the 

 original disposition and state of the colloidal 

 material in the living system. Inferences so 

 drawn require to be controlled by an exact 

 knowledge of the structural appearances in 

 living muscle during both rest and contrac- 

 tion. Hence recent investigators — as Hiirthle 

 and Meigs — have relied mainly on direct and 

 photographic observation of living muscle 

 fibers. But even results so gained show con- 

 siderable disagreement in detail, and unfor- 

 tunately they have proved compatible with 

 diametrically opposed conceptions of the es- 

 sential nature of the contractile process. 



Certain definite conclusions have, however, 

 been reached. It is agreed that the contractile 

 elements are solid fibrils — consisting sup- 

 posedly of water-swollen myosin — which are 

 embedded in a more fluid sarcoplasm; the 

 fibrils are not optically homogeneous, but ex- 

 hibit a regular alternation of longer segments 

 consisting of more refractive and largely 

 doubly refractive (anisotropic) material, with 

 shorter less refractive segments showing little 

 or no double refraction (isotropic). In the 

 voluntary muscle-cell of vertebrates the fibrils 

 are crowded together closely, and in such a 

 manner that the adjacent anisotropic and iso- 

 tropic segments of different fibrils lie at the 

 same level; this level is perpendicular to the 

 long axis of the muscle cell, hence the latter, 

 as a whole, shows the characteristic cross- 

 striation. It is this peculiarity of parallelism 

 in the disposition of the fibrillar segments 

 which Hiirthle characterizes as " schwer ver- 

 standlieh." It plainly suggests that the fibrils 

 are laterally conjoined in a definite manner 

 so as to form a system coherent throughout 

 the cell. Conceptions differ, however, as to the 



ultimate structure of the individual fibrils, 

 and particularly as to the nature of the con- 

 ditions producing the segmented appearance. 



I shall not attempt in this place to dis- 

 criminate minutely between conflicting views 

 regarding structure; my aim is merely to 

 show that a segmented structure is in itself — 

 and largely irrespective of the precise nature 

 of the structural conditions that determine 

 the segmentation — a condition favorable to 

 rapid and quickly reversible changes of form. 

 The questions to be discussed are thus : What 

 mechanical advantage does a rapidly contrac- 

 tile colloidal system like muscle derive (1) 

 from having its fibrils regularly and minutely 

 segmented, and (2) from having the corre- 

 sponding segments of adjacent fibrils at the 

 same level? 



Considered from a physico-chemical point 

 of view, the individual muscle-fibril forms a 

 colloidal system in which regions showing a 

 well-marked contrast in optical and apparently 

 also in chemical properties (evidence from 

 staining) alternate regularly with one 

 another. It is, however, doubtful if the alter- 

 nate segments differ in their essential chem- 

 ical constitution; the evidence seems rather to 

 indicate that the same colloidal material forms 

 the solid substratum of the fibril throughout 

 its entire length, but that this material dif- 

 fers in its state of aggregation in alternate 

 segments. The colloidal particles must be re- 

 garded as forming a coherent system through- 

 out the entire fibril, as in a gel; otherwise the 

 fibril would have no tensile strength. Be- 

 tween the coherent particles of any gel are 

 interstitial fluid-containing spaces.' If the 

 fibril is essentially a gel, as thus assumed, 

 with myosin as the chief colloid, its physical 



' The recent observations of Baehmann, made in 

 Zsigmondy's laboratory, on the ultramiorosoopic 

 structure of gels, show that during gelation the 

 motion of the submicrons changes from one of a 

 free translatory character to one in which the sub- 

 microns undergo only Blight vibratory movements 

 about fixed positions of equilibrium. There is 

 thus in gelation an aggregation of submicrons and 

 amicrons to form a coherent system, with fluid- 

 containing interspaces. Cf. Zeitschrift fur anor- 

 ganisohe Chemie, 1911, p. 125. 



