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SCIENCE 



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



cence of the particles, and if the contraction 

 is to be rapid and quickly reversible the move- 

 ment of the fluid must be equally so. It is 

 this consideration, in my belief, vehich ex- 

 plains the advantage of the segmented struc- 

 ture. The fluid displaced from each aniso- 

 tropic segment during coalescence of its par- 

 ticles gathers in the adjacent isotropic seg- 

 ments. These appear therefore to increase in 

 volume during contraction (Hiirthle), but the 

 apparent transfer of fluid between these re- 

 gions is merely the visible expression of its 

 displacement from the anisotropic segments 

 by the coalescing particles within the latter. 

 Evidently the quickness and readiness with 

 which the transfer of fluid occurs will in- 

 crease with the area of the surface separating 

 the two regions. The disposition of the con- 

 tractile material of the fibril in numerous 

 small denser segments separated by narrower 

 segments of looser texture {i. e., with wider 

 interstitial spaces) allows for the rapid dis- 

 placement of the fluid by the colloidal par- 

 ticles as they coalesce; in other words, mini- 

 mizes the resistance to such coalescence, and is 

 hence a necessary condition for any rapid 

 movement. In the reverse process of relaxa- 

 tion the original relative distribution of the 

 solid and the fluid portions of the system is 

 regained with equal readiness. 



Conditions in a colloidal system of this 

 segmented structure are thus highly favorable 

 to rapid and promptly reversible contractile 

 movements due to a massing or flocking of 

 colloidal particles. The striated muscle-cell 

 as a whole need lose no water in contrac- 

 tion; there is merely a temporary redistribu- 

 tion of the more fluid portion of the tissue 

 within the cells. The case of the vertebrate 

 smooth muscle fiber is different, and there is 

 good evidence that in this tissue fluid does 

 actually leave the cell during contraction. 

 The characteristic slowness of both contrac- 

 tion and relaxation in smooth muscle may con- 

 ceivably be explained on the supposition that 

 the fluid displaced by the contraction of the 

 intracellular fibrillar system can collect only 

 in the relatively large intercellular spaces, i. e., 

 must pass across the plasma membrane, in- 



stead of collecting in numerous minute intra- 

 cellular spaces as in striated muscle. The 

 transfer of fluid is thus necessarily gradual, 

 and the contraction and relaxation are cor- 

 respondingly slow. 



The massing of the colloidal particles in the 

 anisotropic segments results in a shortening 

 and thickening of these segments and hence 

 of the whole tissue. The advantage of having 

 all the segments of the different fibrils within 

 the same cell equal in size and situated at 

 the same level becomes evident on further 

 consideration. In a system of closely crowded 

 contractile fibrils the mechanical processes in 

 contiguous fibrils would clearly interfere with 

 one another were this not the case. The over- 

 flow of the displaced fluid into the isotropic 

 segments in any fibril would be impeded, and 

 the coalescence of the colloidal particles thus 

 cheeked or prevented, if during contraction 

 the isotropic segments were in close contact 

 with the anisotropic segments of other fibrils; 

 as the latter thickened they would compress 

 the isotropic segments of the fibril under con- 

 sideration, and prevent the passage of fluid 

 into these latter. Prevention of the displace- 

 ment of fluid would however retard or prevent 

 the coalescence of the particles, and hence the 

 whole dependent contraction. Such interfer- 

 ence between adjacent fibrils would obviously 

 be minimal with a parallel disposition of fib- 

 rils, such as we actually find in the tissue. 

 This arrangement is thus mechanically the 

 most advantageous conceivable for a tissue 

 whose effective action depends on the simul- 

 taneous contraction of a large number of 

 closely crowded fibrils of the above conceived 

 structure. 



We conclude therefore that the physiological 

 advantage of both the segmented structure 

 and the parallel arrangement of the fibrils 

 consists in the provision thus made for a rapid 

 to-and-fro displacement of the fluid part of 

 the fibrils during contractile activity. This 

 displacement of fluid is, however, to be re- 

 garded as merely incidental to the contrac- 

 tion, and not as its immediate cause; and in 

 this respect the theory supported above differs 

 fundamentally from those according to which 



