442 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



which are demonstrable in muscular tissue are long thin cylinders 

 which are maintained in a condition of elongation by passive stretching. 

 Under the influence of increase of surface-tension at the surface of 

 contact of these tubules and their fluid contents Imbert assumes that 

 the tubules become more spherical and therefore shorter, the simul- 

 taneous shortening and swelling of a number of these elements leading 

 to the contraction of the muscle. According to this hypothesis the 

 work performed in muscular contraction is derived from changes in 

 the surface energy of the fluid contained in the tubules. Bernstein, 

 rather drastically assuming certain magnitudes for the tension and 

 alterations thereof at the surfaces of these tubules, inferred that if the 

 tubules consist of the visible muscle-fibrils then the surface afforded 

 is not sufficient to account, on Imbert's hypothesis, for the amount of 

 work performed in contraction. If, however, we suppose that the 

 fibrils are broken up into a number of separate elements, for example 

 into rows of ellipsoids which become spherical when the tension of 

 their surfaces increases, then the surface presented would be sufficient 

 to account for the observed release of mechanical energy. Now recent 

 investigations by Schafer, McDougall and others on the details of the 

 microscopic structure of muscle, have revealed the presence in the fibril, 

 not exactly of the structure imagined by Bernstein, but one that for 

 the purposes of Imbert's hypothesis is precisely equivalent to it. Schafer 

 describes the contractile elements of the muscle-fiber as fine columns 

 or Sarcostyles which are divided into segments or Sarcomeres by thin 

 transverse discs, known as Krause's Membranes. Each sarcomere 

 contains a relatively opaque portion, the Sarcous Element, while those 

 portions adjacent to Krause's membrane are relatively transparent 

 and seen to consist of a fluid material. The sarcous element itself is 

 double and, if stretched, the two portions separate at a line which runs 

 transversely across the opaque portion of the sarcomere (Hensen's line). 

 On contraction the sarcous elements become shorter and thicker, 

 absorbing the fluid which constitutes the Hyaloplasm or intervening 

 transparent area between the sarcous elements and Krause's mem- 

 brane. We may therefore picture the muscle-fibril as consisting of a 

 series of discs formed by minute tubules packed together and com- 

 municating with spaces separating the discs and filled with fluid (Fig. 

 25). Evidently such a structure as this conforms to every requirement 

 imposed by Bernstein upon Imbert's hypothesis, and it is an exceedingly 

 significant fact that the details of the structure which we have out- 

 lined become clearer and more elaborate as we successively pass from 

 the relatively sluggish and inert smooth muscles, or the striated mus- 

 cles of amphibia, to the muscles of insects with their lightning-like 

 rapidity of contraction and enormous power of performing work. This 

 fact alone prevents us from entertaining any doubt that this elaborate 

 structure is an essential part of the muscular mechanism, and the 

 salient characteristic of this structure is the enormous surface of 

 contact which it brings about between the fluid and the semisolid 



