MUSCLE 265 



the surface relations of the rods to the substance in which they 

 are embedded. Molecules of fluid from the bright bands are 

 drawn in amongst them ; the rods are pushed farther apart ; 

 the fibre broadens with a corresponding diminution in length. 

 This brings muscular contraction into the category of the 

 phenomena which play the most important role in bringing 

 about the varied activities of the animal mechanism. Con- 

 traction is due to osmosis. 



The separation of muscle into fibrils after hardening does not 

 seem to bear out either the rod or the pore hypothesis of the 

 structure of the dim disc. It must be remembered, however, 

 that before the fibrils are teased apart the substance of the 

 fibre has been coagulated. The fluid in the bright disc may 

 thus have become as much a part of the fibril as the rod in the 

 dim disc. The longitudinal striation of plain muscle and the 

 appearance of continuous fibrillation in heart-muscle is more 

 difficult to reconcile with the hypothesis that striped muscle 

 is composed of interrupted rods. 



Muscle transforms the energy supplied to it by the blood 

 into mechanical work. It is doubtful whether any hypothesis 

 as to structure will help us to an understanding of the way 

 in which this transformation is effected. Explanations are 

 seductive, but all attempts at explaining the connection be- 

 tween molecular change and change in shape must be viewed 

 with suspicion. It is quite clear that muscle as a motor is 

 not to be compared with any form of motor with which we 

 are acquainted. It is also clear that the theory of muscle must 

 be applicable to all its varieties striped, cardiac, and plain. 

 It must cover the alterations in form of an amoeba and the 

 streaming movements of protoplasm within a vegetable cell. 

 Probably it must extend farther, ajrid cover the discharge of 

 electricity by an electric organ and the emission of light by the 

 lamp of a firefly. We are on ground so treacherous that we 

 are not sure whether, in crossing it, we may lean with confidence 

 on the laws of thermodynamics ; and doubt as to the applic- 

 ability of these laws to living tissue almost upsets one's mental 

 balance. Until we have evidence to the contrary, we are bound 

 to exclude such a misgiving from our minds. If we allow 

 it to influence us at all, it is merely to the extent of causing us 

 to hesitate to assume that the explanation of muscular con- 



