THE STRUCTURE AND FUNCTION OF STRIATED MUSCLE
Authors: Macdonald J. S. 
Published: December 2014

In no previous hypothesis has a convincing attempt been made to coordinate all the several groups of phenomena known to attend the progress of muscular contraction. There is, therefore, no theory of muscular contraction either offered or accepted.In this paper an attempt has been made to satisfy the demand, which has been based primarily on the suggestion formerly advanced by MacDougall that the process is due to a displacement of water occasioned by differences of osmotic pressure.Reasons have been offered which discredit the opinion that these differences of pressure can be explained as arising from any disturbance in molecular numbers produced by the combustion of organic material. The combustion process does not produce new molecules in quantity sufficient to explain even one‐hundredth part of the force developed, nor is there any evidence of an internal mechanism whereby molecular motion might, as has been suggested, obtain the mechanical advantage required to explain this quantitative deficiency.Reasons have, however, been advanced to show that considerable variations in osmotic pressure are required to explain not only the contraction process as a whole, but also that first item in this process, the electrical change, which is generally admitted to precede the occurrence of the combustion process. It is probable that this particular variation in osmotic pressure is due to a disturbance of the inorganic salts, the main electrolytes of muscle, as also of the other tissues and fluids of the body.Evidence of a histological, that is, of a visible, kind has therefore been sought in support of the statement that disturbances in the inorganic salts of the muscle attend the development of the contraction process.Such evidence has been obtained by the use of a method introduced by Macallum, and necessarily stands or falls with the validity of this method. It has been shown, however, that the indications provided by its use are supported by the results of gold‐chloride staining which can be similarly interpreted.This visible evidence indicates the momentary development of a concentrated solution of inorganic salts within precisely the same unit of structure in the muscle as that into which water is moved during the contraction process.Evidence has been offered in support of the statement that this concentration of salt solution within the effective unit occurs at the moment of onset of the contraction, and is abolished by its further progress.Arguments have been arranged to show that this observation, whilst explaining the electrical change observed on the surface of the muscle at the onset of the contraction—the phenomenon in whose interest, so to speak, it was sought—also explains the whole of the remainder of the contraction process, the displacement and the replacement of water, the mode of onset of chemical change, and the means whereby the combustion process responsible for thermal change is developed.Apart from the nature of these arguments, open to criticism, prevalent opinion will doubtless discover an apparent weakness of the tale they are arranged to support in the fact that the combustion of organic material is indicated as a consequence and not as the determinant cause of the contraction.Evidence collected by other observers is arranged to show that this indirect relationship between contraction and combustion actually exists, and not that direct relationship popularly expected.Arguments have been displayed to support the view that this indirectness of relationship is in no conflict with the fact that the combustion process is the ultimate source of energy for all the work performed in the course of contraction.In the development of this theory of muscular contraction it has been found necessary to consider the nature of colloidal solutions such as are found within the cellular structures of the body, and the manner in which variations in their state affect the functions of these structures.Much use has been made of an hypothesis developed by the author originally in explanation of changes in colloidal state of nerve‐fibres accompanied by variations in salt concentration observed by histological methods; that is to say, directly observed as visible changes in colloidal solution and salt concentration, and also indirectly observed by means of electrical phenomena consequent upon their occurrence serving as an exact measure of the quantities involved. This hypothesis of colloidal solution is very similar in its basal assumptions to that advocated by Posternak, but will be found to diverge widely from this in certain important points. Thus it will be noticed that new views are developed in explanation of the conflicting action of salt solutions of minimal and maximal concentration upon the solubility of “globulins,” and also as to the necessary attendance of changes in reaction upon changes in the solution state of proteins discussed under the heading of “Polarisation.”These divergencies will render criticism of the hypothesis more simple. In the meantime the hypothesis has been used to explain the phenomena of muscular contraction, and, in doing so, seems also to offer a firm basis for an understanding of the development of muscle structure.An attempt has been made to explain certain physical characteristics of the varied structures of the muscle, as, for instance, their transparency or opacity, and all those visible characteristics or relations to the passage of light that are determined by inspection through the microscope, and also that “transparency” to electrical transfer realised by a study of excitation phenomena, as acquired during the life of each muscle‐fibre from the repetition of definite incidents of usage.In presenting this attempt for criticism it has seemed useful to consider the appearances and properties of other tissue elements, and references are made in this paper to the structure of bone as demonstrating physical change differing only in its irrecoverable degree from the physical changes responsible for the function of nerve and muscle, as also reference is made to the excitatory properties of the materials of the nerve‐cell as illuminating the character of sarcoplasm and explaining the structural appearances of the nerve‐cell and the growth of the nerve‐fibre.