By J. W. S. PRINGLE
Department of Zoology, University of Cambridge
No one is likely for a long time to make a working model of a muscle which is complete enough in its properties to satisfy physiologists. On the other hand, there is probably no field of physiological research in which the idea of models has been used to such a large extent. This paradox provides an example of the problem which must have been in the minds of the organizers of this symposium when they chose its title. In experimental biology, when we use the term model, we do not necessarily mean an actual construction whose appearance or properties are similar to the system we are studying; we usually mean that we think that the properties of the system as a whole derive from the properties of certain defined constituent parts, and that from a knowledge of the functioning of these parts and of their interactions with each other we could, if we chose, give an explanation of the functioning of the whole which would be more readily understood than if we tried to describe the whole all at once.
This definition of a model makes it conceptual rather than actual. In the context of muscle physiology there is one notable exception to this use of the term. Weber and his co-workers ( Weber & Portzehl, 1954) designate as models of muscle two types of biochemical system which may be prepared from living muscles by degradative procedures. The first is the artificially oriented thread of actomyosin prepared from certain extracted muscle proteins by drying under tension or through the stabilizing action of metal ions; the second is the naturally oriented and organized system of structural proteins obtained when living muscles are stored at low temperature in glycerol so that the other constituents of the cells are dissolved. Each of these systems has been extensively used for experiments on the energy supply for contraction, and their behaviour is similar in many respects to that of living muscle. They are mentioned here merely in order to make the point that it is possible to construct actual models of biological systems by controlled degradation of the living material as well as by the more usual engineering practice of synthesizing working models from elementary components. In muscle and perhaps in all tissues where the nature of the elementary components is unknown, actual synthetic models are unlikely to have more than didactic value.
In contrast to actual models whose performance can be subjected to experimental tests to determine their degree of similarity to the original biological system, conceptual models have to rely on logic for their justification. In such