^ COMPARATIVE PHYSIOLOGY 



For our present purpose we shall wherever possible seek 

 to relate the properties of living matter to those of inanimate 

 nature ; where this cannot be done in the present state of 

 knowledge, we must proceed with the task of recording our 

 observations, as in the physical sciences, in quantitative 

 terms. 



In so doing we shall consider first the characteristic 

 activities which living organisms display ; second, the sources 

 of energy which lie behind these activities ; third, the way in 

 which the activities of an organism are co-ordinated with the 

 changing conditions of the external world ; and finally, the 

 means by which a new animate unit is brought into being. 



Organisms respond to their surroundings by movements of 

 various kinds — muscular y ciliary ^ amoeboid ; by the elaboration 

 of material secretions ; by the production of light, electrical 

 discharge ; and by changes in bodily colour. Structures which 

 carry out these responses in Metazoa are collectively referred to 

 as effector organs. The first type of effector which will be dealt 

 with is muscle. Muscular activity is a ubiquitous phenomenon 

 in metazoan organisms ; and therefore cannot be excluded from 

 the present survey of the physiology of the lower animals, 

 although our knowledge of muscular mechanism is largely 

 derived from the study of vertebrate animals. 



Of no form of response in organisms is our knowledge more 

 extensive than in the case of muscular contraction. The 

 greater part of this knowledge is based upon the study of 

 amphibian skeletal muscle. Before considering the quantita- 

 tive treatment of the energy changes associated with excitation 

 in muscle, a brief sketch must be given of those elementary 

 phenomena which can be demonstrated when an excited muscle 

 is allowed to lift a weighted lever whose movement is recorded 

 on the surface of a revolving drum. By this method (isotonic 

 contraction) we can arrive at some preliminary insight into the 

 sequence of events in the contraction cycle. 



When a muscle is excited by a single electrical stimulus the 

 curve recorded in this way shows three distinct phases : (i) a 

 period of latency intervening between the applicadon of the 

 stimulus and the beginning of response ; (ii) a period in which 



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