CHAPTER VII 



ENDOCRINE CO-ORDINATION 



Life as understood by the biologist is a term used to denote 

 a certain combination of reactions exhibited by organic systems 

 under the influence of external stimuli. In the opening 

 chapters attention was focussed upon the mechanisms which 

 underlie the characteristic manifestations of vital activity in 

 animals. From these we turned to consider the material 

 exchanges from which the energy of these activities is derived. 

 The activities of an organism are related in a definite manner 

 to external conditions ; and it is this co-ordination or integra- 

 tion of response which gives rise to the conception of the 

 individual as a physiological unit. 



The integration of response presents two aspects. In 

 all multicellular animals other than sponges, visible manifesta- 

 tions of activity involve first a receptive surface upon which 

 the stimulus operates ; secondly, a structure, the effector organ, 

 specialised for the performance of the appropriate response ; 

 and thirdly, intervening between these, a mechanism of 

 co-ordination by which the disturbance is propagated from the 

 seat of stimulation to the region at which the response is 

 carried into effect. Co-ordination of this kind is twofold : 

 all multicellular animals except sponges and a few aberrant 

 organisms (Mesozoa) of uncertain phyletic relationship possess 

 specialised conducting tissue in continuity with both receptor 

 and effector units, which tissue constitutes the nervous system ; 

 in addition stimuli may in some animals give rise to the pro- 

 duction of specific chemical entities which make their way 

 through the body fluids to the organs which they are capable 

 of activating. Apart from providing a means for co-ordinating 



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