6 INTRODUCTION 



suggested (e.g. Yapp, 1942). A sound definition must be such as to 

 include gastrin, which comes from isolated cells and for which 

 the stimulus to secretion is the direct action of mechanical pressure 

 in the gut lumen, and also insulin, which comes from small 

 groups of gland cells and for which the stimulus to secrete is the 

 level of glucose concentration in the blood. It must also include 

 the hormones which stimulate the secretion of other hormones, 

 like the interstitial-cell-stimulating hormone from the 

 adenohypophysis, which stimulates the secretion of testosterone 

 from the testis, as well as those hormones the action of which is 

 inhibitory rather than activating. 



Huxley (1935) suggests that a hormone is ''a chemical substance 

 produced by one tissue, with the primary function of exerting a 

 specific efltect of functional value on another tissue"; but, as he 

 admits, this has the teleological implications of any functional 

 account. Moreover, it loses sight of the fact that some chemical 

 substances, such as adrenaline, are present as by-products with no 

 apparent function in many primitive animals, and seem only to 

 have been salvaged for use as hormones in the more highly evolved 

 phyla. 



It is also well to remember that hormones, or their active 

 constituents, are usually rather stable compounds, able to persist 

 for some time in the blood stream and yet composed of molecules 

 sufficiently small to pass through the walls of blood capillaries and 

 cell membranes to reach their targets. 



The vascular hormones, with which this book is mainly con- 

 cerned, may best be defined as specific organic substances produced 

 by isolated cells, or by a tissue which may form a gland; they activate 

 or inhibit effects of functional value occurring in other cells or tissues, 

 to which they are carried in the blood. 



1.5 Types of hormones 

 Although many actions in a wide range of animals can now be 

 attributed to hormones, it cannot be expected that any functions 

 will belong to hormones alone ; rather must they be recognized as 

 playing but a small part within the complex co-ordination of 

 metabolic and other processes that supply and direct the food and 

 energy as between the multifarious daily activities of the animal 



