§1-4 DEFINITIONS OF HORMONES 5 



Stimulate them to contract in their turn to give a wave of con- 

 traction passing back from segment to segment. This is a primitive 

 method of control which may be supposed to have preceded that 

 by the nervous system. 



A distinct contrast to this is afforded by the use of mechanical 

 distention of the stomach as a means of initiating the secretion of 

 GASTRIN, one of the hormones from the mammalian gut. For in this 

 case mechanical stimulation, like the direct chemical stimulation 

 which acts upon other endocrine cells in the same region, seems to 

 be part of a highly specialized system for harmonizing the succes- 

 sive stages of digestion, and to have succeeded the nervous control 

 that is used for a similar purpose by cold-blooded vertebrates 

 (§4.11). 



1.4 Definitions of hormones 



The simplest and earliest definition of hormones as "chemical 

 messengers" must be amplified, if it is to limit the use of the term 

 "animal hormones" to the circulatory activators. 



A well-estabhshed definition of a hormone is "a physiologic 

 organic compound produced by certain cells and carried by the 

 blood to distant cells, the activities of which it influences" (Selye, 

 1947). This is still rather too loose a definition; it accords more 

 nearly with what has been termed a "humoral mechanism", or 

 "a process which has been demonstrated to be independent of 

 nervous connections between the site of stimulation and the 

 effector site, and is, therefore, considered to be transmitted by a 

 blood-borne substance, but in which the hormonal or non- 

 hormonal nature of the blood-borne substance may be uncertain" 

 (Grossman, 1950). This refers particularly to substances like 

 histamine, which may occur in the blood in the abnormal condi- 

 tions of an experiment and yet play no part in normal physiology, 

 and to the so-called "secretagogues". These last are substances that 

 may be found in extracts of gut cells ; they have the capacity to 

 stimulate enzyme secretion by the gut glands but are not natural 

 secretions (§ 4.11). 



It has already been indicated that a hormone is not necessarily 

 secreted by a gland, nor is its secretion by any means always 

 stimulated by nerves, as some elementary definitions have 



