§ 4.32 CONTROL OF THE SECRETION OF KINETIC HORMONES 155 



peristalsis, the increase in rate of contraction caused by a hormone 

 can persist for some time without further release of hormone, if 

 the latter is only slowly destroyed in the tissues. This type of 

 effect can clearly relieve the nervous system from a considerable 

 expenditure of energy, on repeated impulses. This control of the 

 level of activity may be one of the most important general functions 

 of kinetic, or perhaps of all, hormones. 



4.32 STIMULATION OF THE SECRETION OF 

 KINETIC HORMONES 



Most kinetic hormones are secreted in response to stimulation 

 that is either direct or nervous. As far as the present evidence goes 

 it is rare for the stimulus to come from another hormone or to be 

 absent altogether. 



4.321 Direct control of isolated cells in the gut 



Direct stimulation can be brought about by mechanical or 

 chemical stimuli without any intervention by the nervous system. 

 The hormones from the gut mucosa of mammals are stimulated in 

 this way, either by mechanical distension of the stomach walls or 

 by the presence of acids or the products of digestion in the lumen 

 (Table 17). The integrated, self-regulating control of digestion, 

 which is brought about by a succession of these hormones has 

 already been pointed out as a specialized feature of mammals 

 (§ 4.113). The cells which secrete these hormones have not been 

 identified ; but they are assumed to occur singly in the mucosa from 

 which the hormones can be extracted, as there is no sign of any 

 endocrine glands there. They might be neurosecretory cells, such 

 as secrete so many other kinetic hormones; but no other neuro- 

 secretory cells have been shown to be directly stimulated as these 

 cells are, without any connection with the nervous system. On the 

 other hand, if the cells are endodermal they would be like the only 

 other hormone-secreting cells which can be directly stimulated, 

 i.e. those which secrete metabolic hormones from the parathyroids 

 and the islets of Langerhans (§ 5.521). 



The fact that the action of these hormones is performed by the 

 parasympathetic nerves in cold-blooded vertebrates is in favour of 

 the former postulate. 



