§4.11 DIGESTIVE GLANDS 117 



mid-gut crypts (Day and Powning, 1949); but there is only 

 presumptive evidence for associating this with increase in enzyme 

 formation, as in Astaciis. 



Cuticle formation in the Arthropoda is due to exocrine glands ; 

 but there is little indication of hormones being specifically con- 

 cerned with their stimulation, although the whole process of 

 moulting is under hormone control (Part II, § 3). 



There is as yet no other evidence of hormones in invertebrates 

 controlling the secretion of exocrine glands. 



4.11 DIGESTIVE GLANDS 



Glands which secrete into the gut occur in all vertebrates; but 

 little is known of the means of their control, except in mammals, 

 where there seems to be a progressive change from the purely 

 nervous control of the salivary glands at the anterior end, possibly 

 through mixed nervous and hormonal control of the stomach 

 glands, to purely hormonal control in the duodenum, pancreas 

 and intestine. In mammals, the action of at least five hormones is 

 to stimulate secretion; but that of enterogastrone seems to be 

 inhibitory, though this is less well established (Table 14). It will 

 be easiest to consider the mammals first, and then to comment 

 briefly on the evidence relating to other classes of vertebrates. 



4.111 Increase in secretion by gut glands 



Mammalia. Grossman (1950) has written a comprehensive 

 review of the hormones controlling the secretion of the glands in 

 the mammalian alimentary tract, and has issued a timely warning 

 about the technical difficulties of investigating them. For one thing, 

 the hormones are all secreted by unidentified cells within the 

 mucosa lining the gut (§ 2.21), and not by discrete glands, so that 

 the usual technique of removing the source of the hormone is not 

 available. Moreover, he points out that ''the glands of the aliment- 

 ary tract are capable of responding to, or being inhibited by, a wide 

 variety of substances which cannot be considered to be specific 

 hormones". For instance, when meat is eaten, it yields extracts 

 which are absorbed into the blood and are capable of stimulating 

 gastric secretion. These active extracts are known as "secreta- 

 gogues"; other tissues contain many other pharmacologically 



