140 THE BIOLOGY OF BIRDS 



discoveries (the inquiry is still young) will show that 

 hormones are important in backboneless animals, where 

 as yet we have only hints of their presence ; the actual fact 

 to-day is that it is only in regard to backboned animals 

 that we are sure that hormones play an indispensable role 

 in the internal economy of the body. It should be re- 

 membered that in backboned animals the blood comes to 

 its own in a way that does not hold true for the lower reaches 

 of the animal kingdom. The hormones in regard to which 

 physiologists have securest knowledge are those produced 

 by the thyroid gland, the parathyroids, the suprarenal 

 bodies, the pituitary body, the mucous membrane of part 

 of the digestive tract, certain islands of tissue in the sweet- 

 bread or pancreas, and the reproductive organs. It is 

 obvious from this list that hormones are formed in very 

 diverse parts of the body, and this suggests the further 

 fact that they have very varied properties. 



(i) Deficiency in the activity of the thyroid gland which 

 lies on each side of our larynx spells arrest of development, 

 cretinism, goitre, and the like ; and every one knows of 

 the modern miracle by which these abnormal states are 

 counteracted by giving the patients injections of the extract 

 of the thyroid gland of sheep. An exaggeration of the 

 activity of the thyroid leads to exophthalmic goitre and 

 other disorders, for excess is as dangerous as deficiency. 

 In a general way it may be said that the hormone of the 

 thyroid seems to regulate the metabolism of the body, 

 especially affecting the nutrition of connective and nervous 

 tissues. It tends to keep the activity of nerve-cells up to 

 the mark. 



(2) Of the smaller parathyroids, which are associated 

 with the thyroid, it may be safe to say, although the hormone 

 has not been isolated, that they put a brake on the excita- 

 bility of nerve-cells. In other words, their internal secretion 

 is, in Sir Edward Schafer's terminology, a chalone rather 

 than a hormone. But both would be included in Professor 

 Starling's definition : "By the term ' hormone,' I under- 

 stand any substance normally produced in the cells of some 



