§ 4.221 ENDODERMAL ENDOCRINE GLANDS OF VERTEBRATA 139 



also of a copious neurosecretion, which passes to them within the 

 nerve axons from the brain and the corpora cardiaca, it is not clear 

 whether the inhibition is nervous or secretory (Scharrcr 1952- 

 cf. Fig. 3-2). 



4.22 ENDODERMAL ENDOCRINE GLANDS OF VERTEBRATA 



4.221 Thyroid gland stimidated by TSH 



The thyroid-stimulating hormone or thyrotrophin, TSH 

 (Table 16), can be extracted from the adenohypophysis of most 

 vertebrates, including teleost fish, but not the elasmobranchs ; yet 

 all too often the hormone has been tested on an animal of a differ- 

 ent class from that of the donor (Adams, 1946). In most cases, 

 even when the hormone comes from the same kind of animal, the 

 test of the efficacy of the TSH has been its action in inducing 

 growth of the thyroid gland rather than in stimulating its 

 secretion of thyroxine, which is the aspect under consideration 

 here. 



Teleostei. The most readily observed effect of thyroid secretion 

 is its power to increase the oxygen consumption of animals in 

 which it occurs. This can be used as an indicator of the action of 

 TSH on the thyroid. 



In most teleosts, except Pseudoscarus, it is extremely difficult to 

 remove the thyroid glands, because they are so diffuse; but the 

 same effect as thyroidectomy can be achieved by the injection of 

 methylthiouracil, MTU, which destroys the thyroxine. By using 

 this on goldfish, Cyprinus, before measuring their oxygen con- 

 sumption for several hours under constant conditions, it could be 

 shown that, whereas injections of TSH into normal adult fish in 

 the summer caused an increase of up to 200 per cent in oxygen 

 consumption, similar injections of TSH, following the use of 

 MTU, were without effect (Muller, 1953). This shows that, at 

 least in active adult fish, thyroid glands are responsive to the 

 endocrinokinetic action of TSH. It has also been shown that more 

 TSH is required at lower temperatures to maintain the same level 

 of thyroxine secretion (cf. Pickford and Atz, 1957). Removal of 

 the hypophysis should not affect these results. 



Amphibia. One of the early discoveries about endocrine glands 



