THYROIDS AND PARATHYROIDS 



653 



The most sensitive and accurate test object for estimating the pharma- 

 cological activity of thyroid substance, or of its hydrolytic products, is 

 the tadpole. Its growth is hindered and its differentiation accelerated 

 by the active iodine compound, so that the limbs grow and the tail 

 atrophies, while the tadpole remains small (Gudernatsch) . By means 

 of this test, it has been possible to demonstrate the presence of active 

 and inactive iodine in the gland, and to estimate the rate at which the 

 thyroid can elaborate the active iodine-containing substance from the 

 inactive iodine (Figs, 206, 207). 



Fig. 206. Tadpoles after seven days feeding : A, with iodine-free hyperplastic lamb 

 thyroid in 50 milligramme doses on alternate days; B, with sheep thyroid 

 hydrolytic product (containing 17-9 milligrammes of iodine per gramme) in 10 

 milligramme doses on alternate days; C, with ox thyroid hydrolytic product 

 (containing 14-8 milligrammes of iodine per gramme) in 10 milligramme doses 

 on alternate days. On the other days all the tadpoles received fresh liver. 

 In A, normal growth similar to control. Great emaciation with differentiation 

 in B and C (Rogoff and Marine). 



While the precise r61e played by the thyroid in the economy re- 

 mains obscure, it is evident that in most animals and in man its 

 secretion is of great importance, whether it be solely the quasi- 

 external secretion of ' colloid, ' with its specific iodine-containing 

 substance that collects in its alveoli and slowly passes out of them 

 by the lymphatics, or perhaps, in addition, some other substance, 

 which, like the glycogen of the liver, never finds its way into the 

 lumen of the gland-tubes at all. It may also be admitted that, by 

 aiding in the maintenance of the normal level of general nutrition, 

 particularly that of the central nervous system, the ability of the 

 organism to cope with toxic substances introduced from the outside 

 or manufactured in the body is favoured. There is, however, no 

 evidence that an actual destruction or neutralization of toxic sub- 

 stances occurs in the gland itself. 



It is uncertain whether the secretion of the thyroid is influenced by 

 nerves. Section of most of the nerves entering the gland does not 

 obviously alter its activity. Thyroid tissue may be transplanted into 

 any part of the body, and there exhibit all the morphological, chemical, 

 and functional changes seen in the gland in situ. Transplantation in 

 the same nerve field as the original thyroid has no advantage over 

 transplantation into any other region of the body (Marine and Manley). 

 This is an indication that specific secretory nerves are not indispensable 

 for thyroid activity. 



