486 A MANUAL OF PHYSIOLOGY 



not, as a rule, survive the operation long enough for the 

 changes in the skin to be developed (p. 530). In the dog 

 and cat muscular weakness soon becomes marked ; tremors 

 of central origin appear, and increase in severity until at 

 length they culminate in general spasmodic attacks. The 

 tissues waste, the temperature becomes subnormal, and this 

 is associated with changes in the heat regulation (p. 512). 

 Dogs and cats often die in a few days after the operation ; 

 occasionally they survive some months, and in rare cases a 

 year. In young animals the symptoms come on more 

 rapidly, and are more severe than in old. If a portion of the 

 thyroid be left, or a graft be made, these effects are entirely 

 obviated. Not only so, but the administration of extracts 

 of the thyroid glands by subcutaneous injection, or the 

 glands themselves by the mouth, brings about a cure, per- 

 manent so long as the thyroid treatment is continued, in 

 cases of myxoedema in man, and sometimes, but with far less 

 certainty, prevents the development of the symptoms in 

 animals or removes them when they have appeared. The 

 same is true, although in a minor degree, of certain com- 

 pounds rich in iodine, for instance the so-called iodothyrin 

 or thyroiodin, which have been extracted from the organ. 

 While the precise role played by the thyroid in the economy 

 remains obscure, it is evident that its secretion is of the 

 utmost importance, whether it be solely the quasi-external 

 secretion of ' colloid,' containing the iodothyrin, 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. 



As in the case of other glands forming an internal secre- 

 tion, it has been debated whether the function of the thyroid 

 is to destroy toxic bodies or to form substances indispensable 

 or advantageous to the organism. It is probable that it does 

 both. The intra-vascular injection of iodothyrin (or extracts 

 of the gland) increases the excitability of the inhibitory 

 mechanism of the heart and of the depressor, and diminishes 

 that of the augmentor and vaso-con stricter apparatus. A 

 fall of blood-pressure takes place, partly due to the slowing 



