128 PHYSIOLOGY FOR DENTAL STUDENTS. 



unchecked, mental symptoms become apparent, first of all, a 

 dulling of the intellect with sleepiness and lethargy, and later, 

 muscular twitchings and tremors. Just as in cretinism, so in 

 myxcedema, administration of thyroid extract causes these symp- 

 toms to disappear, so that in a month or so the patient may have 

 returned to his or her normal condition, to maintain which, how- 

 ever, the thyroid extract must continue to be given. 



When the gland is removed surgically, either in lower animals 

 or in man, very acute symptoms ending in death usually super- 

 vene. These include a peculiar form of muscular tremor called 

 tctany, passing into actual convulsions, which, by involving the 

 respiratory muscles, ultimately cause dyspnoea and death. It 

 is, however, probable that these nervous symptoms are due to the 

 unavoidable removal of the parathyroid glands. The tetany is 

 removed by giving calcium salts. These conditions associated 

 with deficiency of the thyroid are grouped together as hypothy- 

 roidism. 



Even in healthy individuals thyroid extract taken by mouth 

 excites a more active metabolism, and may cause increased heart 

 activity. One result' of this increased metabolism is disappear- 

 ance of subcutaneous fat and increased appetite, thus rendering 

 the administration of moderate doses of thyroid extract a not 

 uncommon method of treatment for obesity. Such treatment 

 should never be attempted except under the control of a physi- 

 cian, for it is very easy to take too much of the extract and cause 

 palpitation and nervous excitement. 



When the thyroid (and parathyroid) glands become excess- 

 ively active in man, the condition is called hyper thy roidism, and 

 the symptoms are very like those above described as produced 

 by taking thyroid extract. To be exact, they are palpitation, 

 wasting of the muscles and consequent weakness, extreme ner- 

 vousness and protrusion of the eyeballs. On account of this last 

 mentioned symptom the condition is usually called exophthalmic 

 goitre. This acute and often fatal disease is to be distinguished 

 from chronic goitre, in which there are very few general symp- 

 toms, but great enlargement of the thyroid gland, indeed an en- 

 largement which may be so pronounced as practically to obliter- 



