INTERNAL SECRETIONS 515 



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There U a considerable divergence of opinion as t\> whether the 

 thyroids and parathyroids are one and the same tissue, or whether 

 they are quite distinct glands with markedly different functions. 

 According to the first view, the two tissues are essentially the same, 

 and all grades from normal thyroid to normal parathyroid tissue may 

 be found when the glands of a series of different animals are studied. 

 The difference in appearance is to be attributed to the presence of 

 colloid. After excision of the thyroid colloid appears in the para- 

 thyroid (Fig. 252). 



Developmentally, the parathyroids arise from the third and fourth 

 gill clefts, the thyroid from a median remnant in the ventral wall of 

 the embryonic pharynx. These glands arise at different times, and 

 come therefore only secondarily into relation with each other. 



FIG. 252. PARATHYROID OF A DOG EIGHTY-THREE DAYS AFTEK THYROID ECTOMY,. 

 SHOWING VESICLES, SOME OF WHICH CONTAIN COLLOID. (Swale Vincent.) 



c. re*., Colloid vesicle; e. ves., epithelium of colloid vesicles; 

 e. interves., intervesicular epithelial .cells. 



Attention was first drawn to the function of the thyroid (including 

 the parathyroid) by clinical observations. In certain districts the 

 gland enlarges in adults, forming what is known as a " goitre." This 

 appears to be due to an infection through the drinking-water of the 

 district of the alimentary tract by a living organism. Goitre has 

 been experimentally produced by drinking the residue filtered from 

 such water, and cured by the taking of thymol. Goitre is very localized, 

 hence the name " Derbyshire neck." The thyroid enlargement is 

 pathological in nature, and often entails a deficiency of thyroid 

 function. The children of mothers suffering from such a deficiency 

 of thyroid function suffer from a condition known as '' cretinism.' 7 

 Such endemic cretinism is common in Switzerland, and in the 

 central and Gilgit valleys of India. It varies with the prevalence of 

 the " endemic goitre." The " cretinism " is due to toxic agencies 

 acting upon the thyroid and parathyroids of the unborn. 



