STRUCTURE OF THE VERTEBRATES 317 



the second gill pouch, and the gland soon loses its connection 

 with the pharyngeal cavity. 



The thyroid was one of the first endocrine glands brought to 

 the attention of research workers; for goiter, which is very 

 prevalent in certain regions, is a pathological condition of the 

 organ. The gland lies across the thyroid cartilage of the larynx 

 and is composed of secreting vesicles surrounded by connective 

 tissue. The thyroid secretion influences differentiation. In the 

 mammal the lack of sufficient secretion results in a form of 

 dwarfism (cretinism) which keeps the individual more or less 

 infantile throughout life. Although the condition is corrected 

 by feeding thyroid extract, there is evidence that the effect is 

 caused by the influence of the thyroid upon the pituitary, and 

 not a direct influence upon the tissues. The first experiments 

 relating to differentiation were upon frog tadpoles (Guder- 

 natsch). Feeding tadpoles bits of thyroid caused rapid meta- 

 morphosis. Larvae of bull frogs which normally would have 

 remained as tadpoles for two years can be metamorphosed into 

 small frogs in ten or twelve days. The thyroid gland also acts 

 as a metabolic accelerator. Effects upon metabolism are greater 

 when mammalian thyroid tissue is used, for the amphibia ap- 

 pear to lack, to a large extent, the essential radical in the 

 -thyroxin molecule. 



4. Parathyroids. The parathyroid glands received their name 

 from their position in juxtaposition with the thyroid gland. They 

 arise as proliferations of cells from the gill pouches, and consist 

 of four discrete structures. They secondarily become partially 

 embedded in the thyroid, but there is no relationship between 

 the two types of glandular tissue. The recent extraction of the 

 effective hormone and the experiments which have followed, 

 show that these glands influence calcium metabolism in the 

 body. So far as the mammals are concerned, the proper secretion 

 of the glands is not only necessary for the development of bone 

 and teeth, but for the function of the muscles as well. In some 

 animals accessory parathyroid tissues are present and carry on 

 the function of the glands in extirpation experiments. 



5. Thymus. The thymus arises as a paired gland from the 

 branchial pouches. The two halves usually unite, and in the 

 mammal the gland migrates posteriorly to a position near the 



