THE FROG 103 



of a number of small closed vesicles filled with fluid, whose walls 

 are composed of a single layer of cubical epithelial cells. Between 

 these vesicles is a fibrous connective tissue framework with a rich 

 vascular supply. It arises from the floor of the pharynx of the 

 embryo. The secretion of this gland appears to be a protein sub- 

 stance rich in iodine (lodothyrin) , and it exerts an influence over the 

 metabolism of the body. In man a certain disease of this organ 

 leading to its enormous enlargement is known as goitre. Inefficient 

 thyroid functioning in the young leads to a condition known as 

 cretinism, in which both bodily and mental activities are severely 

 upset, giving rise to a state of arrested growth and defective mental 

 development. In the adult a condition, known as myxoedema, ensues 

 when the thyroid fails to perform its function the hair falls out, 

 the skin becomes puffy, and there is considerable mental deteriora- 

 tion. A marked alleviation of these symptoms occurs if the patient 

 is fed with animal thyroids or injected with an extract of the gland. 

 Thus in some way or other the secretion of the thyroid gland pro- 

 foundly affects the metabolism of the body, presumable hindering 

 katabolism, and so stopping the normal growth or activities of its 

 parts. 



Certain other small glandular nodules, termed the Accessory 

 thyroids and Parathyroids, are also present. 



Supra-Renal or Adrenal Bodies. 



The position of these glands on the ventral surface of the 

 kidneys has already been pointed out. In the higher vertebrates they 

 are solid bodies composed of two layers, an outer or cortical layer 

 and an inner or medullary layer, and the cells composing these 

 layers are quite different. Both kinds of cells are to be found in 

 the frog, but they are more or less intermingled and without any 

 definite arrangement. The cortical cells are arranged in groups 

 or columns of various sizes, and the cells themselves are of an epithe- 

 lial character with large spherical nuclei. The medullary cells, on 

 the other hand, do not appear in such definite columns, have smaller 

 nuclei, are more granular, and are of the type termed chromophil 

 cells. The internal secretion of the supra-renal bodies has been 

 isolated, and from it a white powder termed adrenalin can be obtained. 

 When a solution of this is injected into the blood stream it brings 

 about a contraction of the non-striate muscles in their walls, and so 

 causes a great rise in the blood pressure and at the same time gives 

 tone to muscles as a whole. The secretion normally is considered 

 to maintain the tone of the body, and one of the most marked 

 symptoms of Addison's disease, a malady of these glands, is a loss 

 of this tone. 



