774 PHYSIOLOGY OF DIGESTION AND SECRETION. 



usually described includes those cases in which both thyroids and 

 parathyroids were removed, while probably many of the apparently 

 negative results obtained after excision of the thyroids are expli- 

 cable on the supposition that one or more of the parathyroids were 

 left in the animal. It should be stated, however, that two recent 

 observers, Vincent and Jolly,* as the result of numerous experi- 

 ments made upon different varieties of animals, throw some doubt 

 upon these conclusions. They contend that in herbivorous animals 

 fully half of those operated upon survive complete removal of all 

 thyroid tissue, showing no evil symptoms except perhaps a di- 

 minished resistance to infection. Carnivorous animals, on the con- 

 trary, usually die after such an operation. Assuming, however, 

 that the distinction made by Moussu and others is well founded, 

 the interesting question arises as to the functional relationship be- 

 tween the thyroids and the parathyroids. Myxedema, cretinism, 

 and similar conditions in man may be referred to an atrophy or loss 

 of functions of the thyroids, but there is no knowledge of effects 

 attributable to pathological changes in the parathyroids. The old 

 view that the parathyroids represent embryonic thyroid tissue which 

 after removal of the thyroids replaces the functions of the latter 

 is not supported by experimental or histological evidence. The 

 two bodies represent different structures, histologically, embryo- 

 logically, and physiologically, and yet there seems to be some cor- 

 relation in function between them. The parathyroids, like the thy- 

 roids, contain a large percentage of iodin; when the parathyroids 

 are removed the thyroids exhibit a change in structure in that the 

 colloid material disappears from the vesicles, and finally the tetany 

 following removal of the parathyroids may be ameliorated by sub- 

 sequent excision of the thyroids. The exact nature of the functional 

 connection between these two organs is, however, as yet quite un- 

 explained; there is need for further investigation. 



The General Nature of the Functions of the Thyroids and 

 Parathyroids. Disregarding the difference in function between 

 these two bodies, it is quite evident from the facts given that they 

 exercise an important control over the processes of nutrition of the 

 body, and especially perhaps over those of the central nervous 

 system. How is this control exerted? Two general points of view 

 have been advocated. According to one theory, the thyroid tis- 

 sues elaborate a special internal secretion, characterized by its 

 contents in iodin. This secretion is given off to the lymph or blood, 

 is carried to the tissues, and there exercises a regulating action of 

 an important or indeed essential character. Excision or atrophy 

 of these bodies results in a loss of this secretion and a consequent 

 malnutrition or perverted metabolism in other tissues of the organ- 

 * Vincent and Jolly, " Journal of Physiology, " 32, 65, 1904. 



