212 E. V. COWDRY 



In fishes ^hey form an organ called the suprapericardial body. They 

 are to be regarded merely as rudiments in man and usually do not 

 persist 'at all in the adult, though they have occasionally been found in 

 close association with the parathyroid glands. Schilder and Erdheim 

 believe that they form true thyroid tissue in cases of thyro-aplasia. In 

 thyro-aplasia it would appear that the parathyroids are often present and 

 normal, though Siegert and others have recorded their absence. Mcker- 

 son has described the replacement of the lost thyroid tissue by thymus in 

 a calf. 



Growth. The thyroid gland, according to Jackson (a), "increases 

 slowly but steadily in relative size during the prenatal period." At birth it 

 averages about 0.125 per cent of the total body weight. Numerous ob- 

 servations have been made on the subsequent growth of the thyroid in 

 man, but they are scattered and uncorrelated. The white rat is the only 

 animal whose growth phenomena are being standardized, chiefly through 

 the researches of Donaldson and his associates at the Wistar Institute. 

 Jackson finds that the thyroid attains its greatest relative size at birth, 

 and then progressively decreases relative to body weight, though, of 

 course, its absolute weight increases. Hatai and Jackson agree that in 

 the rat there are no differences in weight of the thyroid between the two 

 sexes ; this is an interesting observation when we remember that in man 

 the organ is generally thought to be relatively heavier in the female. A 

 comparison of the laws of growth in the rat and man has been made by 

 Donaldson, and it would seem that there are many points of similarity. 

 Since the weight of the gland is so dependent upon the amount of stored 

 colloid, there is always this element of error in comparisons by weight 

 or volume with other ductless glands. The method advised by Bensley 

 for estimaitng the relative amounts of parenchyma and colloid is labori- 

 ous, but gives valuable information. He photographs sections and makes 

 tracings on cardboard of the contained colloid and parenchyma, cuts them 

 out and weighs them, and thus obtains a ratio. 



Some idea of the growth energy possessed by the thyroid gland is given 

 by Jackson's experiments of holding young white rats at maintenance of 

 body weight for several weeks by underfeeding. It was found that this 

 treatment produced a well defined loss in weight in the thyroids, as com- 

 pared with the hypophysis and suprarenals, which showed a considerable 

 increase. Similarly, castration brings about a reduction in the rate of 

 growth of the thyroid and an increase in that of the suprarenals and 

 hypophysis (Schafer), from which we would likewise conclude that the 

 growth energy of the thyroids is relatively small. As age advances retro- 

 gressive changes normally occur in the gland, characterized by a rela- 

 tive increase in the connective tissue at the expense of the secretory epi- 

 thelium. 



