ANATOMY A]\ T D HISTOLOGY OF THE THYMUS GLAND 365 



At birth the thymus weighs about thirteen grams and has a light pink 

 color, due to its high vascularity. It usually reaches its maximum weight 

 about the time of puberty, when involution begins. Involution may, how- 

 ever, commence before puberty or may be delayed for many years. This 

 general association in time between involution and puberty is not generally 

 found among animals (Hoskins (&)). It would rather appear from the 

 table that the decrease in the size of the gland is closely related to the 

 diminution in the number of the lymphocytes in the circulating blood, 

 which is one point in favor of attributing a lymphogenic function to the 

 thymus. 



Involution. The involutionary changes are briefly as follows. The 

 color of the tissue becomes gray, and finally yellowish, owing to the in- 

 crease in the amount of connective tissue and of fat and the relative 

 decrease in vascularity. The thymic parenchyma decreases in amount, 

 as does all lymphatic tissue, as age advances. This loss is in a measure 

 compensated for by an increase in the fatty tissue, so that the absolute 

 weight of the gland as a whole is not greatly changed. 



Variation under Different Conditions. The growth energy of the 

 thymus is difficult to measure. In rats it is small, since the gland becomes 

 considerably reduced in weight in young animals held at maintenance 

 (Jackson (&)). We have evidence also that the weight of the thymus in 

 man is reduced by starvation, (Hart (#)). Castration is thought to bring 

 about an increase in size and to delay involution. According to Tandler 

 and Gross the thymus is hyperplastic in eunuchs. Feeding thyroid to preg- 

 nant animals is said to increase the development of the thymus in the 

 fetus. In very rare cases aplasia, or complete lack of development, of the 

 thymus has been recorded (Clark). 



Comparative Anatomy. 



A representative of the thymus gland is found as far down the scale 

 as the Lampreys. In some fishes it is claimed that the thymus maintains 

 in adult life the epithelial character, which is restricted to the earlier 

 stages of development in man. In birds, reptiles, and the majority of 

 fishes, however, it is for the most part lymphatic, as in man. Phylogenet- 

 ically the thymus may be regarded as the descendant of a gland originally 

 pouring its secretion into the alimentary tract, but which has undergone a 

 complete lymphadenoid metamorphosis. 



Histology. 



The lobes of the gland are divisible into primary and secondary lobules, 

 and in the latter cortical and medullary portions may be distinguished 



