THE ORIGIN OF THE HUMAN BODY 299 



formerly developed and functional in the fcetus, and need not 

 necessarily be interpreted as traces of organs that functioned 

 formerly in remote racial ancestors. That there should be such 

 things as special fcetal organs, which atrophy in later adult 

 life, is a possibility that ought not to excite surprise. During 

 its uterine existence, the fcetus is subject to peculiar condi- 

 tions of life, very different from those which prevail in the 

 case of adult organisms — e.g. respiration and the digestive 

 process are suspended, and there is a totally different kind of 

 circulation. What, then, more natural than that the foetus 

 should require special organs to adapt it to these special condi- 

 tions of uterine life? Such organs, while useful and functional 

 in the earlier stages of embryonic development, will, so soon 

 as birth and maturity introduce new conditions of life, become 

 superfluous, and therefore doomed, in the interest of organic 

 economy, to ultimate atrophy and degeneration, until nothing 

 is left of them but vestigial remnants. 



The thymus may be cited as a probable instance of such an 

 organ. This organ, which is located in front of the heart and 

 behind the breastbone, in the region between the two lungs, 

 consists, at the period of its greatest development in man, of 

 a two-lobed structure, 5 cm. long and 4 cm. wide, with a thick- 

 ness of 6 mm. and a maximum weight of 35 grams. It is 

 supplied with numerous lymphoid cells, which are aggregated 

 to form lymphoid follicles (c/. Gray's "Anatomy," 20th ed., 

 1918, pp. 1273, 1274; Burton-Opitz' "Physiology," 1920, p. 964). 

 This organ is a transitory one, well developed at birth, but 

 degenerating, according to some authors, after the second year 

 of life (c/. Starling's "Physiology," 3rd ed., 1920, p. 1245) ; 

 according to others, however, not until the period of full ma- 

 turity, namely, puberty. (C/. Gray's "Anatomy," loc. cit.) 

 W. H. Howell cites both opinions, without venturing to decide 

 the matter (c/. his "Physiology," 8th ed., 1921, pp. 869, 870). 

 It was at one time classified as a rudimentary or functionless 

 organ. Later on, however, it was thought by certain observers 

 to be an endocrine gland, yielding a secretion important for 



