THE ORIGIN OF THE HUMAN BODY 301 



alimentary canal and are therefore endodermic in origin. These 

 include the tonsils, the thymus, and thyroid glands, the asso- 

 ciated epithelial bodies, and, perhaps, the spleen. ... In their 

 function as formative nidi for the cellular elements of the blood 

 these organs form physiologically important auxiliaries to the 

 vascular system as a whole, but belong elsewhere in their ana- 

 tomical developmental affinities." ("History of the Human 

 Body," 2nd ed., 1923, p. 395— italics mine.) 



This being the case, it is much more reasonable to interpret 

 the thymus as an ontogenetic (embryonic), rather than a 

 phylogenetic (racial) rudiment. It has been observed that, in 

 the case of reptiles which lack definite lymphatic glands (which 

 function in man as formative centers of lymphocytes or white 

 blood corpuscles), the thymus is extraordinarily developed 

 and abounds in lymphoid cells. It has also been observed 

 that the formation of lymphocytes in the lymphatic glands is 

 regulated by the digestive process; for, after digestion, the 

 activity of these glands increases and the formation of leuco- 

 cytes is accelerated. Since, then, the lymphatic glands appear 

 to require the stimulus of the digestive process to incite them 

 to action, it is clear that in the foetus, which lacks the diges- 

 tive process, the lymphatic glands will not be stimulated to 

 action, and that the task of furnishing lymphocytes will de- 

 volve upon the thymus. After birth, the digestive process 

 commences and the lymphatic glands become active in re- 

 sponse to this stimulus. As the function of forming lympho- 

 cytes is transferred from the thymus to the lymphatic glands, 

 the former is gradually deprived of its importance, and, in 

 the interest of organic economy, it t)egins to atrophy, until, at 

 the end of the child's second year, or, at latest, when the child 

 has reached sexual maturity, nothing but a reduced vestige 

 remains of this once functional organ. "The thymus," says 

 Starling, "forms two large masses in the anterior mediastinum 

 which in man grow up to the second year of life and then 

 rapidly diminish, so that only traces are to be found at 

 puberty. It contains a large amount of lymphatic tissue and 



