164 THE STRUCTURE OF MAN 



people, however, epithelial, lymphoidal, and fatty vestiges of 

 it always occur. 



We cannot at present determine what was the original signi- 

 ficance of the thyroid and thymus glands, and the like is true of 

 an allied body, the so-called carotid-gland (glandula intercarotica), 

 which is found at the bifurcation of the common carotid artery. 



[Concerning the thymus, however, Beard, working chiefly at 

 the lower Fishes, in which it attains its greatest development, has 

 recently been led to the brilliant suggestion 1 that it may be 

 in them primarily protective of the branchial organs of respira- 

 tion, by a process of phagocytosis, in a manner akin to that in 

 which the tonsils and associated cytogenous tissues are protective 

 of the main respiratory passages of the pulmonary organs of the 

 terrestrial Vertebrata.] 



BURSA PHARYNGEA 



The primitive history of this organ cannot at present be 

 certainly determined. In Man it appears at about the third 

 month of foetal life, on the posterior pharyngeal wall, as an 

 epithelial evagination, directed upwards and backwards towards 

 the occipital bone. During embryonic life this structure becomes 

 shifted in the course of its growth ; its canal lengthens, and 

 finally approaches the tonsils ; after this it participates in all the 

 changes which affect these organs. Chief among these is degenera- 

 tion, which normally takes place before the time of puberty. The 

 degenerative processes bring about shrinkings, fusions, the formation 

 of crypts and cysts, and other modifications so diverse that hardly 

 any two cases are alike, and the most different accounts are con- 

 sequently given of them in the literature of the subject. 



The following lower Mammals are known to possess a bursa 

 pharyngea ; the Alpine Marmot (Arctomys marmota), the Pig 

 (Sus scrofa), the Eoebuck (Capreolus), and the Bear (Ursus). In 

 no other Mammals examined has anything of the kind been 

 found, and since no traces of the organ are to be observed in the 

 lower Vertebrata, its primitive history and physiological signifi- 

 cance remains problematical (Killian). 



(ESOPHAGUS AND STOMACH 



In their fully developed condition the cesophagus and 

 stomach show no anatomical peculiarities which need be specially 

 1 [Anat. Anzeiger, Bd. ix. p. 482.] 



