164 THE STRUCTURE OF MAN 
people, however, epithelial, lymphoidal, and fatty vestiges of 
it always occur. 3 | 
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’ 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 feetal 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 1t 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 Roebuck (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.] 
