10 ON THE BURSA FABRICII IN BIRDS. 



P.Z.S.1877, interior of the bursa is raised shows the following constituent parts: 

 (1) an external layer of connective tissue, covering (2) a thick layer of 

 elastic membrane ; inside this is (3) a thin layer of mucous membrane, 

 which unites together and connects the "lymphatic follicles." These 

 " lymphatic follicles " consist of masses of minute rounded cells, on an 

 average 0-04 millim. in diam.., enclosed in capsules of connective tissue, 

 in which ramify their nutrient vessels &c. The lymphatic follicles are 

 bound together by processes of the connective mucous membrane into 

 raised processes, which project on the interior of the bursa, forming 

 ridge-like " crests," and are covered with epithelium internally, the cells 

 of the latter being lanceolate with oval nuclei. In Rhea, however, the 

 follicles are not closely bound together in masses forming ridges, but are 

 attached by peduncles of elastic tissue to a central stem, the whole having 

 somewhat the appearance of a bunch of grapes with a few berries on it. 

 As we have already seen, Tannenberg in 1789 was the first to point out 

 that the bursa was more developed in young than old birds, it being 

 gradually reduced and obliterated in the latter. This process of atrophy 

 seems to obtain in all birds, so far as I can make out, though the periods 

 of final disappearance seem to vary much in different groups. M. 

 Martin St.-Ange found that the bursa began to lose its functional activity 

 in Pigeons after six months, and in Fowls after eight ; as a rule it seems 

 to atrophy at about the period of full growth. On the other hand, in 

 some cases it persists for long periods, and probably throughout life ; for 

 I found it well developed and quite open in a specimen of Platycercus 



P. Z. S. 1877, icterotis that had lived in the Society's Gardens for eleven years. As a 

 p. 311. rule, the central cavity of the bursa becomes diminished and the com- 

 munication with the cloaca obliterated in the process of atrophy; and 

 finally, in some cases at least, the whole organ completely disappears, 

 losing its characteristic form and structure, and becoming fused with 

 the tract of abdominal aponeurosis that covers the back wall of the ali- 

 mentary canal, ultimately forming a flat riband-like, or round cord-like, 

 ligamentous structure. Spangenberg observed the conversion of the 

 bursa Fabricii into a round cord-like ligament in the Duck ; and Alesi 

 has observed it (and described it minutely, with figures showing the 

 histological changes that go on) in the Fowl. He sums up the process 

 of atrophy in the last-named bird as follows : The epithelium which 

 covers the internal surface of the bursa becomes, as it were, invaginated 

 into the thickness of the mucous layer, becoming pressed in between the 

 lymphatic follicles. The connective tissue between the latter increases 

 rapidly in bulk, and becomes full of numerous small oval corpuscles; 

 at the same time the follicles become absorbed, and the whole of the 

 bursa becomes obliterated and fused with part of the abdominal 

 aponeurosis. The epithelium gradually becomes confined to certain 

 irregular tracts in the thickness of the mass of connective tissue ; but 



