OCR Text |
Show 314 MR. W. A. FORBES ON THE [Apr. 17, In Larus I found the bursa represented by a small pouch. In the young Uria troile it is large and sac-like, and slightly curved from side to side (see fig. 2, p. 310). The walls are very glandular, and so thick that the central cavity is but small. There are no crests. In an adult Alca torda it was reduced to a pore-like opening. In none of the Tinami that I have dissected have I found any bursa; on the other hand, the posterior wall of the third cloacal chamber is covered with numerous glands arranged in a tree-like manner. In all the birds hitherto mentioned (with the exception of Plotus) the bursa, as we have seen, opens by a more or less constricted aperture into the general cavity of the cloaca. In the Struthious birds, however, the very opposite is the case. The cloaca (or at least as much of it as corresponds to the first and second chambers) opens into the bursa Fabricii. This will perhaps be best explained by looking at fig. 4, p. 313, representing the cloaca and bursa of a not full-grown Cassowary (Casuarius uniappendiculatus) from behind. The bursa is, as one sees, a large, somewhat triangular sac, attached above by a broad riband-like muscle to the posterior wall of the alimentary canal. Most of the back wallof the bursa has been cut away (a), to show the opening into its cavity of the cloacal chamber (b), out of which a pointer (di) is seen passing up into the rectum above through the recto-cloacal valve. From this, I think, it will at once be evident Fig. 5. Vent of young E m u (Dromceus novee-hollandice) with the parts still in situ, viewed from the outside. A. External sphincter. B. Cavity of bursa. C. Wall of cloaca. D. Opening of cloaca into the bursa. E. Clitoris. F. Glandular pores. that the cloaca does not open directly to the outer surface, but indirectly through the bursa by means of its large posterior and inferior aperture b. A similar condition of things is seen in fig. 5, in a young Emu (Dromceus novee-hollandice)-where the parts are undisturbed |