OCR Text |
Show 308 MR. W. A. FORBES ON THE [Apr. 17, If the cloaca of an ordinary bird be taken and laid open in front (i. e. on the ventral surface) along the middle line, the rectum will be seen to terminate at the top of a more or less well-marked chamber, which usually is more capacious than the terminal part of the alimentary tract, from which it is generally separated off by a more or less plain valvular constriction, the different nature of the two parts being also frequently shown by the differences in the character of the mucous membrane lining them. Below the entrance of the rectum, on the posterior wall of the cloaca, and disposed symmetrically as regards the median line on the sides of which they lie, are the openings of the uro-genital ducts, frequently elevated on papilla?, which vary in shape in different birds. Of these the ureters open nearest the middle line ; externally to them are the openings or opening of the vasa deferentia or oviduct, which latter is fully developed only on one side, though sometimes a trace of it may be seen on the other also. Above the uro-genital papillae in many birds (e. g. Meleagris mexicana and Tetrao urogallus) is a well-marked valvular fold of mucous membrane, which thus separates off the upper portion of this " cloaca " from that part into which the urino-genital ducts open. Below these, again, is a similar, but usually better-marked fold (the vesical sphincter of St.-Ange), which is present in all the birds I have examined, in a more or less developed state. Below this in the middle line, and also on the posterior wall, is seen (if tbe specimen be not too old) a circular hole of varying dimensions. This is the opening of the "bursa Fabricii." This third part of the " cloaca " is bounded below by the external sphincter muscles, which often form in birds a great thick fleshy mass; this chamber, from lodging the penis or clitoris, is denominated "loge copulatrice " by M . St.- Ange. In many birds additional glands, opening by large pores, or forming irregular arborescent patches, are developed in the walls of the cloaca in this region. It results therefore from the above-described disposition of parts that, in its most developed form, a bird's " cloaca " is divided into three chambers communicating with each other, and into which open respectively (from above downwards), first the rectum, secondly the urino-genital ducts, thirdly the bursa Fabricii1. This disposition of parts is seen in fig. 1 (p. 309). The bursa Fabricii, when well developed, consists of a sac-like pouch, usually with thick glandular walls : it is blind above and constricted below, and opens by a simple rounded orifice without any valve2 1 Prof. Huxley describes it (< Anatomy of Vertebrate Animals,' p. 308) as opening into the "anterior and dorsal region of the cloaca." If "anterior" mean here, as I suppose it does, " anterior " as regards the whole length of the digestive tract, this is an error; for, as I have above stated, the opening of the bursa is in the lowest chamber of the cloaca (i. e. that nearest the vent), and therefore into the most " posterior " one as regards the general direction of the alimentary canal. 2 Barkow, I. s. c, describes the lower part of the peduncle of the bursa as projecting into the cloaca in the common Fowl, and there forming a conical sort of flap CLipfcV) over the opening. I have never myself seen any thing like this " Zipl'el" in any bird I have examined; and all other writers are equally silent on the point, Most, indeed, say expressly that there is no valve or flap' of any kind. * J |