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Show 460 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, named the " recessus superior sacci omenti" in Mammalia, whose homologues in the Bird are much more conspicuous, that it occurred to me that these recesses and their bounding walls might serve as landmarks, and enable us more definitely to contrast the diaphragmatic structures in the two classes (cf. infra). III. (c). On the Development of the Pulmohepatic Ligaments and Recesses. In this connexion it will be convenient to consider the developmental changes in the chick classed under head (D) on p. 456. Referring to figures 1-4 (of which I and 2 are transverse sections of a 6th-day chick, 3 of an embryo of Lacerta vivipara, 4 of an embryo mammal*), we see running down the centre of the sections the median vertical sheet of tissue (m) which in all the types under consideration forms a complete vertical septum in the thoracic and anterior abdominal regions. On either side of it are set the lungs and the two halves of the liver, while the alimentary canal runs in its midst. It will further be noted that in all three cases the right liver-lobe is attached to the lung of its own side by a vertical ligament (a), which closes on the outside a space (2). The latter is blind anteriorly and opens into the general peritoneal cavity behind. In fig. 1, which is a section anterior to fig. 2, we see that in the chick there is, on the left side also, a corresponding ligament (a) and included recess (2'), while in the case of the Mammal and (in this instance) of the Lizard there is no such ligament traceable on the left side, and therefore no recess, properly speaking, though 2' in figs. 3 and 4 marks where it should be. Now the recess on the right side (2) is the "recessus superior sacci omenti" of His, as quoted by Ravn (9, p. 141), and the ligaments (a) are those which in the Bird can be clearly traced developing into the pulmohepatic ligaments ; while the recesses, with the addition on the right side of all that remains of the omental sac proper, develop into the pulmohepatic recesses. According to Ravn (op. cit. figs. 15, 16, and text) this "recess" on the right side is continuous with the main omental sac as late as about the 15th day in the Rabbit, but by the 17th day (p. 146) it has become constricted off from the latter cavity and persists as a separate closed peritoneal sac, which comes to wrap round the oesophagus. By a reference to the Plates, the development, in the chick, of these pulmohepatic recesses and ligaments may be followed. Figs. 1 and 2 are transverse sections of the 6th day (and the relations are much the same even on the 4th day). Figs. 5, 6, 7 may be called transverso-longitudinal sections (cf. fig. 25) of a 7th day embryo. Of these fig. 6 shows the foramen of Winslow (/. TV.) leading from the main peritoneal cavity into the sac (2) of the right side, part of which corresponds to the omental sac of Mammals. 1 See also figs. (3-9, 11, 12, 14-18 of Eavn's paper (9). |
