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Show 458 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, rosis," while the "intermediate" or " diaphragmatic " air-sacs lie posterior to this in a layer of tissue which, at this stage, forms the thicker element of the avian diaphragm. At the beginning of the 12th day (cf Plates XLVI., XLVII. figs. 14 and 15) we find that these air-sacs have increased in proportional size. Compare, for instance, the relative extent of the anterior diaphragmatic sacs (s.a1) in figs. 12 and 15, which are taken through an approximately corresponding region of embryos of the 10th and 12th day respectively. As these air-sacs develop, they, so to speak, split the layer of the diaphragm in which they lie, part going, with the darkly staining layer and tissue in front, to form the " pulmonary aponeurosis," and part, that lies postero-ventrally to the air-sacs, forming the "oblique septum." In the end the two air-sacs referred to come into contact, and any membranous diaphragmatic tissue that may lie between the apposed walls is quite insignificant. The avian diaphragm is thus seen to be completed as a single structure, and its separation into its two laminae is a secondary detail arising in connexion with the development of the two pairs of intermediate or diaphragmatic air-sacs, which first penetrate it at a time when no distinct line can be drawn between the tissue that goes to form it and that of the developing lung itself. III. (b). Certain previous Opinions with regard to the Avian Diaphragm. With regard to the question of the homology, if any, between the Mammalian and Avian diaphragms, Sappey, whose most excellent monograph on the respiratory apparatus of the adult bird (1) was published in 1847, expresses his opinion as follows (p. 21) :-"Dans les oiseaux le diaphragme comprend deux plans qui se confondent a leur point de depart, mais s'isolent bientot pour suivre l'un une direction transversal, l'autre une direction oblique." "Le premier a pour analogue dans l'homme et les mammiferes toute la partie de ce muscle qui s'insere a la face interne des cotes ; le second repre'sente les piliers du diaphragme ; " and he gives his reasons for regarding the attachment of the paired partly muscular fibrous tracts in the anterior dorsal region of the oblique septum as homologous with those of the pillars of the diaphragm to the lumbar vertebrae. The presence or absence and the distribution of muscle-fibre seems to have considerable weight with him and others in dealing with questions of homology1; and with this the questions of nerve-distribution must be also considered. With regard to this latter, Sappey says (1, pp. 25 and 26) that (what Huxh *• afterwards called) the " pulmonary aponeurosis" is supplied from the intercostal nerves, and suggests that this mode of supply is sir.i.ar to that by the phrenic nerve, both being by branches 1 The question is apt to present itself, however, is muscle-fibre of great importance in such a case ? Might we not have a septum homologous to the mammalian diapheagm without any muscle developed in it at all, simply consisting, so to speak, of the pleural and peritoneal membranes back to back ? |