458 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, 



rosis," while the " mtermediate " 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 (c/. Plates XLVL, XLVII. 

 figs. 14 and 15) we find that these air-sacs have increased in propor- 

 tional size. Compare, for instance, the relative extent of the anterior 

 diaphragmatic sacs {s.a) 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 jjenetrate 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. (6). 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 I'un une 



direction transversal, I'autre une direction oblique." " Le 



premier a pour analogue dans I'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 homology ' ; and with this the questions of nerve-distri- 

 bution must be also considered. 



With record to this latter, Sappey says (1, pp. 25 and 26) that 

 (what Huxl( •/ afterwards called) the " pulmonary aponeurosis " is 

 supplied from the intercostal nerves, and suggests that this mode of 

 supply is siir.Uar to that by the phrenic nerve, both being by branches 



^ 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 diaphuagm without any muscle develoiDed in it at all, simply con- 

 sisting, so to speak, of the pleural and peritoneal membranes back to back ? 



