1889.] THE BODY-CAVITY IN LIZARDS, ETC. 459 



from spinal nerves ; while the nerves that supply the muscle in the 

 antero-dorsal region of the " oblique septum " seem comparable to 

 parts of the sympathetic of Mammals, in which group the diaphragm 

 is partly supplied from the solar plexus \ 



Uskow (5, p. 214), in giving a resume of the different types of 

 diaphragm, says that the condition in Man is like that in the Rabbit, 

 except that a part of the diaphragm is fused with the pericardium ; 

 and that the diaphragm of the Fowl is the same as that in Man, but 

 the diaphragm has no muscle, and its ventral portion is entirely 

 fused with the substance of the pericardium. 



I do not, however, think, in the light of the clear description he 

 gives elsewhere of the important diti'erences between the mammal 

 and bird, in the regions of the diaphragm (see below p. 462), that 

 such remarks are necessarily to be taken as implying a belief in an 

 approximate morphological homology. 



Huxley, on the other hand (4, p. 567), says " neither iu Apteryx 

 nor iu any other bird has either of these [pulmonary aponeurosis 

 and oblique septum] the slightest real resemblance to a Mam- 

 malian diapliragm. For, as has been seen, the heart lies altogether 

 behind both, and tlie muscular digitations of the pulmonary 

 aponeurosis are supplied by the intercostal nerves, the phrenic being 

 absent. The vertical and oblique septa' really answer to the fibrous 

 tissue of the posterior and middle mediastinum in Mammals. In 

 this, as in all other cases, the meaning of ornithic peculiarities of 

 structure is to be sought, not in Mammals but in Reptiles." And 

 he goes on to mention certain avian characteristics which are 

 elsewhere only represented in Reptiles, and to compare the Crocodile 

 with the Bird. 



Huxley's verdict on the question of the diaphragm, as thus 

 tersely stated, failed to remove the suspicion that while the more 

 central part of the avian diaphragm doubtless corresponded to 

 mediastinal tissue, a considerable portion thereof, more lateral in 

 position, might be homologous with the diaphragm of Mammals. 

 And it was only after reading Ravn's paper (9), in which, pp. 139- 

 147, he goes at some length into the development of what His 



^ A rather similar line of reasoning occurred to the writer independently. 

 For instance, in investigating the nature of the nerve-supply, the question at 

 once presented itself — Where is one to look for the homologue of the phrenic 

 nerve ? What is the phrenic nerve ? In Mammals it appears as a specialized 

 trunk (supplying a specialized muscle), composed of factors from a rather in- 

 definite number of spinal nerves of the cervical region. But seeing that there 

 is this indeflniteness, and that the division into regions (thoracic, cervical, &c., 

 &c.) of the vertebral column in Birds and Mammals is so very different, a 

 definite answer was not reached. I was rather inchned, however, to regard as 

 possibly to be reckoned in the same series with the phrenic nerve, those nerves 

 which are connected with the spinal nerves in the thoracic region (of the Duck) 

 rather after the manner of the sympathetic, and which, I presume, are the 

 nerves referred to by Sappey, in his second category, as supplying fibres to the 

 "oblique septum." In spirit-specimens, however, I did not detect any nerve- 

 fibres passing from them to that structure. 



- It will be noticed that he does not refer to the pulmonary aponeurosis 

 with the oblique septum as homologous with the mediastinal tissues. 



31* 



