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Show 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 plexusl. Uskow (5, p. 214), in giving a resume of the different types of diaphragm, says that the condition in M a n 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 M a n , but the diaphragm has no muscle, and its ventral portion is entirely fused with the substance of the pericardinm. I do not, however, think, in the light of the clear description he gives elsewhere of the important differences 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 in Apteryx nor in any other bird has either of these [pulmonary aponeurosis and oblique septum] the slightest real resemblance to a M a m malian diaphragm. For, as has been seen, the heart lies altogether behind both, and the muscular digitations of the pulmonary aponeurosis are supplied by the intercostal nerves, the phrenic being absent. The vertical and oblique septa2 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 1 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 indefinite number of spinal nerves of the cervical region. But seeing that there is this indefiniteness, 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 inclined, 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. 2 It will be noticed that he does not refer to the pulmonary aponeurosis with the oblique septum as homologous with the mediastinal tissues. 31* |