Nerves
Spinal nerves.–Each spinal nerve issues originally immediately behind the neural arch of the vertebral segment to which it belongs. This intra-vertebral position is ultimately modified into a more inter-vertebral one, owing to the predominant share of the neural arches, basidorsalia, in the composition of the whole vertebra. Consequently the nerves issue behind their corresponding vertebra.
The first spinal nerve, or N. suboccipitalis, is exceptional in several respects. It develops a dorsal and a ventral root like a typical spinal nerve, but the dorsal root soon degenerates in all Amphibia, while in the Phaneroglossal Anura the whole nerve disappears. The first spinal nerve reduced to its ventral half persists therefore only in the Apoda, Urodela, and the Aglossal Anura. It issues originally between the occiput and the atlas, but in the adult it is partly imbedded in the anterior portion of the atlas. Its own vertebra is lost, having probably been added to the cranium.
In the Urodela the first spinal nerve either remains separate, or it joins the second spinal, forming with it and with a branch from the third nerve the cervical plexus, which supplies the muscles of the cervical region. The third, fourth, and fifth nerves, and sometimes also the sixth, form the brachial plexus.
In the Aglossal Anura N. spinalis I. mostly sends a fine thread to the second spinal nerve, the rest supplies chiefly the M. levator scapulae, in Pipa the abdominal muscles also. In all the other Anura this N. spinalis I. is lost; occasional vestiges have been reported in Bufo vulgaris and Rana catesbiana, and remnants of it may possibly be found in Pelobatidae and Discoglossidae. The first actually persisting nerve of the Phaneroglossa is consequently N. spinalis II.
The brachial plexus is composed as follows:–Pipa, N. spinalis II. and III.; Xenopus and Phaneroglossa, N. spinalis III. and IV., with a small branch from the second; the next following three nerves, numbers V., VI., and VII., behave like ordinary trunk nerves.
The pelvic plexus of the Phaneroglossa is formed in Rana by the VIII. + IX. + X. + XIth nerves, the tenth issuing between the sacral vertebra and the coccyx. In Bufo and Hyla the plexus is composed of five nerves, the seventh spinal sending a branch to it. Occasionally the twelfth nerve contributes a small branch to the posterior portion of the plexus. This and the eleventh nerve leave the coccyx by separate holes, thereby indicating its composition. The rest of the spinal cord gives off no more recognisable nerves, owing to its reduction during the later stages of metamorphosis; its terminal filament passes out of the posterior end of the coccygeal canal.
Concerning the cranial nerves it is necessary to draw attention to one point only. The last nerve which leaves the cranium of the Amphibia is the vagus or tenth cranial nerve. There is consequently no eleventh, and no twelfth or hypoglossal, pair of cranial nerves. Their homologues would be the first and second spinal nerves, but the whole tongue of the Amphibia, with its muscles, is supplied by the glossopharyngeal, or ninth cranial pair, and is morphologically not homologous with the tongue of the Amniota.
Respiratory Organs
A very important and characteristic feature of the Amphibia is the development of two sets of respiratory organs: Gills and Lungs. It is as well to give definitions of these organs. Lungs are hollow evaginations from the ventral wall of the pharynx, and their thin, vascularised walls enable the blood to exchange, by osmosis, carbon dioxide for oxygen from the air which enters the lungs by the mouth or the nostrils, and the windpipe. The latter is unpaired, the lungs themselves are paired. Gills are highly vascularised, more or less ramified excrescences, covered by a thin epithelium of ecto- or endo-dermal origin, which permits of the exchange of carbon dioxide for oxygen from the air which is suspended in the surrounding water. It is obvious that this definition applies to all sorts of well-vascularised organs whose thin surface comes into contact with the water. Various recesses of the pharyngeal cavity, the dorsal and ventral folds of the tail-fin, nay, even any part of the skin of the body can, and does occasionally, assume additional respiratory functions. The proper definition of gills, in Vertebrates, requires, therefore, the restriction that they must be developed upon and carried by visceral arches.