The milky secretion of toads protects them against many enemies, although not always against the grass-snake. A dog which has once been induced to bite a toad, suffers so severely that it will not easily repeat the experiment. The handling of tree-frogs also irritates both nose and eyes. The hind limbs of the Water-frog, _Rana esculenta_, have a very bitter, acrid taste. In short, most, if not all, Amphibia are more or less poisonous, and it is significant that many of the most poisonous, e.g. _Salamandra maculosa_, _Bombinator_, _Dendrobates_, exhibit that very conspicuous combination of yellow or orange upon a dark ground, which is so widespread a sign of poison. Other instances of such warning colours, protective in a defensive sense, are the Wasps and _Heloderma_, the only poisonous lizard. 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. {39}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.