38 AMPHIBIA 



the parotoids and larger dorsal glands, the secretion of which 

 is acid, and acts as a convulsive. 



The Indians of Colombia are said to employ the secretion of 

 Dendrobates tinctorius for poisoning their arrows. The poison is 

 obtained by exposing the frog to a fire, and after being scraped 

 off tlie back is sufficient for poisoning fifty arrows. It acts on 

 the central nervous system, and is used especially for shooting 

 monkeys. Concerning the use of this poison for " dyeing " 

 parrots, see p. 272. 



Tlie 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. 

 ^^(dammidra macidosa, BomMnator, Dcndrohates, 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. 



Nekves 



Spinal nerves. — Each spinal nerve issues originally immedi- 

 ately 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 vertebrae. 



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. 



