The Common Toad (Bufo vulgaris), in which the skin, which is very thick and rugose, is covered on the back with large rounded tubercles with reddish summits. This species is a great destroyer of insects, and, as such, is very useful to agriculturists.
The Natter-Jack (Bufo calamita), in which the digits are palmate at the base. When irritated it contracts its skin and covers itself with a white frothy exudation, which gives off an odour of burnt powder.
The Green Toad (Bufo viridis), which is especially abundant in Southern Europe, the Levant, and North Africa.
The Musical Toad (Bufo musicus), a species distributed throughout North America as far south as Mexico, and in which the back is covered with pointed conical tubercles resembling spines.
The Brown Pelobates (Pelobates fuscus), common in the neighbourhood of Paris, the skin of which is almost entirely smooth. Although it appears to be nearly destitute of glands, this animal secretes a very active venom, which has a penetrating odour and kills mice in a few minutes, producing vomiting, convulsions, and tetanic spasms of the muscles.
The toxicity of the venom of toads was long ago demonstrated by the experiments of Gratiolet and Cloëz.[154] It is manifest only in the case of small animals, and in man merely produces slight inflammation of the mucous membranes, especially of the conjunctiva.
That this venom preserves its toxic properties for more than a year in the dry state was shown by Vulpian, and satisfactory studies of its composition and physiological action have been made by Fornara,[155] G. Calmels,[156] Phisalix and Bertrand,[157] Schultz,[158] Pröscher,[159] and S. Faust.[160]
Toad-venom was prepared by Phisalix and Bertrand in the following manner: Holding the head of one of these batrachians under water, they expressed the contents of the parotid glands with the fingers or with a pair of forceps. They repeated the same operation with a second, and then with a third toad, until they had sufficiently impregnated the water, which serves to dissolve the venom. In this way they obtained an opalescent, acid liquid, which they filtered with a Chamberland candle under a pressure of from four to five atmospheres. There remained on the filter a yellowish substance, with a highly acid reaction and partly soluble in ether and chloroform, while there passed through the pores a clear, reddish, and slightly acid liquid, which on being evaporated left behind a greyish-white precipitate. This precipitate was separated by filtration, washed in water, and redissolved in absolute alcohol or chloroform. The albuminoid matters were thus separated, and the liquid, after being rendered limpid by filtration, was evaporated away. The substance obtained in this way represents one of the two active principles of the venom. It acts on the heart of the frog, and arrests it in systole. It assumes the appearance of a transparent resin, the composition of which roughly corresponds to the formula C119H117O25. It is the bufotalin of Phisalix and Bertrand, and is probably identical with that obtained by S. Faust, the formula of which, according to the latter author, is said to be C11H23O5.
Bufotalin is readily soluble in alcohol, chloroform, acetone, acetate of ethyl, and acetic acid. When water is added to a solution of it in alcohol it is precipitated, giving a white emulsion, which has a very bitter taste.
From the aqueous extract whence the bufotalin has been separated, it is possible to separate a second poison, which acts on the nervous system and causes paralysis. In order to obtain it in a pure state, the extract is treated with alcohol at 96° C., filtered and distilled; the residue dissolved in water is defæcated with subacetate of lead and sulphuretted hydrogen. The solution thus obtained is successively exhausted with chloroform to extract the cardiac poison, and with ether, which removes almost the whole of the acetic acid. The second neurotoxic principle, called bufotenin, remains in the residue of the solution after being evaporated in vacuo.