48 VENOMOUS SNAKES AND THE PHENOMENA OF THEIR VENOMS 
a typical mucus-epithelium and a granulated cell, but considered them to 
be identical only in different stages of function. They considered the gland 
in the corner of the mouth not identical with the parotis of the mammalian, 
and named it glandula angularis oris. Cholodokowsky (1892, 1893) declared 
that the salivary glands of the birds are purely mucus-secreting. Wieders- 
heim (1898) hesitated to conclude the identity of the gland of the mouth 
corner of birds with the posterior supralabial gland or the poison gland of 
snakes, while Gadow (1879) recorded the occurrence of the parotids in 
various species of birds. As mentioned before, snakes possess the supra- 
labial glands throughout all families. It was also remarked that only certain 
snakes have well-developed poison glands. The question may be asked 
whether the poison gland may be phylogenetically related to the parotid of the 
mammalia or the glandula angularis oris of the birds. What relation has it 
to the supralabial glands in general? The situation and the serous character 
of the secretion and the single excretory duct of the poison gland suggest 
strongly a possible homology with the mammalian parotid. The histological 
and the anatomical examinations of the supralabial glands of many innocuous 
snakes brought out certain interesting facts which seem to help to trace back 
the origin of the poison gland. The accounts of the anatomical studies of 
various glands of the oral cavity by the early investigators are mostly macro- 
scopic. Meckel (1826) stated that the non-poisonous snakes possess far 
larger salivary glands than the poisonous ones. A similar finding is in 
Stannius’s work (1846). The supralabial gland (glandula labialis supe- 
rior, Leydig; glandula maxillaris superior, Oppel) has been exhaustively 
examined by Tiedemann (1813), Meckel (1826), Cuvier (1810), Cloquet 
(1821), Dugés (1827), Duvernoy (1832), Schlegel (1837), and Leydig (1873). 
Leydig made the important discovery that the glandula labialis superior of 
non-poisonous snakes is composed of two distinct parts. The anterior part 
is reddish-gray in color and consists of many minute glandular grains, with 
numerous excretory ducts. The posterior part appears yellowish-white and 
is of coarser glandular grains. It possesses only one excretory duct and is 
homologized with the poison gland of the venomous species. Reichel (1882) 
demonstrated an exactly similar condition of the gland with Tropidonotus 
natrix, one of the harmless, solid-toothed colubrine snakes. 
According to Leydig (1873) the poison gland is not a proper gland, but a 
modification of one of the lobes of the supralabial gland, and may be present 
in non-poisonous snakes. It may be here stated that, prior to Leydig, Meckel 
(1826) announced that the poison gland originates in an enlargement and 
development of the yellowish portion of the supralabial gland. As early as 
1833 Duvernoy demonstrated the occurrence of the poisonous gland in many 
species hitherto considered non-poisonous, and declared thereby that the 
simultaneous presence of the poison fang is one of the essential characteristics 
of the poisonous snakes. Phisalix and Bertrand (1894) look upon the toxic 
property of the adder blood as deriving from the supralabial gland through 
the internal secretion, and the natural immunity of the same animal against 
the viperine venom as the result of constant immunization by the same prin- 
