NATURAL IMMUNITY 265 
The action of snake venom, especially that of Cobra, is quite powerful upon 
different species of non-poisonous snakes. The bite of Cobra was fatal 
within 30 minutes to several hours to the following snakes: Passerita mycteri- 
zans (green whip-snake), Tropidonotus quincunciatus (grass-snake), Den- 
drophis picta (tree-snake), and Dryophis (green tree-snake). Ptyas mucosus 
is much more resistant to cobra venom and often escapes death from several 
successive bites. If death occurs it usually comes over 24 hours after the 
infliction of the venomous bite. 
EXPLANATION OF THE MECHANISM OF NATURAL IMMUNITY. 
On what does the relatively high natural immunity of venomous snakes 
depend? Why are the venomous species of snakes more resistant to venom 
than their innocuous congeners, and why do the latter possess a greater resist- 
ance than various mammals and birds? Bearing on these interesting ques- 
tions numerous experiments were performed. 
The work of Leydig, Phisalix and Bertrand, Jourdain, and other anato- 
mists and physiologists established in the non-venomous snakes the existence 
of poison-secreting glands and made the differences in the non-venomous and 
venomous snakes appear as a matter of grade in the evolutional phase. (See 
Phylogeny of snakes.) The physiological analogies between the venomous 
and innocuous species have been shown by the toxic properties of the parotid 
glands of the latter to be somewhat comparable to the poisonous action of 
venom (Alcock and Rogers). Again, the poisonous properties of the blood 
serum of various poisonous snakes, as well as those of the innocuous kinds, 
came to light, and it was shown that these serums are rather powerfully 
poisonous, being strongest in the serum of the snake with the most active 
venom. 
These facts have formed the basis on which Phisalix and Bertrand built 
their theory that the non-susceptibility of snakes, especially of the venomous 
species, to venom is due to the constant internal secretion of venom. 
In 1893 Phisalix and Bertrand’ studied the relation of the poisonous 
properties of the blood of viper and its venom, and concluded that they are 
identical in their physiological actions. The source of the toxic principle 
in the blood was sought in the constant absorption of the venom. 
Calmette ? found that the blood of Cobra is highly toxic for the rabbit. 
The fact that certain non-venomous snakes sometimes enjoy a compara- 
tively high immunity to venom demands explanation. Phisalix and Bertrand® 
investigated this point carefully and have shown that while adders have no 
venom apparatus by which it is possible to produce a poisonous wound, the 
secretion of the parotid glands resembles venom in the effects it produces 
when artificially introduced into animals. Extracts of the various organs 
1 Phisalix and Bertrand. ‘Toxicité du sang de la vipére. C. R. Soc. Biol., 1893, 10 ser., V, 997; and 
C. R. Ac. Sci., 1893, CXVII, 1099. 
2Calmette. Sur la toxicité du sang de cobra capel. C.R. Soc. Biol., 1894, ro ser., I, rr. 
8 Phisalix and Bertrand. Sur la présence de glandes venimeuses chez les couleuvres, et la toxicité du 
sang de ces animaux. C. R. Soc. Biol., 1894, ro ser., I, 8. C. R. Acad. Sci., 1894, CXIII, 7. 
