420 



THE SNAKES OF SOUTH AFRICA. 



Horses, and sometimes donkeys and mules, are used for the 

 purpose. The normal fatal dose of snake venom for a horse is 

 carefully ascertained, and the immunizing operations are begun 



Fig. i6o. — i. A dissection sliowinj^ the remarkable venom ^;laIKIs oi the Night or Demon 

 Adder {Causus rhombcalus). These are 3J inclies long. 

 2. When dissecting this Night .Adder, only one venom gland was found. There 

 was no trace whatever of another, although both fangs were intact. The 

 specimen was an exceptionally large one. The dissection shows the single 

 gland and the space where the other should have been. 



by injecting the animal with considerably less than a fatal dose. 

 The dosage of venom is gradually increased until the animal is 

 able to withstand eighty to one hundred ordinary fatal doses at 



