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 



A dissection showing tiie remarl^ablc venom glands oi the Night or Demon 

 Adder (Causus rhombcatus). These are 3) jnclies long. 



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 graduahy increased until the animal is 

 able to withstand eighty to one hundred ordinary fatal doses at 



