98 BRITISH SERPENTS. 
bone, has the fang alone upon it. The adder-fang 
is abont one-third of an inch in length, curved back- 
wards, the extremely fine point directed to the throat. 
The fangs lie horizontally in a groove when at rest, 
and only assume the vertical attitude when the adder 
is about to strike. They can be moved independently 
of each other, one or both being brought into play as 
the adder wishes. When the adder is about to strike 
the lower jaw is opened, the pterygoid goes forwards, 
the os transversum rotates the maxilla (or upper jaw), 
and the fang is brought into the striking position. 
The gland which secretes the venom is situated 
at the base of each fang, and has a duct leading from 
it to the narrow canal through the fang, which has 
its point of exit just above the sharp point of the fang. 
Tn the adder this canal is a closed one, while in some 
other poisonous species it is merely a groove on the 
surface of the fang. The poison, which is a chemical 
one, is driven through the duct and down the canal by 
the action of the muscles of the lower jaw, the whole 
process being the work of a moment. 
When broken off or worn out, the fangs are quickly 
replaced by others, the new ones being already de- 
veloped and lying in the fang-sheath until required. 
Tf the adder’s jaw be opened widely (after death) and 
the fangs carefully moved into the striking attitude, 
the reserve supply of new fangs can be readily seen 
with the naked eye. (For the effeet of the bite see 
later.) 
