The glands:

Salivary gland:
Some snakes have salivary glands that produce a toxic saliva.
But this are exceptions.

Duvernoy's gland :
Some Colubrids have Duvernoy's glands. They don't produce a venom stricto sensus, but a liquid between venom and saliva (Chippaux, 2002).
The role and the substances are variable according to species.

Roles :

- lubrication of the prey
- help to digestion
- immobilization of the prey
- killing of the prey (venom sensu lato)

With some species, the role can be limited to a lubrication and/or help to digestion. That's why the term "venom" is utilised in sensu lato for the secretions of Duvernoy's glands.

These glands cannot store venom (sensu lato) and no muscle compress it to facilitate the flow. This one is done in small quantity by gravity and capillarity.

Some examples:

Philodryas sp.: important envenimation near to Bothrops (potentially mortal for P. olfersii). Hemorrhagic syndrome. However some species have never caused real envenimation (P. aestiva, P. psammophidea).

ARAUJO, Maria Elisabeth de and SANTOS, Ana Cristina M.C.A. dos. Cases of human envenoming caused by Philodryas olfersii and Philodryas patagoniensis (serpentes: Colubridae). Rev. Soc. Bras. Med. Trop., Nov./Dec. 1997, vol.30, no.6, p.517-519. ISSN 0037-8682.

Dispholidus typus: discrete inflammatory syndrom. Massive coagulation, then haemorrhage after exhaustion of the plate system. Risk embolisms or infarction due to the clots thus formed.

Boiga sp.: local enzymatic activity without gravity. Except Boiga dendrophila: hemorrhagic activity.

Malpolon monspessulanus: loco-regional inflammatory syndrome (pain, oedema, lymphangite) and sometimes neurological disorders (paraesthesia, disturb swallowing, ptôsis, dyspnea).

all this species are opisthoglyphous. For the aglyphous, we can mentionne the genus Thamnophis with several Cases of human envenoming.