Rear-fanged snakes don't inject venom. Instead, the toxin flows down a telltale groove in a fang's surface and into the bite wound, inducing a state of shock.

In Sinornithosaurus fossils, researchers discovered an intriguing pocket, possibly for a venom gland, connected to the base of a fang by a long groove, which likely housed a venom duct, the study says. Sinornithosaurus fangs also feature snakelike grooves in their surfaces.
“The ductwork leading out of the venom gland gave the venom a way to travel to the base of the teeth, where the venom welled up in the grooves,” said study co-author paleontologist David Burnham of the University of Kansas Natural History Museum and Biodiversity Research Center.
“So when they sank their teeth into tissue of the victim, it allowed the venom, which was really enhanced saliva, to get into the wound.”
Turkey-size Sinornithosaurus, which likely had feathers, lived in the forests of what's now northeastern China, and was a member of the family Dromaeosauridae, as was another Jurassic Park baddie, Velociraptor. Birdlike Sinornithosaurus probably used its longish fangs to put the bite on prehistoric birds, Burnham said.
Like rear-fanged snakes and some lizards, the dinosaur probably had nonfatal venom that could shock its victims into a defenseless stupor—allowing Sinornithosaurus to eat in peace.
Burnham's research was inspired by the 2000 find of another possibly venomous dinosaur fang and by a recent discovery that the today's top lizard predator, the Komodo dragon, has a venomous bite that weakens victims so they can be eaten later.
Though believed to have descended from dinosaurs like Sinornithosaurus, today's birds are toothless and so lack a venom delivery system (though some birds do have toxic skin and feathers).
“How primitive is venom really? Does it go all the way back to the archosaurs?” he said, referring to reptiles thought to have predated dinosaurs by 30 million years or more. “These are things people haven't really tested yet.”
(Source: National Geographic)