What is the only venomous shark, and where is the venom injected from?

The Port Jackson Shark

The Port Jackson Shark has two similar-sized dorsal fins. Each fin has a spine at the leading edge, which is reputed to be venomous. The spines of juveniles can be quite sharp, but those of the adults are usually blunt.

The spines are sometimes found washed up on beaches and have been mistaken for all sorts of things from bird beaks to goat horns. These spines are believed to have given rise to the common name of the family, 'Horn Sharks'.

One observer noted:

"PJs are not dangerous but like all marine life, they are wild animals and should be treated with respect. PJs have a venomous barb in front of each dorsal fin. Some books I read indicated that this barb is very sharp in young PJs but it grows blunt as the shark matures. Apparently this barb even falls off sometimes and can be found washed up on the beach".

https://www.sharkwater.com/shark-database/sharks/port-jackson-shark/

http://www.abyss.com.au/PJs.html

The spiny dogfish shark is the most abundant shark. This small shark is also known as the piked dogfish, skittledog, spotted dogfish, white-spotted dogfish, codshark, and thorndog. Its dorsal fin has spines that are mildly poisonous.

http://www.enchantedlearning.com/subjects/sharks/species/Dogfish.shtml

The class Chondrichthyes is made up of cartilaginous fishes such as sharks, rays and chimaeras. Among them, only a few species of sharks are known to be venomous. To date, only two families, Heterodontidae and Squalidae, are reported to include venomous species. These two families are characterized by sharks with dorsal fin spines, which are not typical of most other families.

The horn shark (Heterodontus francisci) from the Gulf of California, possesses two venomous spines on the anterior portion of the dorsal fins. Envenomations from this species
occur when anglers handle struggling fish while attempting to remove the hook.

The spiny dogfish (Squalus acanthias) is another venomous shark from the Eastern Pacific. Again, injuries are the result of stings while live specimens are handled.

The toxin of venomous sharks is contained within the tissue embedded in a groove along the spine. Almost nothing is known about the toxic properties of the venom. Experiments
conducted on laboratory animals indicate the venom is comparatively mild. Nonetheless, human envenomation can produce intense pain. Swelling of the injured area can persist for several days. Clinical reports claim some species of Squalus can induce fatal stings. This, however,
remains unconfirmed.

https://txmarspecies.tamug.edu/fishfamilydetails.cfm?famnameID=Carcharhinidae