Thick and sticky thistle No. 3: Milk Thistle, Silybum marianum. This one is the best of the bunch. It has a modest number of spines nicely arranged around a more generous and showy flower than the others. Dried ones are used to decorate floral bouquets. It’s been used for food. Wikipedia says, but without backup citations:
The roots can be eaten raw or boiled and buttered or par-boiled and roasted. The young shoots in spring can be cut down to the root and boiled and buttered. The spiny bracts on the flower head were eaten in the past like globe artichoke, and the stems (after peeling) can be soaked overnight to remove bitterness and then stewed. The leaves can be trimmed of prickles and boiled and make a good spinach substitute or they can also be added raw to salads.
However, oddly enough, it isn’t good food for cattle and sheep; something about oxygen deprivation; see the Wikipedia article.
Milk thistle extract is not made from the milky sap but from the seeds, which contain the complex compound silymarin. This has been used for 2,000 years in efforts to cure a wide range of ailments from cancer to hepatitis, diabetes to cirrhosis, but current medical opinion says, generally, that more research and better quality research is needed before any health effects can be established with confidence. See, e.g., the Mayo Clinic report.