Why Did People "Ejaculate" in Old Books?

Also, is our 's for possessives just a shortened form of "his"?

Twelfth Night celebrations In London, next to the Globe Theater.

Bob Woolley is reading Shakespeare’s “Twelfth Night,” and in Act 3 Scene 3 came across this passage:

Once in a sea fight ’gainst the Count his galleys
I did some service,

He notes that his annotated edition says that “Count his” simply means “Count’s,” and wonders, as many have, whether our “apostrophe-s” is short for his.