Something that's described as "deltoid", including your deltoid muscle, is shaped like this
[DOC] [TLE] How Muscles Are Named - Ground Up Strength [PAR] Nov 9, 2011 ... Incuded are Latin and Greek word roots that signal location, action, ... Get GUS 
on your RSS Reader ... Let's look at some of the basic words used to describe 
muscles: .... in calling a muscle "deltoid" because it is shaped like a triangle.