Well being a Doctor he'd have seen it direct but he didn't think of it as anatomical, he was into serpents big-time and establishing a connection with proto-Christianity.

"The form of that stupendous work is a picture of the Deity, more particularly of the Trinity, but most particularly what they anciently called the Father and the Word, who created all things ... A snake proceeding from a circle is the eternal procession of the Son, from the first cause ... My main motive in pursuing this subject is to combat the deists from an unexpected quarter, and to preserve so noble a monument of our ancestors' piety, I may add, orthodoxy."