Why all the instruments in Medieval images?

We know that most all Church music from the middle ages was vocal music, the exception being music heard in processions at large Cathedrals. Why, then, do so many Medieval manuscripts portray music with instruments? Modern artists pick up on this and constantly portray Church musicians with drums and tamborines and the like, with the implication that if we were truly doing what we should do, we should have an entire percussion and brass section at Mass each week.

Well, I had never thought much about this before, but I was reading a nice introduction to music in Medieval manuscripts by Nicholas Bell (Univ. of Toronto Press, 2001) and, in passing, he points out that the artists often depicted not instruments in current use but instruments as they imagined various Old Testament figures might have used. Actually, he is quite emphatic on this point as a way of underscoring the reality that Church music is in fact vocal music.

We might add another point. From the point of view of the artist, instruments are just better choices than open mouths if the goal is to portray music being made. Instruments are just more elegant, so, given the choice, imaginative Old Testament renderings are just better. Then they became artistic conventions. But that does not mean that you can drag out your bongos and play them in Mass.

It's a small point but an interesting one, and, these days, everything has to be explained.