Dear Christoph,

In reply to your question: 
>  I was wondering about the general image of dogs in the MA. Was it 
> all derived from biblical sources? If so, how is the biblical image, 
> generally? Jews were often called "dogs", and I don't know whether it 
> all goes back to Ps 21.

Dogs in the MA could be regarded as a positive symbol, like the 
dog at the feet of tomb effigies (the classical idea of faithfulness, 
as in Pliny's Natural History), or as the emblem of St Dominic 
('domini canes' fighting off the wolves [heretics] who are 
attacking the sheep [the faithful]).

I think you should look at the Bestiary for other medieval ideas 
about dogs.  One famous image was that of dogs returning to their own 
vomit, symbolising (a.o.) the Jews who turned away from Christ to 
adhere to their own faith but no doubt other list members can point 
you to more detailed examples.

Best regards,

Sophie Oosterwijk