[Zope] How do I eval a variable within the acquisition path?

I apologize for the question (the zope area of my brain must need coffee) but I can't find the answer in the zope book or in any of the scripts posted to the zope cookbook. I want to use a variable in a acquisition path. So for example, from the zope book on scripts, imagine that vaccinateplan() printed out a specific animals vaccination plan. Now instead of saying 'LargeAnimals' and 'hippo' like so: print context.Vet.LargeAnimals.hippo.vaccinateplan() I want to do like this: category = REQUEST.form['category'] animal = REQUEST.form['animal'] print context.Vet.category.animal.vaccinateplan() but I get an Attribute Error. How do I eval a variable within the acquisition path? Thanks, Tim