A hollow in an aging face invites a quick fix. The easy miss is to fill it without asking where the volume was actually lost.
Where the volume goes
PubMed Central hosts a review of the aging face. It states: "Volume loss occurs within the subcutaneous fat compartments of the face, leading to a deflated appearance."
The key phrase is "within the subcutaneous fat compartments." The loss has a location, and that location is the thing a new injector has to learn to identify.
Terms to know
Hyaluronic acid comes up again and again in filler training, and Wikipedia has an entry on hyaluronic acid for the basics. Anatomy training builds on that, and Wikipedia's entry on the cadaver covers the term for anatomy study.
A check before treating
Before you plan any filler, name the compartment you think has lost volume and say why. In Parsippany, New Jersey or elsewhere, a patient is better served by an injector who can locate the loss than one who simply sees a hollow.
For the muscle layer that sits over these structures, Wikipedia's entry on facial muscles is a plain reference to keep nearby.