First of all, borax (sodium tetraborate decahydrate) in water produces boric acid and raises the pH. So the studies on boron toxicity that refer to boric acid are absolutely relevant for Borax as well. (crunchybetty.com)
Brains were fixed with 4 % paraformaldehyde in 100 mM sodium tetraborate, pH 9.5, for 3 h, cryoprotected with 20 % sucrose - potassium - PBS (KPBS), and sectioned into coronal (30 μm) sections using a sliding microtome (Leica Microsystems Inc, Buffalo Grove, IL, USA). (alzres.biomedcentral.com)
Borax, of the mule team variety, is sodium tetraborate or sodium borate (to get all official for a second) and NOT boric acid (hydrogen borate), which is a common misconception on the internet, apparently. (wellnessmama.com)