Foxglove in Scalia's spinal fluid may not have been a sign of intentional poisoning. He was known to have a heart condition and Foxglove is where the heart medication Digitalis (Digoxin) comes from. Digitalis is known to cross the blood-brain barrier & into the cerebrospinal fluid to some degree so it wouldn't necessarily be unusual to find it on his post-mortem.
Also, Digitalis if taken without awareness…ie. just taking every day without checking your pulse rate beforehand…can provide enough toxicity that your heartrate could be slowed to a dangerous or fatal level. This is why Digitalis should ALWAYS be held (not given) if the pulse is less than 60 beats per minute.