Minutus

Horizontal ridges may occur from infection, disease, or severe stress
(physical or psychological). A horizontal line (indentation) may occur
after a heart attack, major illness, or surgery. One of my sources
indicates you need to check for arsnic poisoning when you see horizontal
ridges - but most agree that it is a white horizontal line (not ridge) that
indicates arsnic poisoning. Vertical ridges indicate poor nutrient
absorption, malnutrition, kidney disorders, etc. Coloration and shape are
important in making some of the more specific determinations. Before
jumping to any conclusions, ask a few questions about use of gloves,
lotions, detergents, cleaning agents, pesticides, etc. - anything that
they come in contact with.
from side to side (=from left to right on the width of the nail).
which.