All nail technicians are taught in school that they are voids in the nail plate cells that fill up with blood from the underlying tissues.
Since it is in both, finger nails and toe nails, you might want to consider this:
Splinter hemorraghes can be a sign of heart problems, but generally are a different thing all together. They are mostly caused by tiny voids in the nail plate cell production. They fill up with blood from the nail bed and appear as tiny black lines that look much like you have a splinter under your nail. Your nail plate will ride forward on the nail bed much like a train rides forward on its tracks. The rails are on the nail bed, and the grooves are on the underside of the nail plate. The voids are usually on the underside of the nail plate which allows them to fill with blood from the nail bed. They usualy are nothing to worry about unless there is a history of heart problems in your family. If so, then I would have a stress test perfromed if your doctor believes it to be
I've been wondering about the vertical ridges in the finger nails (not black) and what it may be an indication of too- I wondered about some kind of deficiency, lack of something? Internal stresses?
I've noticed them for some time now, but I've also not known how much of a parasite infection I've had, or how much junk has been in my liver (no gallbladder) until 7 months ago, and have been getting cleaned up internally.
My concern is that I have gotten more of these lines only lately. Before, I had only the one on my left ring finger. I started developing more in the past few months; roughly around the time when my eczema got much worse.
They dont really look like splinters though. They run the whole length of the nail and they differ in width so some are thicker and darker than others.
Theres no history of heart problems in my family. My mum though, has the same thing on her nails.