Surrounding this very soft, fluid filled center is a pulp, inner nail bed, or layers of soft and moist tissue that helps to protect and cushion the sensitive vein and nerves much as our fatty tissue and subcutaneous makeup does for our own bodies. This area is slightly harder than the layers beneath it, yet still cannot be counted as the nail itself because it cannot protect the quick of the nail when exposed. This area is also what is visible in a light colored nail as the darker circle or half moon shape when we trim back the nail and get closer to the quick. On dark nails it can be nearly impossible to see, but it does make a different sound in the nail trimmers when clipped into. This area feels pressure and will often cause the dog to begin to pull back as it feels this pressure and anticipates possible pain.
What @PinkCupcake is talking about is the pulp
Inside your dog's black toenails, there is a "dot" or pulp that will tell you when to stop cutting your dog's nail. This dot will start out white or light-in-color and will get gradually darker the further into the nail you go. When this "dot" is gray or black-in-color, stop cutting as you are getting close to the quick.
nails cut - Dog Breed Info Center
Toenail length must be checked frequently. Most dogs need to have their nails shortened regularly by use of either a nail cutter or a nail grinder, as they do not get worn off sufficiently by ground contact. Old dogs need especially frequent nail care. If your dog has dewclaws (5th toe), these toenails will definitely have to be shortened regularly as they do not wear down at all; if neglected they will eventually ingrow , causing great pain. For cutters use the big scissors action type, rather than the "Resco" type : the latter does not work as well on really large nails. For a grinder , I like to use a Dremel Moto-Tool with a drum sander attachment with coarse sanding drums. You are much less apt to invade the sensitive "quick" or pulp inside the nail using the grinder; and you can create a bluntly rounded tip to the nail, which is less apt to split. If the outer shell of the nail does get split, this can be very painful and can lead to infection of the inner pulp. Cutting off a split nail and cauterizing the cut end is usually best done by your vet.
Cutting your Dogs Nails, Nail Trimming Dogs, Cut a Dogs Nails.