What are the lines called that run lengthwise through the nail?

The lines that run lengthwise through the nail are called striations. Striations are vertical or lengthwise lines that can appear on the surface of the nail due to various factors, including aging, nutritional deficiencies, or other health issues. Understanding these lines is important for identifying the overall health of the nails and for proper nail care.

Corugations, on the other hand, refer to wavy or ridged surfaces, which typically occur across the nail rather than lengthwise. Creases may refer to folds or indentations, but they do not specifically indicate the linear markings that occur parallel to the growth of the nail. The term "lines" is too general and does not specify the lengthwise characteristic necessary for accurate identification. Thus, striations is the most precise term to describe the lines running lengthwise through the nail.