The medulla is the innermost layer of the hair shaft. This nearly invisible layer serves as the pith or marrow of the hair. Scientists are still uncertain about the exact role of the medulla, but they speculate that it is primarily an air space that is more prominent in in-pigmented (grey or white) hair. ref: Haircutting for Dummies J. Elaine Spear
Human hairs are unique in that its medullary index (the fraction of the hair shaft's diameter that the medulla occupies) is very small: generally less than 1/3. Other species will have ratios much larger, usually at least 1/2. The medulla's presence in human hair and the patterns of which differ both from person to person and from strand to strand on a single person's head. One hair may have an absent medulla while another from the same person may have a complete or fragmented medulla. For this reason, medulla patterns are not very useful as forensic evidence and are typically only used for determining the species of the subject. The medulla contains large amounts of mitochondrial (as opposed to nuclear) DNA. Mitochondrial DNA is passed from the mother to her offspring. As such, mDNA may be of forensic value.