Medulla (hair)

The medulla is the inner most layer of the hair shaft.

Importance in forensics
Human hairs are unique in that their medulliary index (the fraction of the shaft's width that the medulla takes up) is small: generally less than 1/3. Other species will have ratios much larger, usually at least 1/2. One individual will have hairs with differing medulla patterns. One hair from the scalp may have an absent medulla while another will have a complete medulla. Unfortunately, medulla patterns are not very good as individual evidence and are typically only used for determining species and ruling out suspects.