The innermost layer of the hair is the Medulla.
The innermost part of the hair (medulla) and the outside layer of the hair (cuticle) do not react with hair dye.
It's called the lower mantle.
The innermost part of an object is called its core.
The medulla is the inner most layer of hair.
The innermost part of the bone in called the Bone Marrow or inner bone
Because medulla means the middle or interior part of something. With regard to hair, the medulla of a piece of hair is the innermost layer of the hair shaft; with regard to the brain, the full term is the medulla oblongata, which is often shortened to just medulla when it's clear that the reference is to the brain, and I think the term is used in this instance to mean the first part of the brain, between the spinal cord and the rest of the brain.
the pistil and stamen
the innermost part of something
Hair Shaft
The endocardium is the innermost layer of the heart wall.
The hair shaft.
The middle.