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 innermost layer of a carrot is called the core or heart. This part of the carrot is typically darker in color and has a slightly different texture compared to the rest of the carrot.
The medulla is the inner most layer of hair.
The innermost and hottest part of the sun is called the core. It is where nuclear reactions occur, producing immense heat and energy through the fusion of hydrogen atoms into helium.
Connective Tissue Sheath
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.
Hair Shaft
The three main layers of the hair shaft are the medulla (innermost layer), cortex (middle layer), and cuticle (outer layer). The medulla is the innermost part of the hair shaft, often absent in fine hairs. The cortex provides strength, color, and texture to the hair, while the cuticle is the protective outer layer made up of overlapping scales.
A single hair is called a "hair follicle" when referring to the structure from which the hair grows. The visible part of the hair itself is known as the "hair shaft." Each hair is composed of a protein called keratin and is part of the integumentary system in mammals.
the innermost part of something