The medulla is found only in the terminal hair on our bodies, and it is located in the middle of the hair shaft. It is composed of a protein called trichohyalin, and the degradation and metabolism of this protein contributes to the water holding properties of our hair. Interestingly enough, the medulla is pretty much a series of empty spaces and contributes very little to the chemical or mechanical properties to our hair. You'd think it would be this strong backbone upon which everything else is built, but it's really an area filled mostly with air and protein.
The cortex is the star of our hair when it comes to internal structure and the mechanical properties. Our hair shaft is composed of columnar cortical cells called hard keratins (soft keratins are found in desquamating tissues like skin and mucous membranes, which you might remember from the chemistry of your skin). The cells are embedded in an amorphous matrix rich in high sulfur proteins, and are held together by disulphide bonds. surrounded by the outer cuticle. The cortical cells are normally cylindrical - about 50 to 100 micrometres long and 3 to 6 micrometres in diameter. The cells are closely packed together as the cortical cells interlock (this is called "interdigitation" because the little cells have finger like structures that come together like two hands).
We find the melanin - the high molecular, water insoluble pigment that gives our skin and hair colour - in the cortex. There are two types of melanin in our hair - eumelanin is brown to black, while pheomelanin gives us yellow-red to red colours.
The colour of our hair is an optical phenonomenon that has to do with the reflection and refraction of light from various interfaces. (Yep, now we're into physics!) The way our hair shines is also a function of physics, but we'll get into that more when we get to the cuticle.
Well, that's enough for today! Join me tomorrow for the hair strand, part 2 - the cuticle!