4.4: Ridges and furrows
- Page ID
- 52974
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Imagine if you will poor caveman Og sliding down the face of a boulder, relying on his friction ridge skin to save him, but suddenly his fingertips become glassy and smooth by the rough surface of the rock. Og would need a plan B, perhaps landing on a plump woolly mammoth. This would be the case if Og’s finger surfaces were not flexible. How do we therefore keep the surface of our fingers flexible? To answer this question, we must look at the formation of friction ridge skin, which is composed of a raised area, known as the ridge, and a depressed area, known as the furrow. Think of a field that has just been plowed with the planting area having been raised above a channel for watering. The epidermis is that plowed field. It is the surface layer of our skin. The dermis is the bottom layer of the skin. The two layers are held together by the papillae by interlocking papillae pegs.
In friction ridge skin, sweat glands provide secretions that allow the skin to be flexible and moist, and to facilitate grip without constantly wearing smooth. The sweat glands exit the epidermis at the pores.
It is the secretions from the sweat glands that deposit a latent mirrored image of the ridges of the finger. What are these secretions? The technical answer is that secretions are composed of fatty acids, amino acids, lipids, sodium, chloride, potassium, inorganic salts, sebum, proteins, cholesterol, carbohydrates, and organic and inorganic environmental contaminants. The easy answer is as follows:
Eccrine – Secreted by the eccrine gland and consists of approximately 98% water, electrolytes, and organic and inorganic solids.
Apocrine – Secreted by the apocrine gland and consists of water, protein, carbohydrates, sodium chloride, carbohydrates, and other organic compounds
Sebaceous oils – Generally associated with hair follicles, sebaceous oils contain sebum, the waxy and oily secretion that keeps hair and skin flexible. Sebum consists of triglycerides, wax esters, and squalene, a polyunsaturated hydrocarbon. Other components include cholesterol esters, cholesterol, and free fatty acids.
Yuck – Organic and inorganic environmental compounds make up the last category. These include anything that your hands have contacted throughout the day including hamburgers, cosmetics, grease, dirt, etc.
Take the first letter from each word and you find this is an EASY way to learn the basic secretions in a ridge print impression. Just be sure to say “contaminants” instead of “yuck” when testifying in court.
It is interesting to note that even latent fingerprints can be seen by the human eye when deposited on non-porous (hard) surfaces like glass or metal because of an inherent luminescent or “glistening” quality of the secretions or contaminants. You may have to turn the object slightly or use a flashlight at an oblique angle to see them, but it may provide a helpful hint as to what development medium to use.