Fingerprints

The use of fingerprints began in earnest as a method of identification in the late 1800s and early 1900s with the establishment of fingerprint classification systems. To thoroughly understand fingerprint identification and the methodology associated with the process, it is necessary to understand the terminology associated with fingerprints that has been established. Let us begin by setting forth definitions associated with fingerprints.

Definitions Associated with Fingerprints

Historical terms associated with fingerprints come from various parts of the world.

Dactylography, a Greek term that translates as “finger writing.” Dactyloscopy, a Greek term that translates as “to view the fingers.” Dermatoglyphics, a Latin term that translates as “skin carving.”

How, one might ask, do these terms apply to fingerprints? The terms are all associated with the fingers in one way or another and demonstrate that the pursuit of knowledge of fingerprints has been around for centuries. The modern day equivalent or usage of these terms can be converted into the scientific study of fingerprints for the purpose of identification.

How is a fingerprint defined? The following definitions will assist in understanding what we call a fingerprint, palm print, or footprint.

Fingerprint—The impression or reproduction left on any material by the friction skin of the fingers. Most often we see the impression left by the fingertips or the bulb of the fingers. But the friction skin covers the entire area of the inner hand (the palm) and fingers.

Palm print—The impression or reproduction left on any material by the friction skin of the palms.

Footprint/toe print—The impression or reproduction left on any material by the friction skin of the foot (feet) or toe(s).

It should be noted that friction skin is found on the hands and the feet of individuals. The friction skin is the outer layer of skin that contains many of the elements and characteristics we use to identify and individualize a print.

Friction skin—The skin on the inner hands and fingers, and on the bottom of the feet and toes, which is characterized by alternating strips of raised ridges and furrows arranged in a variety of patterns. The friction skin is found on both humans and anthropoids. In lower mammals, friction ridge patterns are sometimes similar to ours. Friction surfaces are sometimes padded in apes. The purpose of the friction skin, as the name implies, is to provide resistance so that those surfaces containing friction skin will be able to grasp objects. The friction skin forms during the third or fourth month of fetal growth (approximately 120 days). The process by which friction skin develops is through the formation of small islands surrounding pores, which then develop to form ridges.

Friction ridge—The raised portion of the skin that leaves the impression or reproduction.

Furrow—The portion of the skin lower and between the ridges.

A cross-section of friction skin (Figure 2.4) illustrates various parts and layers of the skin.

1. Epidermal layer: Outer layer

a. Stratum corneum: Surface skin

i. Friction ridges, furrows, and pores

ii. 1- to 2-mm thick

b. Stratum mucosum (Malpighi—inner skin)

i. Programs/forms outer skin

Figure 2.4 Cross-section of the structure of friction skin.

2. Dermal layer: (inner layer/second layer)

a. Dermal papillae

i. Determines ridge structure

b. Sweat glands and ducts

c. Nerves of touch (sensors)

d. Fat

The constancy (persistence) and uniqueness of friction ridges are:

Friction skin is persistent. That is, the skin does not change under normal conditions from the time of formation until decomposition after death. The exception is that, like other parts of the anatomy, the fingerprints or friction skin will get larger as the body grows. However, the specific characteristics will remain the same. Friction skin will deteriorate with age as well as all skin, but classification and identification normally will not be affected. There is an adage that is often used to describe the persistence: under normal wear and tear, friction skin will remain unchanged throughout one’s life.

Friction skin destruction (temporary or persistent) encompasses: An injury penetrating into the dermal layer (second layer of skin), through the dermal papillae, will result in the ridges not being regenerated. Scar tissue will form to the extent that the damage occurred, and only those ridges in the path of the injury should be permanently affected.

Injuries to the epidermal layer (first or outer layer) will repair themselves as they were prior to the injury, for example, paper cuts.

There are many instances in legend where allegedly people have sanded, burned, or surgically altered their fingerprints with persistent results. Research has demonstrated what effect different techniques have on the appearance of fingerprints.

Self-induced injuries cannot remove all ridges or the hands would be too severely injured to be used. What one must understand is that friction ridges cover the entire surface of the inner hands and bottom of the feet.

If the pattern area alone were disfigured, classification might be affected, but identification or individualization would not. In all likelihood, the pattern would be made more unique, which would make identification and individualization that much easier.

Other alterations to the friction skin whether it be surgical, occupational, or medical can have an impact on the appearance. Examples are as follows:

Skin grafts would result in either the old pattern being regenerated as the graft skin wore away or the graft area remaining smooth.

A new pattern would not occur.

Occupational wear might wear down the ridges, but the cessation of the work will result in the ridges becoming distinct again.

Disease can have an effect on fingerprints as well; such as in the latter stages of leprosy, the skin may flake off and the pattern may be lost. Allergic reactions may have an effect on the ridges in that a temporary change may occur but when the reaction disappears, ridges should return to their configuration. Other conditions such as warts, creases, or calluses may be present but seldom affect classification and rarely, if ever, affect identification.

Based on the foundation and fundamentals of the aforementioned information, a basis of the science of fingerprints was established:

• 1. Every finger contains ridge detail that is unique to that finger and no other.
• 2. Unique ridge details do not change (except in size) from approximately one hundred twenty days after conception until decomposition after death.

Figure 2.5 Fingerprint patterns (Source: NIST).

3. A set of fingerprints lends itself to classification and therefore can be filed and searched. Whether the system be manual or automated does not matter.

Fingerprint classification—The process of assigning a formula, consisting of numbers and letters, to a set of fingerprints determined by the pattern interpretation and ridge detail of the fingerprints themselves.

Fingerprint identification (individualization)—The process of determining that the same finger made two or more impressions based on the friction ridge details of both impressions (to the exclusion of all others).