STRUCTURE OF HAIRS AND DIFFERENTIATING HUMAN FROM NON-HUMAN HAIR

• Jurisdiction: Australia

Introduction ................................................................................................... [88B.300]

Common animal hairs ................................................................................... [88B.320]

[88B.300] Introduction

As we saw in Figure 4, the hair shaft comprises three distinct components: the medulla, cortex and cuticle. This is regardless of whether the hair is human or non-human in origin. Human hair is simply a subset of animal hair. For animals (for simplicity from now on, this means non-human) hair is present in much greater numbers than in that in humans. The highest estimate for humans is 300 hairs per cm² against 6,200 hairs per cm² for the Australian red kangaroo. Merino sheep have hair densities in the order of 5000 hairs per cm². This results in animals having a peltage or fur. Terminal animal hairs can be subdivided into a number of types. The "fur" is comprised of two types, longer and coarser guard hairs and shorter and finer under hairs. Guard hairs have the widest range of microscopic features and are the hairs which are more useful for forensic examination.

Table 1 shows the major differences between human and animal hairs.

TABLE 1 Comparison of features of human and non-human hair

Feature	Human	Non-human
Colour	Relatively consistent along shaft	Often showing profound colour changes and banding
Cortex	Occupying most of width of shaft - greater than medulla	Usually less than width of medulla
Distribution of pigment	Even, slightly more towards cuticle	Central or denser towards medulla
Medulla	Less than one-third width of shaft. Amorphous, mostly not continuous when present	Greater than one-third width of shaft/ Continuous, often varying in appearance along shaft, defined structure
Scales	Imbricate similar along shaft from root to tip	Often showing variation in structure along shaft from root to tip

Visually human scalp hairs are relatively straight compared to animal hairs which are often described as being shield-shaped. These may also be wavy and unevenly thickened. Hence, visually, or at low power stereo microscope level, it is usually possible to separate animal and human hairs.

The most distinguishing microscopic features between animal and human hairs are the cuticle layer and the medulla. To assess these features requires high power microscopic examination.

The cuticle layer of human hair has an overlapping, imbricate scale pattern which is largely the same from the root to tip. The appearance of the scales towards the tip end can display "wear and tear" damage to the scales. By contrast animal hairs display a wide variety of scale pattern and these usually also vary from root to tip in a systematic way.

To see the scale features of animal hairs, it is usual to make a caste of the outer cuticle, or a "scale caste". There are a number of methods used to do this. A simple method is to apply a layer of clear nail varnish onto a glass microscope slide, place the hair carefully on the varnish and leave to dry. The latter can be accelerated by placing the slide on a mildly warm hot plate. The hair is carefully peeled away from the varnish to leave an impression of the outer cuticle. The hair can then be mounted for further examination of its internal structure.

A more expensive method to record scale patterns is to examine the hair in a scanning electron microscope (SEM). However, this has limitations as, unless you have a very new environmental SEM, it is necessary to coat the hair with metal. Furthermore, only a very short length of hair can be examined at one time.

Figure 9 shows one classification scheme for scale patterns in animal hairs. It is beyond the scope...