In animals hairs cover nearly the entire body surface, except the
1) planum nasale,
3) vulvar lips and
4) limb pads.
Hairs are cornified filiform structures that are formed by the skin. The hair is subdivided into
1) the shaft which projects beyond the surface of the skin,
2) the root which is obliquely oriented within the dermis and
3) the hair bulb, the proximal expanded part.
Hair root and hair bulb are in a divided epithelial root sheath (Vagina epithelialis radicularis). The outer part of the sheath is continuous with the superficial epidermis. Its inner part cornifies above the mouth of the sebaceous gland and will be shed.
The connective tissue root sheath (Vagina dermalis radicularis) is continuous with the surrounding connective tissue. The epidermal and dermal root sheaths together with the bulb of the hair constitute the hair follicle.
The parts of the hair are:
2) the cortex and
3) the superficial hair cuticle.
The arrector pili muscle terminates below the mouth of the sebaceous gland, attaching obliquely to the dermal sheath of the root of the hair. Its contraction results in erection of the hair (in human beings, this brings about the phenomenon of ‘goose pimples’). Contraction of the arrector pili muscle compresses the sebaceous glands and, in erecting the hair, increases the air space between the hairs and the skin surface for thermo-isolation.
The hair coat depends on the breed and is characterized by the individual and group-like arrangement of the hairs, the different portions of the individual hair types (lead hairs, guard hairs, wool hairs) as well as by the density, length and color of the hairs.
There are basically three types of hairs:
1) The ‘lead’ hair or ‘main’ hair is long, stiff, and slightly curved. It is independent of other hairs and in the dog occurs only rarely.
2) Guard hairs are shorter than the lead hair, arched near the tip and thickened. Both lead and guard hair types form the hair coat (Capilli).
3) The third and shortest type of hair is the wool hair. It is very thin, pliable and in its course slightly or strongly undulated. Guard and wool hairs pass in a bundle or tuft together from a compound hair follicle, in which case one guard hair is surrounded by the six to twelve wool hairs that accompany it. The wool hairs predominate in the coat of the puppy. In most canine breeds they lie under the hair coat and only in a few breeds such as the Puli and Commodore, do they project above the hair coat and form a superficial ‘wool coat.’
The color of the hair is effected by the melanin content of the cornified cells as well as the inter- and intracellular air bubbles, especially of the medullary cells.
The direction of the hairs characterizes the coat. That part of the coat in which the hairs have a uniform direction is called the Flumina pilorum. In a vortex, the hairs are arranged divergently or convergently with respect to a central point. By the crossing of converging lines of hairs, hair ‘crosses’ are formed.
Sinus or tactile hairs are remarkably long, special forms of hair in the vicinity of the opening of the mouth (Rima oris). To receive tactile stimuli, the root of the hair is ensheathed by a blood sinus that is contacted by numerous sensory nerve endings. Owing to the great lever action of this long hair even the finest tactile stimuli result in stimulation of this receptor. The length of the hairs varies considerably and is dependent on breed. In the ancestors of the dog, who lived in the wild, the longest hairs are found on the dorsum and the shorter ones on the belly and head. But this pattern is mostly lost with domestication. In wild Canidae, the thickness of the hairs increases toward the belly (thickness is about 0.1 mm).