» Conformation and External Features
The age, the sex, and the breed of a horse influence the morphology of the head. The cranial vault of the skull in young foals is shaped to accommodate the brain and projects above the face (Fig.
18.1). The age-related changes in complement of teeth and expansion of paranasal sinuses alter the conformation of the skull, including lengthening of the face. The enlargement of the frontal sinus smoothes the dorsal profile at the junction of the face and cranium. A longer face is a characteristic of the adult compared with the juvenile, the stallion with the mare, and the heavy draft horse with the pony. The other very obvious breed difference concerns the dorsal profile; a relatively straight profile is generally preferred but some convexity ("ram's head") is characteristic of certain heavy breeds, whereas concavity ("dishing") is the rule in Arabians and common in horses with admixture of Arabian blood (Fig. 18.1). As part of the normal developmental process, sometimes the root of an unerupted permanent cheek tooth may create rounded swellings on the ventral margin of the lower jaw (p. 505).The skin of the face is thinner and more firmly bound down than that over most other parts of the body and is especially tight where it lies directly on bone. The coat is generally short, but a forelock continuing the mane may be prominent; a "mustache" is a feature of some animals, especially the larger breeds. Tactile hairs are numerous and widely scattered on the lips and chin and about the margins of the nostrils.
The nostrils are large and widely spaced, especially in the Thoroughbred (Fig. 18.2), which is largely due to the supporting alar cartilages (Fig. 18.3B/1' and 2'). The upper part of the opening leads to a blind nasal diverticulum (Fig. 18.3/1") that occupies the nasoincisive notch (Fig. 18.3/6) and is without counterpart in other domestic species.* The lower part leads directly to the nasal cavity and provides a passage for the stomach tube.
The flexibility of the margins of the nostrils allows them to dilate during strenuous breathing or clinical procedures. The dilated nostril is rounded, and the change in form is achieved by apposition of the walls of the diverticulum. The pliancy of the tissues facilitates examination of the nasal vestibule and exposure of the opening of the nasolacrimal duct, which is found on the floor about 5 cm internal to the entrance and near the mucocutaneous junction. Occasionally the duct has more than one opening.FIG. 18.1
Variations in the profile of the equine head. (A) The common straight profile. (B) The dished Arabian profile. (C) The domed contour of the foal.
FIG. 18.2 Functional variations in the form of the nostril.
FIG. 18.3 (A) Left nostril opened laterally to expose nasal diverticulum. (B) Nasal cartilages. 1, Alar fold, supported by the lamina (1') of the alar cartilage; dorsal to the alar fold is the nasal diverticulum (1"); 2, floor of nostril supported by the cornu of the alar cartilage (2')—the floor leads into the nasal cavity; 3, probe in nasolacrimal duct; 4, dorsal lateral nasal cartilage; 4', nasal septum; 5, nasal bone; 6, nasoincisive notch; 7, incisive bone; 8, canine tooth.
FIG. 18.4 Left equine eye; note implantation of eyelashes on lateral side of upper eyelid.
The entrance to the mouth is small, and the commissure is a short distance in front of the first cheek teeth (P2). The skin of the lips and adjacent part of the muzzle is sparsely covered by short, fine hairs that impart a velvety texture. The lips are both mobile and sensitive and are used in the selection and prehension of food.
The sensitivity of the upper lip is exploited when a twitch is applied to control a horse during procedures (e.g., injections) elsewhere on the body. The acupressure created by the twitch lowers the heart rate and may cause release of endorphins that diminish pain. The lower lip surmounts the chin swelling, which is based on a pad of fatty fibrous tissue.The eyes are prominent and placed to each side of the head, providing the horse with a panoramic field of vision. This ability to survey widely—perhaps through 330 degrees—is obtained at the expense of the binocular field, which is limited to some 65 degrees. The field of overlap is further reduced by the length and shape of the muzzle, which creates a blind area directly to the front (see Fig. 9.1).
The upper and lower eyelids and adjacent skin carry a few scattered tactile hairs. The palpebral skin is thin and, being loosely attached, is thrown into folds when the eye is open. The lid margins carry numerous lashes, longer and more prominent on the upper than on the lower lid (Fig. 18.4). The tarsal glands, which open at the junction of the skin with the conjunctiva, number about 50 in the upper lid and rather fewer below and are clearly visible in palisade formation when the lids are everted. The palpebral conjunctiva is well vascularized, the bulbar part less generously. The bulbar conjunctiva is strongly pigmented toward the corneoscleral region. The third eyelid (Fig. 18.5/1) in the medial angle can be exposed in the usual way by pressing on the eyeball through the upper eyelid; a small accessory lacrimal gland is associated with it. The lacrimal caruncle is prominent. The features of the eyeball are considered later (p. 516).
FIG. 18.5 The right conjunctival sac. 1, Third eyelid; 2, lacrimal caruncle; 3, lacrimal puncta; 4, openings of the tarsal glands.
A depression caudal to the eye (behind the palpable postorbital bar of bone) is prominent in the animal at rest. It disappears and reappears during feeding in rhythm with the movements of the jaws; these effects are due to the displacement of a pad of fat interposed between the temporalis and the periorbita. The fat is depleted in horses in poor condition, and exaggeration of the hollow contributes significantly to the haggard appearance.
Deposition of fat above the upper eyelid may produce a conspicuous swelling seen in animals suffering from Cushing disease.
Little need be said concerning the external ears, which are prominent and capable of being swiveled when attempts are made to locate the origin of a sound. Their carriage is also very expressive of emotion.