Sebaceous glands produce a fatty secretion (sebum) that lubricates and waterproofs the skin and coat.
It also promotes the spread of sweat, retards bacterial growth, and, in certain instances, serves as a territorial marker that is recognized by other members of the species. The odor of the wet dog is due to these glands.
Certain substances (pheromones) present in sebum are known to be sexually attractive. Their rate of production is controlled by steroid hormones (androgens generally promote secretion, and estrogens retard secretion). A good illustration of a selective effect of androgens is found in the reaction of the so-called acne region of the human adolescent. The sebum of the fleece of sheep, known as lanolin commercially, is used as a base for ointments, in cosmetics, and as a cleansing agent in soaps. The secretions of certain specialized glands (e.g., the preputial glands of musk deer and the anal glands of the civet) have long been collected for use by the perfume industry.
FIG. 10.23 Horn glands caudomedial to the base of the horns in the goat.
The major localized accumulations of sebaceous glands found in domestic animals and large enough to be visible to the naked eye are listed; several are associated with skin pouches.
Circumoral Glands (Fig. 10.11)
Circumoral glands are large glands found in the lips of cats, which use them to mark their territories. The secretion is deposited directly by the animal's rubbing its head against an object or ingratiatingly against its owner and indirectly after transference to the body during grooming.
Horn Glands (Fig. 10.23)
Horn glands are musk or scent glands present in goats of both sexes, caudomedial to the horn base (or at the corresponding site in polled animals). They are larger and more productive in the breeding season; stimulated by testosterone, those of males produce a secretion with an odor so pungent that some owners insist on their surgical removal.
Glands of the Infraorbital Pouch (Fig. 10.24)
Some glands in sheep are contained in a cutaneous pouch, called the infraorbital pouch, rostral to the eye and opening Ventrolaterally on the face. The pouch wall contains both sebaceous and tubular serous glands whose mixed secretion stains the skin when it escapes from the pouch. The glands, which serve as territorial markers, are larger in rams.
FIG. 10.24 (A) and (B) Infraorbital pouch (arrow) of the sheep.
FIG. 10.25
Carpal glands (arrows) of the pig, palmar view.
Carpal Glands (Fig. 10.25)
Pigs and cats have carpal glands. In pigs they surround several cutaneous invaginations on the mediopalmar aspect of the carpus. They are found in both sexes and serve to indicate territorial claims; boars are said to make particular use of them when "marking" sows during copulation.
FIG. 10.26 Interdigital pouch (1) of the sheep and its opening (2).
The location of the glands in the cat is marked by a tuft of a few tactile hairs proximal to the carpal pad. The site is betrayed by a palpable thickening of the skin (Fig. 10.15B/5).
Glands of the Interdigital Pouch (Fig. 10.26)
Interdigital pouches are found on the forelimbs and hindlimbs of sheep of both sexes. The pouches are tubular invaginations of the skin whose walls contain branched sebaceous and serous glands. The waxy secretion is discharged through a single opening above the hoofs and serves as a "trail marker." Many gregarious wild species have similar glands.
Glands of the Inguinal Pouch (Fig. 10.27)
Inguinal pouches, found near the base of the udder or scrotum of sheep, contain both sebaceous and sweat glands. The secretion escapes as a brown waxy substance whose odor may assist the lamb to find the udder.
FIG. 10.27 Inguinal region of the ram. 1, Inguinal pouch; 2, rudimentary teat; 3, scrotum.
FIG. 10.28 Location of the tail glands of the dog (dark pink area with dotted outline).
Preputial Glands (Fig. 35.11)
Sebaceous and apocrine sweat glands within the prepuce produce secretions that combine with desquamated cells to form the crumbly substance known as smegma. They are best developed in the boar, in which they are massed within a dorsal diverticulum of the preputial cavity (see Chapter 35). Their secretion gives the boar its characteristic odor. They are present but less offensive in other species (which lack the diverticulum).
Tail Glands (Fig. 10.28)
Collections of large sebaceous and serous glands are found in an oval patch on the dorsal surface of the tail of certain carnivores. The skin over these glands is often defined by its sparser hair and yellowish color. Activity is greatest during the breeding season. The patch is situated more proximally in cats, toward the root of the tail, than in dogs (Fig. 10.28).
Circumanal Glands (Fig. 10.29)
Some sebaceous glands are restricted to the perianal skin of certain carnivores (circumanal glands), including dogs, where they drain into (and are believed to influence) special sweat glands. It is probably their secretion that excites the particular attention paid to the anal region when dogs confer.
Glands of the Anal Sacs (Fig. 10.30)
Sebaceous and serous glands are found in the walls of the anal sacs, cutaneous pouches that open beside the anus of carnivores (Fig. 10.29/2). The secretion, which is particularly foul smelling, is expressed during defecation and apparently serves as a marker. It is well known that skunks can forcefully expel the contents of the sacs to fend off aggressors.
FIG. 10.29 (A) Cutaneous zone of the canine anal canal. (B) Feline anal canal opened dorsally. 1, Cutaneous zone with circumanal glands forming a ring around the anus of the dog; 2, opening of the right anal sac; 3, anocutaneous line; 4, columnar zone; 5, right anal sac.
FIG. 10.30 Exposed right anal sac of a dog. 1, Anus; 2, anal sac; 3, opening of excretory duct of anal
sac (emphasized; see Fig. 10.29A/2).