The purpose of this lab is to examine certain structures and features of the common integument and then to reflect the skin off the left side of the cadaver as directed.  Team members are to rotate dissection responsibilities for this lab, to allow all students a chance to use instruments and to reflect skin.

Prelab reading of this introduction is recommended and portions of it are highlighted in the common integument lecture.

Introduction to the common integument

(adapted from Hudson and Hamilton, Atlas of Feline Anatomy for Veterinarians and from König and Liebich, Veterinary Anatomy of Domestic Mammals)

The common integument ([L] in-tegere – in, to cover, integumentum – a covering) is the vital outer covering and barrier of the animal, interfacing with the environment.  The common integument encompasses the skin and its appendageal structures (e.g. hair, arrector pili muscles, glands, pads, claws, hooves, horns), the subcutaneous tissue, and the vascular and nervous supply. It is the largest organ of mammals.

The common integument fulfills many functions, to include:

1. A protective barrier to external mechanical, chemical, physical and biological factors encountered in the environment, and for preventing unregulated loss of endogenous body substances.
2. A reservoir and/or excretory organ for electrolytes, water, vitamins, fat, and other elements and molecules.
3. A primary sense organ for touch, pressure, pain, itch, heat, and cold.
4. Thermoregulation
5. Immunological defense
6. Communication

The common integument has unique properties with regards to coat color, skin pigmentation, inflammatory response, and specialized integumentary structures.

The ‘skin’

‘Integument’ is commonly used (somewhat inaccurately) when referring to the skin.  Cutis is Latin for skin.  Dérma is Greek for skin, and derm– is the root combining word for skin-related terms (e.g. dermatitis, pachyderm, taxidermy). The skin includes the epidermis ([L] epi – upon, above) and the dermis.  The hypodermis ([L] hypo – below, under, less, and also known as subcutis, subcutaneous tissue, tela subcutanea) is the layer deep to the dermis. In clinical terms, reference to the ‘skin’ implies all three layers, ie epidermis, dermis and hypodermis.  The epidermis of skin is an epithelial tissue (one of the four major tissue types in the body) and specifically is classified as stratified squamous epithelium (one of eight subtypes of epithelial tissue). The dermis is a form of connective tissue (another of the major tissue types in the body). The hypodermis (subcutis or subcutaneous layer) is a loose connective tissue between the dermis and the superficial fascia. At the mucocutaneous junction, skin is continuous with the mucous membranes at the various openings of the body’s digestive, respiratory, urinary and genital tracts. The microscopic structure and function of the skin will be considered in physiology and pathology content.  Below are a few more details regarding these layers for gross anatomy context.

Epidermis

The epidermis consists of four to five distinct layers: the stratum corneum (the most superficial layer we see), stratum lucidum (the regionally present 5th layer), stratum granulosum, stratum spinosum, and stratum basale. The thickest epidermis of the body of carnivores is found on the pads and nasal plane (discussed later). Stratum corneum is Latin for horn layer.  The substance within cornified cells (aka keratinocytes, from [Gk] keras = horn) of the stratum corneum is called ‘horn’. The basal cell layer of the epidermis is referred to as the stratum germinativum or germinal epithelium, because it is the site of epidermal mitotic cell division (i.e. the germination of new cells).  The basal cells lie on the basement membrane, a critical structure that separates the epidermis from the dermis.  Basal cells actively divide and push their progeny upward to replenish the more superficial layers of the epidermis.  Additional cell types found in the basal layer include melanocytes, and macrophages (Langerhans cells).

Dermis (or Corium, from [L] corium = skin)

The dermis is a connective tissue composed of collagen, elastic and reticular fibers, and interstitial ground substance. Nervous tissue, blood vessels, lymphatics, glands, hair follicles, arrector pili muscles, and various cellular elements are present throughout. The dermis provides the nutritional needs of the non-vascularized epidermis. The dermis contributes most to the thickness of skin and forms the familiar leather used to make products.  In our domestic animals, cattle generally have the thickest dermis (which makes IV catheterisation of the external jugular vein challenging!) and sheep and cats the thinnest.  The dermis decreases in thickness from dorsal to ventral on the abdomen and proximal to distal on the limbs.

Hypodermis (subcutis or subcutaneous tissue)

The hypodermis is composed of bands of collagen fibers and abundant, smaller elastic fibers that interweave and enclose fatty adipose tissue. Compared to the dermis, hypodermis is relatively loosely arranged with tissues separated by (microscopically) small spaces.  Hence the term areolar tissue ([L] areola = small space) is used in reference to the hypodermis.  The dog, cat and sheep have abundant hypodermis.  The hypodermis of the horse, ox and goat is thinner and more tightly adhered to the underlying trunk.  Where skin movement is minimal in general, the hypodermis is very thin or absent e.g. lips, eyelids, ears.  Modified hypodermis forms specialized padding in the feet of animals e.g. the digital cushion of the horse.  Closely related on the deep side of the hypodermis is fascia, a stronger connective tissue surrounding the body.

Clinical relevance:

Subcutaneous fluids are deposited in the loose connective tissue of the hypodermis, particularly in carnivores, where it exists in greater amounts e.g. the interscapular region.  Pigs have substantial fat accumulation in the hypodermis (backfat measurements of pigs determine carcass quality and reproductive soundness), and a cresty neck in the horse is due to regional fat accumulation in the hypodermis (a sign of Equine Metabolic Syndrome).  Inflammation of the subcutaneous fat is termed pansteatitis or yellow fat disease and causes painful subcutaneous nodules. Inflammation of the subcutaneous tissues in general is called panniculitis, with various causes.

 

Appendageal structures

Hairs ([L] pili)

Hair is a modified epidermis.  Long, thin columns of elastic horn tissue are produced at the root of the hair in the hair follicle. Hair follicles are invaginations of the epidermis, extending well into the dermis and sometimes hypodermis. Sebaceous and apocrine sweat glands discharge into the space of the hair follicle, around the shaft of the hair.  These secretions are distributed over the skin and coat the hair as it grows, providing the sheen of a healthy coat.  Tiny smooth muscles, arrector pili muscles, acting involuntarily, straighten hairs to increase thermal properties and to give an animal a threatening appearance (raising its hackles). They are present in all haired skin areas, being most highly developed in the dorsal lumbar, sacral and tail regions

Relatively stiff guard hairs (capilli) provide the outer coat of hair in domestic animals (except sheep) and the undercoat consists of fine, wavy wool hairs (pili lanei).  The fleece of sheep is of entirely wool hair.  Guard hairs show body region, and species, and breed variation.  Horses have a mane and forelock, goats have a beard (barba), ungulates have long tail hairs.  Modified guard hairs are present at the nostril (vibrissae), at the entrance of the ear canal (tragi), and form eyelashes (cilia).

Tactile hairs are a particularly specialized and modified guard hair.  They are substantially thicker and longer than normal guard hairs. The tactile hair follicle is surrounded by a venous (blood) sinus. Therefore, yet another name for these specialized hairs is sinus hairs. In the walls of the venous sinus are nerve endings responsive to movement of the tactile hair, which is amplified by wave action of the surrounding blood. Most tactile hairs are found on the face, principally on the upper lip and around the eyes, though others are scattered on the lower lip, chin, and elsewhere on the head.  The slightest movement of tactile hairs, even by air currents, stimulates the nerve endings and provides information on the animal’s immediate surroundings. The attached arrector pili muscles move the tactile hairs and the hair can be “put on the alert” when required.

Sebaceous Glands

Sebaceous glands are holocrine glands producing sebum and are mostly associated with hair follicles, discharging into the follicular space. Sebum is an oily secretion that (mixed with the secretion of apocrine sweat glands) keeps the skin soft, pliable and waterproof, and gives the hair shafts their glossy sheen. The sebaceous glands of the lips and face are numerous and larger than in other regions. The cat has an increased number of these glands on the chin region, referred to as the circumoral glands (or submental organ).  The dorsal aspect of the tail (caudal glands) is another concentrated site. Contraction of the arrector pili muscles helps express the sebaceous glands. The oil glands of the eyelids (Meibomian glands) are specialized sebaceous glands.  The cerumin glands of the external ear canal are a combination of sebaceous and modified tubular glands, secreting the characteristic waxy substance (ear wax) called cerumen.  Pigs have sparse and rudimentary sebaceous glands.

Clinical relevance:

During periods of illness or malnutrition, the haircoat may become dull and dry as a result of inadequate sebaceous secretions. In contrast, hyperactivity of these glands or abnormal secretions is also possible, as in the case of feline chin acne.

Sweat Glands (tubular glands)

Apocrine sweat glands are coiled or saccular, and discharge their protein-rich (albuminous) sweat into hair follicle sacs. Their secretion provides the individual odor of an animal. They are distributed throughout all haired skin. Apocrine glands are relatively numerous in the horse, with additional openings, and they are responsible for the frothy, ‘lathering up’ of a working horse, due to the rich protein content of the sweat. Apocrine glands are not present in pads or the nasal plane. They are largest and most numerous near mucocutaneous junctions, in interdigital spaces, and over the dorsal neck, and rump. Large apocrine glands may be found on the dorsal tail.

Clinical relevance:

Hyperfunction of these glands on the dorsal tail along with the associated large sebaceous glands may result in the buildup of waxy, greasy debris (“stud tail”).

Eccrine sweat glands are small, tightly coiled merocrine (secretions are discharged directly on the skin surface) glands which are found in the footpads of carnivores and the frog of the horse foot. The excretory duct opens directly to the pad surface. The secretion is watery. In contrast, these glands predominate throughout the body of primates.

Mammary gland

The mammary gland is a series of highly modified sweat glands, occupying the hypodermis, and they are not fused to the body wall (which makes removal of the gland, a mastectomy, less difficult to perform). A mammae is the complex of tissue associated with one teat. This complex consists of skin, glandular tissue and associated connective tissue.  Clinically the terms mammary glands and mammae are often used interchangeably.  Mammary gland is the collective term for the mammae of mammals with thoracic, abdominal and inguinal mammae, i.e. the sow, bitch, queen.  In ruminants and the horse, the mammary gland is restricted to the inguinal region and the collective term in these species is udder. A left and right set of mammary glands is present.  Supernumerary teats are additional teats that are typically nonfunctional. Mammary glands receive extensive blood supply, ramped up during lactation.  Lymphatic drainage is cranial and caudal to regional lymphocenters.

Male mammals typically have rudimentary or vestigial mammae, with teats visible (but not always! Stallions/geldings don’t normally have teats, but jacks frequently do, bucks and bulls do, and bucks may develop functional glands).

Detailed internal anatomy of the mammary gland will be considered in the reproductive system unit.

Cutaneous muscles

Very thin muscles are located in the superficial fascia of the head, neck and trunk, immediately deep to the hypodermis. These muscles directly act on the skin, causing it to twitch and tense.  Cutaneous muscles of the head contribute to facial expressions.  Examples of cutaneous muscles include m. cutaneus faciei, m. platysma, m. cutaneus colli, and m. cutaneus trunci.  A few cutaneous muscles will be highlighted as they are encountered during the dissection (in the musculoskeletal unit).  During skin reflection it is not uncommon for cutaneous muscles to be elevated with the skin, because they are so closely associated.

Lab dissection instructions

Carnivore common integument and additional external body features

Study the cadaver before skin reflection is commenced and consider the following structures, features and terms.  Should you have access to a live companion animal, they are now officially ‘supplementary anatomy resources’ and generally tolerate the curious veterinary student intent on studying anatomy, especially if you combine with a good grooming:)

In the cat, tactile hairs are also found on the caudomedial distal antebrachium, close to the carpus.

The nasal plane (planum nasale) is the hairless, non-glandular portion of the apex of the nose (a moist nose is due to secretions from nasal glands elsewhere, and tear drainage).  A groove (philtrum) divides the nasal plane vertically.  The nasal plane skin is composed of thickened and highly cornified epidermis that is elevated into ridges, creating patterns unique to the individual. In dogs, nose prints may be used similar to fingerprints as a form of identification.

Pad ([L] torus)

The pads (tori) of carnivores are thickened, highly cornified and pigmented, hairless portions of the skin. The dog and cat weight bear on the digital, metacarpal, and metatarsal pads. Carnivores also have a carpal pad on the palmar carpus (which does not normally contact the ground) but no corresponding tarsal pad. The epidermis of pads is the thickest of the body. The dermis of the pad shows prominent dermal papillae interdigitating with epidermal pegs. Very pronounced subcutaneous cushions of adipose tissue are present in the pads that serve as shock absorbers. Sweat glands (eccrine glands) are present.

Claw ([L] Unguicula)

The claw is a specialized, curved, cutaneous structure that is a direct continuation of the dermis and epidermis. The most prominent part is the superficial layers of the epidermis, modified to form the continuously growing, hard, horny claw (or ‘nail’).  Grossly, the claw consists of a central dorsal ridge of thicker, horny material, and two thinner walls and a sole. At the base (or root) of the claw, extending from the distal phalanx, the dermis (corium) is a deeper, vascular, innervated, connective tissue that nourishes the epidermis.  Trimming a claw too short breaches the dermis, resulting in hemorrhage and discomfort.  Previously stated, the stratum basale of the epidermis is the source of the proliferating epidermal cells that constantly produce new growth of the horny claw. A fold of skin covers the base of the claw dorsally, and palmarly or plantarly the digital pad does the same.  One obvious difference in the claws of dogs and cats is that cats retract (ensheath) their claws due to the passive pull of the elastic dorsal ligaments of the distal interphalangeal joint, whereas dog claws are consistently extended.

Umbilicus

The umbilicis (belly button) is represented by a scar that may be either flat or slightly raised and is located on the midventral line, one third to one fourth of the distance from the xiphoid cartilage of the sternum to the scrotum or vulva. It is often hidden by hair. The umbilicus is irregularly oval and may be from a few millimeters to a centimeter in length. The umbilicus serves as a landmark in abdominal surgery and it is where the umbilical cord extending from the placenta was attached to the developing fetus in utero.

Mammary Gland

In the dog, the mammary gland consists of typically five pairs of mammae (i.e. total of 10, with a range of 8 to 12). They are situated in two rows, usually opposite each other. The number is usually reduced in the smaller breeds.  When ten glands are present, the cranial four are the thoracic mammae, the following four are the abdominal mammae, and the caudal two are the inguinal mammae. When the abdominal and inguinal mammae are maximally developed, the glandular tissue in each row appears to form a continuous mass.  Each mamma has a teat, that is partly hairless and contains about 12 teat canal openings, but these vary and are difficult to see if the animal is not lactating.

The mammary gland of the cat consists of generally four pairs of mammae (i.e. eight total). In some male cats only a pair of inguinal teats are obvious. Four to seven teat canal openings can be identified on each teat.

Anal Sacs ([L] sinus paranalis)

The anal sacs (sinus paranalis) of the carnivore are ovoid cutaneous pouches located in a lateral position between the internal (smooth muscle) and external (skeletal muscle) anal sphincters.  Anal sacs are lined by a smooth, keratinized, stratified squamous epithelium that is perforated by the efferent ducts of sebaceous and apocrine tubular glands (the paranal sinus glands). The efferent ducts of the anal sacs are narrow and terminate at the transition between the anus and skin. The anal sacs expel their serofatty, unpleasant smelling secretion during defecation or by voluntary contraction of the external anal sphincter m. The scent is utilized for marking.

Costal arch

A rib consists of a bony head, neck, and shaft with its distal (ventral) end changing to a cartilaginous structure, called the costal cartilage. The costal cartilages of the tenth, eleventh, and twelfth ribs in the carnivore unite with each other to form the costal arch.

Before skin incisions are made through the epidermis and dermis, let’s reconsider what to anticipate in the subcutaneous (hypodermis) layer.

Subcutaneous tissue and fascias

When skin is incised and reflected we are typically cutting through epidermis and dermis and as we peel back the skin we view the subcutaneous tissue.  As previously described the subcutaneous tissue is composed of areolar tissue, a layer of loose, irregularly arranged connective tissue that often contains fat. The areolar tissue is often distended with embalming fluid (and subcutaneous injections are made into this tissue – e.g. subcutaneous fluids provided to a cat with chronic renal disease). Dissection through the areolar tissue plane is quick and easy based on the nature of the tissue being incised. Fascia, on the other hand, is a denser, regularly arranged thin layer of connective tissue. It is more fibrous, and it envelops the body beneath the skin and encloses individual muscles or groups of muscles. Superficial fascia is directly under and associated with the areolar tissue and covers the entire body.  Cutaneous muscles are located in the superficial fascia.  Some superficial fascia (and cutaneous muscles) may be reflected with the areolar tissue.  Superficial fascia blends with the deep fascia, which is more firmly attached to the muscles that it encloses. The superficial and deep fascias are not always easily distinguished from each other.  As a point of reference, the external jugular vein is surrounded by superficial fascia.  Fascia may receive a more specific name based on its location in the body e.g. the thoracolumbar fascia.  When the fatty areolar tissue and fascia with vessels and nerves are removed from a muscle, the muscle is said to be “cleaned.”

Skinning instructions

Ungulate common integument: comparative and unique features

Equine

Tactile hairs are present on the upper and lower lips and around the eyes.  The hairless nasal plane feature of other animals is not present in the horse.

Viborg’s triangle (trigonum viborgi) – this external feature sits over the pharyngeal region and was described by the Danish veterinarian Viborg, in 1795, as a surgical approach to the guttural pouch of the horse (a unique air-filled cavity related to respiratory tract).  Although the surgical approach is less commonly used today, the region and term remain clinically significant.  The triangle is demarcated by the caudal margin of the ramus of the mandible, the tendon of insertion of the sternocephalicus muscle and the linguofacial vein. This anatomy will be considered again in later units.

Jugular groove – This is a linear depression extending down the lateral neck, defined by sternocephalicus and cleidocephalicus muscles. The external jugular vein (an important venous access vessel in ruminants and the horse) passes in this visible and palpable groove. This anatomy will be considered again in later units.

Chestnut and ergot

Chestnuts are unique in structure and can be used as a form of identification, as has been done for certain breed registries.  Donkeys and zebras normally don’t have pelvic limb chestnuts and a few horse breeds don’t either e.g. the Icelandic horse. Chestnuts on horses should not be confused with the Horse Chestnut tree……..

Mammary gland

Mares have an udder, the collective term for the mammae of mammals with inguinal mammae, i.e. cow, ewe, doe, mare.  The mare has two inguinal mammae (like the doe and ewe).  A mammae is the complex of tissue associated with one teat.  The skin of the teat of the mare contains sweat and sebaceous glands. Sebaceous secretions occur just prior to parturition that give the teat a characteristic appearance; this is called ‘waxing’.  A gland complex is the single gland and associated duct system within a mammae.  On each mammae, the number of gland complexes coincides with the number of teat orifices. Mares have two gland complexes per mammae, therefore two teat orifices per teat are visible. Internal mammary gland anatomy will be considered in the reproductive system unit.

Hoof ([L] ungula) – horses bear weight through a single digit per limb, i.e. digit 3.  The critical structure of the horse hoof will be studied in detail in the musculoskeletal unit.  For now, recognize it is a complex, specialized modification of the common integument.

Ruminant

The ox has a hairless nasolabial plane (planum nasolabialis) incorporating the nose and upper lip.  The small ruminant has a nasal plane, like the carnivore.

The skin covers the body forming characteristic folds and grooves that are prominent in various species and breeds.

The dewlap is the pendulous midline fold of skin in the distal neck of the ox, very prominent in certain male breeds of cattle.  Merino sheep have large skin folds on their neck.  Appendages of skin, containing cartilage, hang from the ventral cranial neck, commonly in goats, and are called wattles.

The flank fold connects the ventrolateral abdomen caudally to the medial thigh region.  It is well developed in ungulates due to the cutaneus trunci m. contained within, and can be grasped to help position a small ruminant in lateral recumbency (‘flank a calf down’).

Jugular groove – This is a linear depression extending down the lateral neck, defined by sternocephalicus and cleidocephalicus muscles. The external jugular vein (an important venous access vessel in ruminants and the horse) passes in this visible and palpable groove.  In cattle, the dermis of the neck is particularly thick, making intravenous catheterization in the adult ox difficult.  A ‘cut down’ procedure may be pursued whereby the skin is incised through most of its dermal thickness to facilitate catheter insertion.  Sheep lack a part of the sternocephalicus m. and therefore have a less defined groove, which is also difficult to see when the wool coat is long!

Skin glands of note

Goats have cornual (horn) glands, one at the caudomedial base of each horn. These are sebaceous glands, and are typically difficult to see grossly.

Sheep have numerous specialized skin pouches or sinuses, similar to the anal sacs of dogs and cats. These sinuses are lined with sebaceous and sweat glands. The resulting secretion is considered a scent marker. Location and number in the sheep include: Infraorbital (aka lacrimal) sinus (2), interdigital sinus (4), inguinal sinus (2).

Horns

The horns (cornu) of domestic ruminants consist of a bony process (cornual process, extending from the frontal bone) encapsulated in a heavily cornified epidermal tissue, i.e. horn tissue.  Horns are hairless and glandless.  Particularly in wild ruminants, horns are used as weapons during breeding season and to establish and maintain hierarchies.  Horns are present in both sexes in domestic ruminants, typically larger in males.  Polled breeds, by definition, do not have horns.

Horns are permanent, continuously growing structures, following appearance post-natally, with shape and size strongly characteristic of the species, breed, age, and sex. As an aside, antlers are not horns.  The mature antler is a construct of bone, having developed from a cartilaginous model covered in finely haired tissue, known as velvet.  The antlers of the deer family are grown (rapidly!) and shed on an annual basis, under hormonal influence.

Horns have a base, body and apex.  The cornual process becomes pneumatized over time, until the base and body are primarily hollow, while the apex remains solid.  This air-filled space is called the cornual diverticulum of the frontal sinus.  Dehorning practices or fracturing of the horn can lead to open communication of the frontal sinus and an infection (sinusitis) as a consequence.

There is no hypodermis present in the horn. Horn dermis directly adheres to the underlying bone and projects papillae into the overlying epidermis. In the neonate, an epidermal bud (hillock) can be identified as the horn development site, usually indicated by a hair whorl at birth.  Effective debudding at this early age destroys the germinal epidermis, preventing horn and cornual process growth.

Horn growth is primarily from the base, with newer horn pushing older horn layers towards the tip, elongating the structure.  The epidermis, layered over the scaffold of dermal papillae, forms tubular horn (akin to tubular horn in equine hooves).  Horn growth rate is tied directly to nutritional status of the animal, and energy demands elsewhere (e.g. lactation, pregnancy).

Horn dermis receives a significant blood supply (cornual a.) to nourish the highly active germinal epidermis and is innervated by the cornual branch (sensory nerve that ties back to the Trigeminal n. [CNV].  The nerve can be blocked before dehorning.

Small ruminant horns:

• Have the same basic anatomy as the ox
• Arise much closer to the orbit
• Sheep horns grow a helical form, goats are more straight and directed caudally
• Growth is more intermittent resulting in a corrugated surface
• Blood supply and innervation are more direct based on the horn location.
• The goat horn has additional innervation that should be blocked before dehorning.
• Goats have a sebaceous cornual gland at the caudomedial base of each horn

Mammary gland

Ruminants have an udder, the collective term for the mammae of mammals with inguinal mammae, i.e. cow, ewe, doe, mare.  The cow has four inguinal mammae, more commonly referred to as quarters.  The doe and ewe have two inguinal mammae (like the mare).  A mammae is the complex of tissue associated with one teat.  A gland complex is the single gland and associated duct system within a mammae.  On each mammae, the number of gland complexes coincides with the number of teat orifices. Ruminants have one gland complex per mammae, therefore one teat orifice per teat is visible. Previously mentioned, the ewe has cutaneous inguinal sinuses on the dorsolateral aspect of her udder that contain secretions thought to scent mark her lambs.  Internal mammary gland anatomy will be considered in the reproductive system unit.

Hoof ([L] ungula) – the ruminant bears weight on digits 3 and 4, per limb, with non-weight bearing, vestigial digits 2 and 5 persisting as dewclaws.  The critical structure of the ruminant hoof will be studied in detail in the musculoskeletal unit.  For now, recognize it is a complex, specialized modification of the common integument.

Swine

The pig has a rostral plane (planum rostralis) that is the thickened disk-shaped rostral margin of their snout, supporting rooting behavior.  The rostral plane has a high density of sweat glands, responsible for the moist snout.

On the ventral mandible, rostrally, the mental gland of the pig is identified as a slightly raised circular mound, with a few elongated mental hairs.  Sebaceous and sweat glands secrete a scent marker.

Hair coat

The course, stiff, sparsely scattered guard hairs of the pig are a species-specific modification.  These hairs are referred to as bristles ([L] setae).

The pig lacks a distinct jugular groove along its neck.  Instead, a jugular fossa is observed distally in the ventral neck as a depression adjacent the point of the sternum.

Carpal glands (the ‘carpal organ’) are located in a linear array on the palmaromedial carpus/distal antebrachium.  These glands secrete a seromucous substance, likely a sexual marker. They may appear as a series of dark dots on the skin.

Mammary gland

Sows typically have 14 mammae, i.e. 7 pairs, a row each side of ventral midline, similar to carnivores. Thoracic, abdominal and inguinal mammae are present.  A mammae is the complex of tissue associated with one teat.  A gland complex is the single gland and associated duct system within a mammae.  On each mammae, the number of gland complexes coincides with the number of teat orifices. Sows, like mares, have two gland complexes per mammae, therefore two teat orifices per teat are visible. Internal mammary gland anatomy will be considered in the reproductive system unit.

Hoof ([L] ungula) – the ‘trotters’ of pigs represent digits 3 and 4, per limb, along with the non-weight bearing, but complete, paradigits 2 and 5. The paradigits are also known as dewclaws. The critical structure of the pig hoof will be studied in the musculoskeletal system unit.  For now, recognize it is a complex, specialized modification of the common integument.