By Les Sellnow he evolution of the horse from a tiny, four-toed an­imal, T perhaps no more than one foot tall, to the variety of equines in existence today, is one of the wonders of nature. During that process of change, the horse evolved over many thousands of years from an animal­ that predators hunted for food to an animal that became a servant and friend for mankind. Today’s horses are designed to do one of two things— pull a load with their shoulders or carry riders on their backs. The type of horses utilized for these respective tasks varies a good deal; one is large and pon­d­erous and the other is lighter-boned with less mu­scle­ mass. Even within these two types, there are significant differences. For example, the conformation of a roping or cutting This first article of a 12-part series on equine anatomy and horse is different from that of the American Saddle- physiology discusses basic terminology, the horse’s largest bred. Yet there is a basic sameness to anatomy. organ, and how horses and humans are alike (and different) Robin Peterson illustrations

2 www.TheHorse.com The Horse January 2006 January 2006 The Horse www.TheHorse.com 3 Saudi Arabia as chief medical illustrator closer to the tail (cauda). Example: The Median Plane—This divides the horse’s for the King Faisal Specialist Hospital and horse’s back is caudal to his neck. body into right and left halves (median Research Center in Riyadh. Through the Rostral—That part of the structure means in the middle). years, he has received many honors and located­ closer to the nose (rostrum). Sagittal Plane—Any plane parallel to the awards of excellence for his work. Example:­ The eyes are rostral to the ears. median plane. Example: A plane dividing Kainer joined the anatomy faculty at Proximal—A location toward the the right and left sides of a hoof. Colorado State in 1961 and remained there attached­ end of a limb. Example: The Medial—Structures located closer to the for 27 years. He, too, has won a variety of proximal end of the cannon bone connects median plane. Example: When you look awards and honors for his work. with the knee. at a horse’s left side, you see the medial In this opening article­ of our 12-part Needless to say, since there are entire Distal—Indicates a location toward the surfaces of his right limbs. series on equine anat­omy and physiology, books written on equid anatomy and phys- free end of a limb—the part that is farther Lateral—Structures located away from we will attempt to paint a picture of how iology, it would be impossible to address away from the body. Example: The distal the median plane. Example: When you today’s horse is constructed and what this the entire animal in one article. Therefore, end of the cannon bone connects with the look at a horse’s left side, you see the lateral means in the realm of form to function. We this month we look in-depth at the horse’s long pastern bone or third phalanx. surfaces of his left limbs. will not be quoting a lot of sources, for the largest organ, its skin, and details of the most part, because the information to be nomenclature used to discuss anatomy presented is an amalgamation of what has and physiology, as well as lightly touch on been recorded­ in thousands of scientific structures that will be covered in-depth papers, textbooks, and manuals as knowl- in the coming year. In the months ahead edge has been gained and disseminated we’ll have articles on joints, the foreleg, the through the years. However, we would be hind limb, the hoof, the head and neck, the remiss in not calling attention to two valu- back, muscles, tendons and ligaments, and able sources upon which we shall draw There are some similarities between human and equine anatomy, but there are a great many differ- the digestive, circulatory, respiratory, and heavily. Both are writ­­ten at the layman ences as well. For example, the human knee and the equine stifle have a lot in common. The equine reproductive­ systems. level. They are Horses and Horsemanship radius is similar to the human forearm. The equine carpal or knee bones are similar to human wrist by the late M.E. En­sminger, BS, MA, PhD, bones. And the equine splint bones compare to human fingers. A Glossary to Get You Started and The Coloring Atlas of Horse Anatomy by Colorado State University. has authored seven books, and today he We will begin this discussion of equine Robert A. Kain­er, DVM, MS, and Thomas McCracken, who resigned from Colorado­ is president of Biographix, LLC. Early in anatomy with Kainer’s and McCracken’s O. McCracken, MS, both of whom were at State in 1994 to enter private business, his career, McCracken spent two years in explanation of terminology that is routine for veterinarians and researchers, but can be confusing to some horse owners. Along SUCCEED can help you achieve your with the two authors’ terminology, we’ll No Guts... No Glory. goals with measurable results. add examples to demonstrate­ how the Use SUCCEED and see the results for yourself, sometimes terminology might be employed. Under- in a matter of days. Soon, you’ll be able to measure the standing the terminology is essential when Highest scoring U.S. Dressage rider at 2006 WEG difference in the show ring. discussing various conditions with a veter- SUCCEED delivers highly specialized nutrients to support inarian. a healthy digestive tract, the fuel line to every other system in the horse’s body. Use the product that gives real Here is some terminology, accompanied results – use SUCCEED. by illustrations, to help you visualize: On Side—The horse’s left side; also called Available in oral paste, top-dress granules or SmartPaks. near side. Example: Normally, the side on which we mount and dismount. Off Side—Right side of the horse; also called far side. Example: One theory of why Congratulations Floriano trainer and rider Steffen we mount from the left is that warriors and Peters and owners Stephen and Laurelyn Browning. soldiers of old generally carried a sword on the left side and this would get in the way ® SUCCEED helps Steffen Peters get the most out of his horses. if swinging up from the right or off side of the horse. Available from tack and feed stores, Dorsal—Parts of the horse’s anatomy “SUCCEED is the best single product I’ve ever used.” or ask your veterinarian. toward his back (dorsum). Ex­ample: The “With SUCCEED my horses are consistently trainable and relaxed both at home and at shows. point of the croup is dorsal to the stifle. Trainability has been the biggest factor. There was a defi nite improvement. SUCCEED will Anatomy toward­ the belly continue to be a part of our program.” Ventral— (venter). Example: The stallion’s reproduc- Steffen Peters; trainer of Floriano, 2006 USDF Grand Prix Horse of the Year, on SUCCEED since August 2005. tive organs are ventral to his flank. • Highest scoring U.S. rider in all three dressage events at the 2006 World Equestrian Games Cranial—That part of the horse’s struc- (WEG) in Aachen, Germany. ture above the knees and hocks located • Led the U.S. to a Bronze Medal fi nish in the team competition at WEG. • Maintained a commanding lead during the 2006 USEF National Grand Prix Championship 866-270-7939 ext.115 closer to the skull (cranium). Example: competition in Gladstone, NJ. The withers are cranial to the tail. www.SucceedDCP.com Caudal—That part of the horse’s struc- Photos © 2007, Stuart Vesty SUCCEED® and Digestive Conditioning Program® are registered trademarks of Freedom Health, LLC. ©2007. All Rights Reserved. PATENT PENDING. ture above the knees and hocks located

4 www.TheHorse.com The Horse January 2006 Month 2006 The Horse www.TheHorse.com 5 Planes of the Horse structure is located away from the axis. 5) A simple stomach followed by a long better place to start than with the skin, the small intestine and a large, complicated horse’s largest organ, ranging from 12-24% Classifying the Horse large intestine where fermentation of of the animal’s total weight depending on Cranial Dorsal Caudal Before leaving our Atlas authors, we’ll feed occurs. age. It serves a number of functions. It pass along their explanation of proper 6) Well-developed skin glands. protects underlying tissues from injury, Frontal plane ter­minology for this animal we are 7) Large heart and lungs. drying, water absorption, and bacterial dis­cussing, along with what they consider 8) A uterus with short horns and a relatively invasion. Another important role involves to be identifying characteristics: large body, and a prominent de­pression thermoregulation (regulation of body Cranial “Horses and their close relatives, don­keys in each ovary where the egg cells are temperature). The skin also excretes water Transverse Plane—Passes through the Rostral and zebras, are in the mammalian order of released. and salts through sweat glands, senses the head, trunk, or limb perpendicular to the odd-toed, hoofed animals (Perissodactyla) 9) A large, vascular penis and a complete environment, and synthesizes vitamin D in part’s long axis. Example: An ultrasound as are their distant relatives, rhinoceroses­ set of male accessory sex glands. response to sunlight. image gives a transverse view of the limb. and tapirs. The horse, Equus caballus, is There are similarities between human The skin consists of various cellular and Dorsal Plane—Passes through a body an equid, a member of the horse family and equine anatomy, but there are many tissue components. There are two layers, part parallel to its dorsal surfaces. Dorsal plane Equidae. The adjective equine is frequently differences too. For example, the human the epidermis and the dermis (subcutis), Proxi- There are subtle shifts in terminology as mal Axial used improperly as a noun.” knee and the equine stifle have a lot in with the epidermis being the outer layer. Frontal plane we move about the equine anat­omy. For Abaxial Characteristics of equids include: com­mon. The equine radius is similar to The two are attached by collagenous and example, the two writers point out, “Distal Sagittal plane 1) Highly specialized limbs, each with the hu­man forearm. Equine carpal (knee) elastic connective tissue. to and including the carpus (knee), dorsal Transverse plane or one digit (the third) and with the main bones compare to human wrist bones. And The main activity of the epidermis is to replaces cranial and palmar replaces Distal cross-section muscle mass of the limb situated close equine splint bones compare to human produce two types of protein—keratin and caudal.­ Distal to and including the hock, Dorsal Plantar to the body’s trunk. fingers. melanin. Keratin, the principal compo- Dorsal Palmar dorsal replaces cranial, but plantar replaces 2) Large paranasal sinuses in the skull. There is also a similarity in the cooling nent of the epidermis, is a simple protein Median plane caudal.”­ This means that when you are Lateral 3) Guttural pouches, which are large process during exercise—both humans characterized by its insolubility (it won’t Medial talking about the lower limbs, you use the Solar out­pocketings of the auditory tubes and horses cool their bodies via sweating. dissolve)­ and fibrous structure. It serves word dorsal to mean frontward instead Ventral that extend from the nasopharynx to Yet, the two are worlds apart when looking a sup­portive and protective function, of cranial,­ and instead of using caudal to the middle ears. at the digestive process. includ­ing the shedding of water. Melanin mean rearward, you use palmar (for the 4) High-crowned permanent teeth that is the dark, shapeless pigment of the skin front limb) or plantar (for the hind limb). that contacts the ground. front, an axial structure is located toward grow for a long time—a feature used to Skin and hair. The solar surface of the hoof is the part When looking at a horse’s limb from the the axis (center of the limb) and an abaxial determine the age of a horse. When discussing equine anatomy, what Another cell type in the epidermis is the

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6 www.TheHorse.com The Horse January 2006 January 2006 The Horse www.TheHorse.com 7 Anatomy of the Skin longer periods of light. in wet, humid climates than in arid areas. sentence­ for the horse. There is no known Melanocyte Again, we turn to McCracken and The condition is caused by a fungus that is treatment. Sweat pore Kainer for an explanation of what occurs: capable of creating infection when condi- Keratin Cells “Horses shed hair mostly in the spring, tions are right, such as during hot and Stay Tuned… beginning with the belly and sides and humid periods. The infection results in There, in short form, is a description continuing up onto the back. The hair scabby spots that are devoid of hair when of the equine skin and its functions. In Stratum cor- cycle includes an actively growing stage rubbed free. up­coming articles, we will concentrate on } neum Epidermis (anagen) followed by a stage (catagen) Mange—This condition is caused by different parts of the horse’s anatomy with } during which the hair matrix sort of several species of mites. Mange is charac- an in-depth look at how they are struc- Langerhans’ cell, a non-neural cell that is shrinks (atrophies) and peels away from terized by loss of hair, itching, and intense tured, the functions they serve, and what is active in the immune response and possibly its blood supply. A longer, quiescent irritation to the skin. involved in keeping them healthy. in the regulation of keratin formation. period (telogen) then occurs in which the Lice—Horses can harbor two species of Here is a brief look at those upcoming The most important part of the epider­mis Dermis hair (club hair) separates from the hair lice. One species, Damalinia equi, is the article topics, to whet your appetite. is the superficial layer—known as the matrix, but remains in the follicle. The horse biting louse and the other, Haema­ The equine skull houses the brain and Shaft stratum corneum—since much of the func- matrix later becomes active and begins to topinus asini, is the horse sucking louse. the most important organs of sense. There tional activity of the skin resides here. The grow a new hair that pushes out the old Afflicted horses will show signs of itching are 34 bones in the equine skull, most of proper functioning of this superficial layer Sebaceous Root club hair (shedding).” by constantly rubbing and biting at them flat. During the birthing process, is dependent on the structural arrangement gland } There are health conditions that can affected areas. Lice are most often found these bones yield and overlap, allowing the of the keratin it contains and on its lipids— Subcutis overpower the forces of nature in the shed- in un­thrifty horses and quickly add to the skull to be somewhat compressed and thus fats and fat-like substances characterized } ding department. For example, horses poor condition status. In severe infesta- allowing for easier parturition. Sweat gland by being water-insoluble. Lipids and keratin Hair follicle Smooth muscle Nerve suf­fering from Cushing’s disease often tions, the horse might lose patches of hair. The horse has unique eyes—they are combine to waterproof the skin and prevent The skin protects underlying tissues from injury, drying, water absorption, and bacterial invasion. carry a heavy coat even during long-light, both monocular and binocular. Monocular various agents from entering the body. If Another important role involves thermoregulation (regulation of body temperature); the skin also hot summer days. means that the horse can see objects with excretes water and salts through sweat glands, senses the environment, and synthesizes vitamin D this superficial layer of the epidermis were The color of a horse’s hair is determined Hyperelastosis cutis one eye. As a result, the brain often gets removed, the skin would be like a mucous in response to sunlight. by a rather complicated genetic process two images simultaneously. With binocular membrane that is easily permeated by involving the amount of pigment in the often results in a vision the horse can focus with both eyes water and a variety of agents. Nerve endings in the dermis, just under result can be deadly. hair. There are three basic hair pigments— and the brain is receiving only one signal. The hair follicles in a horse’s skin are the epidermis, are called sensory nerves The problem develops something likes black, brown, and yellow. The great varia- death sentence. Humans have binocular vision only. quite simple, with a single hair emerging and carry sensations of pressure, pain, this: As heat accumulates, blood flow to tion in color has to do with combinations From the equine head, we move to the from each pore. The hair that emerges heat, and cold. In contrast, motor nerves the skin is increased to speed up the trans- or absences of these three pigments. neck. There are seven cervical vertebrae from these pores serves as frontline protec- cause the sweat glands to secrete. portation of heat from the core of the body in the horse’s neck, with the spinal cord tion for the horse. First, a heavier growth When sweat reaches the surface of the to the skin surface. As exercise increases, Skin Problems Dandruff—This is a condition where running through them. The neck blends in cold weather helps to provide warmth skin, it evaporates. This has a cooling however, sweat loss leads to dehydration The skin, as the first line of defense the horse’s skin becomes scaly and the into the back at the withers. and a lighter coat during summer facili- effect on the horse’s body. Since sweating and the loss of plasma water from the against the elements, insects, and antigens, hair coat often is dry and dirty. In some We come now to the horse’s rib struc- tates the cooling process. The hair also is the horse’s primary method of cooling bloodstream. This results in a decrease in is under almost constant attack from one instances, the hair will fall out. Grooming ture. Normally, there are 18 pairs of ribs serves as an important filtering system. the body during exercise, cooling can be the volume of blood that is carrying heat source or another. In many cases, the condi- can help to stimulate the sebaceous glands; in the horse, but a nineteenth rib on one For example, ultraviolet light is filtered by severely compromised when evaporation to the surface. The result is an increased tions are not life-threatening, but produce a exercise and a proper diet can prevent this side or both is not considered all that rare. the hair coat and is absorbed by melanin of that sweat does not take place, such as heart rate as the horse’s body tries to make great deal of irritation to the horse and can problem and can work to solve it if it does Eight pairs of ribs are known as true ribs. granules in the epidermis and hair. Hair is on a hot, humid day. Unfortunately, nature up for the decreased volume of blood. lead to more serious consequences. occur. They join the segments of the sternum composed of keratinized epithelial cells. didn’t provide much in the way of backup Very quickly, body temperature rises and Here are some of the attackers that make Biting Insects—During the summer or breastbone. The other 10 are known Each hair consists of a shaft and a root measures­ for hot, humid weather, and it extreme fatigue sets in. war on the skin: months, horses are under almost constant as floating ribs—merely overlapping and that is contained in a depression known as is up to the humans involved to help out A problem that afflicts some horses and Dermatitis—This is an inclusive term attack by one type of biting insect or another, being at­tached to each other. The seventh the hair follicle. Associated with hair folli- by pro­viding plenty of water to drink and compromises the thermoregulatory system for inflammation of the skin that can be including flies, mosquitoes, and ticks. Not and eighth ribs are the longest. Those cles are sebaceous (oil) glands and bundles cool mists or baths to cool the body when is anhidrosis or dry coat. Horses with this produced­ by many agents, such as external only are these pests irritating to the horse, toward the rear are much shorter. of smooth muscle connecting the side of horses must be exercised in such condi- affliction simply can’t sweat. It appears that irritants;­ burns; allergens; trauma; bacteria; but they also spread diseases such as West Next we switch to the horse’s thoracic or the hair to the dermis. The sebaceous tions. anhidrosis is climate-specific; the signs are viral, parasitic or fungal infections; and Nile virus, equine infectious anemia, and front limbs. Each of these consists of the glands secrete an oily substance into the Research has shown that a horse’s body found in areas with a hot, humid climate, systemic­ diseases. When dermatitis encephalitis. Responsible horse owners scapula, humerus, radius, ulna, seven or follicles and thus to the epidermal surface. temperature can rise to a dangerous level such as Florida during the summer. surfaces, the important thing is to deter- should take all steps that are possible to eight carpal­ bones, cannon bone, two splint The oil lubricates the skin to prevent very quickly when adequate cooling does No one is sure what causes anhidrosis, mine the underlying­­ cause, as treatment can prevent these disease-carrying pests from bones, two proximal sesamoid bones, large excessive evaporation that can result in not occur. Without proper cooling, body and there is no effective way to treat it vary a great deal. An example of dermatitis attacking the animals in their charge. pastern bone, small pastern bone, navic- dry skin. temperatures during endurance rides, for other than to move affected horses to a is ringworm, which is actually an infection Hyperelastosis cutis (HC)—This is an ular bone, and coffin bone. The structure There are two types of sweat glands in the example, can rise to 106ºF (normal temper- cooler climate. Often, racehorses that of keratinized tissue by one of several types inherited condition of Quarter Horses of these bones and their functional rela- horse—the apocrine and the eccrine glands. ature is 99.5-101.5ºF), and the ultimate suffer from anhidrosis in Florida will show of fungi. The most common form of ring- from a particular bloodline. Earlier, we tionship to each other helps determines a The apocrine glands are spread throughout result can be heat stroke. no signs of the affliction when moved to worm—which is easily transmitted from mentioned­ that the epidermis and dermis horse’s athletic potential and its capability the skin, while the eccrine glands are found There is more involved in the thermo- the cooler Midwest. animals to humans—produces skin lesions are held together by collagenous mate- of remaining sound even under an exacting only in the frog of the hoof and, thus, play regulation department than just sweating. The growth and shedding of hair coats that appear as small rounded spots in the rial, sort of like glue that can bind two exercise and performance regimen. a very limited role in the cooling process. An intricate system of blood vessels must follows a definite cyclical pattern. A skin. The lesions eventually form blisters substances. When a horse inherits the The angles at which the bones connect The horse sweats the most of any of the first carry heat from the core of the body heavier coat grows when days shorten and break, leaving scabs in their wake. genes that cause HC, the collagenous mate- have much to do with the horse’s physical domestic species. The sweat glands are to the skin so that it can be dissipated via and weather turns colder, with shorter Rain Rot—This condition carries the rial is defective and unable to hold the two capability and its continued well being. simple, coiled, tubular glands that open sweating.­ When heat buildup overpowers periods of day­light, and is shed when days scientific name of dermatophilosis. It is a skin layers together; the result is tearing The leg bones, as well as other bones, independently of the hair follicle. the horse’s ability to dissipate it, the become longer and warmer and there are form of dermatitis that is more common of the skin, which often results in a death are connected at joints. There are three

8 www.TheHorse.com The Horse January 2006 January 2006 The Horse www.TheHorse.com 9 articular cartilage. The cartilage within the long pastern bone; next is the short pastern joint is smooth and resilient, allowing for bone, and lastly the coffin bone. Both front frictionless movement. Ligaments keep the and rear limbs are con­nected and sup­port­ed bones in proper support and relationship. by ligaments, ten­dons, and muscles that In the rear legs, we’ll start at the spine provide attachment and mobility. where the ilium, the largest of three bones Nature didn’t do the horse many favors in the pelvis, connects with the spinal when designing the way in which it col­umn. The ilium angles down and rear- pro­cesses food. Its stomach can hold only ward to blend into the femur or thigh bone. small amounts of food—eight to 16 quarts types of equine joints—synovial, which are The femur angles slightly forward to the at a time. The stomach empties into the movable; cartilaginous, which are slightly stifle. Beginning at the stifle (comparable small intestine, a tube that connects with movable; and fibrous, which are immov- to the knee in humans) is the tibia, which the large intestine. The small intestine is able. The joints that connect the horses’ continues­ downward to the hock. From the approximately 70 feet long, with a capacity front leg bones are synovial. A sy­nov­ial hock to the pastern is the metatarsus or of about 12 gallons. The large intestine of joint consists of two bone ends, covered by rear cannon bone, which connects with the the horse is divided into four parts—the cecum, which is about four feet long and one foot wide; the great colon, which is about 12 feet long and 10 inches wide; the small colon, which is about 10 feet long and four inches wide; and the rectum. Required Reading In the reproductive department, the main sex organs of the mare are two ovaries that produce eggs, which are for Responsible Horse Owners. housed in follicles. When an egg is mature, it leaves the follicle and travels through the infundibulum and oviduct to the uterus. If fertilized, it grows within the uterus from For over 10 years The Horse: Your Guide to Equine a minute structure into a foal that is born has been essential reading by responsible Health Care through the vagina. horse owners. The primary sex organs of the stallion Subscribe today, and along with your full access to are the two testicles, which produce semen TheHorse.com, you’ll receive 12 issues of The Horse and testosterone, and the penis. A horse’s respiratory system is designed—­ filled with essential information you need to know to as is the case with other mam­mals—to keep your horses healthy and strong. Order now and exchange oxygen and carbon dioxide. Air SAVE OVER 72% off the cover price. Plus, you’ll get the enters the nostrils, passes through the long next edition of The Horse Source FREE. nasal cavity over the pharynx and larynx, then enters the trachea that carries it to NO RISK. NO OBLIGATION. the lungs. Your satisfaction is absolutely guaranteed. If you’re A horse’s cardiovascular system consists not completely satisfied, we’ll send you a refund for all of blood, a series of blood vessels through unmailed issues! which blood flows, and a heart that pumps blood throughout the horse’s body. Blood is comprised of red blood cells, white blood cells, and platelets. The red blood cells have a protein known as hemoglobin that binds oxygen and conveys it to the SUBSCRIBE NOW! muscles. The white blood cells function in the immune system by defending against CLICK HERE or foreign invaders, such as harmful bacteria, and the platelets are involved with blood call 1-800-582-5604 clotting. H

About The Author Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and he operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, includ- ing Understanding Equine Lameness, Understanding The Young Horse, and The Journey of the Western Horse, pub- CQ06Z236TH lished by Eclipse Press and available at www.Exclusively Equine.com or by calling 800/582-5604.

10 www.TheHorse.com The Horse Month 2006