Editing Horse (section)

==Anatomy==
{{Main|Equine anatomy|Muscular system of the horse|Respiratory system of the horse|Circulatory system of the horse}}

===Skeletal system===
{{Main|Skeletal system of the horse}}
[[File:Horse anatomy.svg|thumb|upright=1.4|The skeletal system of a modern horse|alt=Diagram of a horse skeleton with major parts labeled.]]
The horse skeleton averages 205 bones.<ref name=Evans90>{{cite book|author=Evans, J. |title=The Horse |edition=Second |publisher=Freeman |location=New York |year=1990 |isbn=0-7167-1811-1 |oclc=20132967 |page=[https://archive.org/details/isbn_9780716718116/page/90 90] |url= https://archive.org/details/isbn_9780716718116/page/90}}</ref> A significant difference between the horse skeleton and that of a human is the lack of a [[Clavicle|collarbone]]—the horse's [[Equine forelimb anatomy|forelimbs]] are attached to the [[Vertebral column|spinal column]] by a powerful set of muscles, tendons, and ligaments that attach the [[Scapula|shoulder blade]] to the torso. The horse's four legs and hooves are also unique structures. Their leg bones are proportioned differently from those of a human. For example, the body part that is called a horse's "knee" is actually made up of the [[carpal bones]] that correspond to the human [[wrist]]. Similarly, the [[hock (anatomy)|hock]] contains bones equivalent to those in the human [[ankle]] and [[heel]]. The lower leg bones of a horse correspond to the bones of the human hand or foot, and the [[Equine anatomy|fetlock]] (incorrectly called the "ankle") is actually the proximal [[sesamoid bone]]s between the [[Equine anatomy|cannon]] bones (a single equivalent to the human [[Metacarpus|metacarpal]] or [[metatarsal bones]]) and the [[proximal phalanges]], located where one finds the "knuckles" of a human. A horse also has no muscles in its legs below the knees and hocks, only skin, hair, bone, [[tendon]]s, [[ligament]]s, [[cartilage]], and the assorted specialized tissues that make up the [[horse hoof|hoof]].<ref name=Ensminger21>[[#Ensminger|Ensminger]], pp. 21–25</ref>

===Hooves===
{{Main|Horse hoof|Horseshoe|Farrier}}

The critical importance of the feet and legs is summed up by the traditional adage, "no foot, no horse".<ref name=Ensminger367>[[#Ensminger|Ensminger]], p. 367</ref> The [[horse hoof]] begins with the [[distal phalanges]], the equivalent of the human fingertip or tip of the toe, surrounded by [[cartilage]] and other specialized, blood-rich soft tissues such as the [[Horse hoof#Internal structures|laminae]]. The exterior hoof wall and horn of the sole is made of [[keratin]], the same material as a human [[Nail (anatomy)|fingernail]].<ref>[[#Giffin|Giffin]], p. 304</ref> The result is that a horse, weighing on average {{convert|500|kg}},<ref>[[#Giffin|Giffin]], p. 457</ref> travels on the same bones as would a human on tiptoe.<ref>{{cite web |url= http://www.cvm.uiuc.edu/petcolumns/showarticle_pf.cfm?id=118|title= Yes, The Shin Bone Is Connected to the Ankle Bone |access-date=2008-04-05 |author= Fuess, Theresa A.|website= Pet Column |publisher= University of Illinois |archive-url = https://web.archive.org/web/20060909161307/http://www.cvm.uiuc.edu/petcolumns/showarticle_pf.cfm?id=118 |archive-date = September 9, 2006}}</ref> For the protection of the hoof under certain conditions, some horses have [[horseshoe]]s placed on their feet by a professional [[farrier]]. The hoof continually grows, and in most domesticated horses needs to be trimmed (and [[horseshoe]]s reset, if used) every five to eight weeks,<ref>[[#Giffin|Giffin]], pp. 310–312</ref> though the hooves of horses in the wild wear down and regrow at a rate suitable for their terrain.

===Teeth===
{{Main|Horse teeth}}

Horses are adapted to [[grazing]]. In an adult horse, there are 12&nbsp;[[incisor]]s at the front of the mouth, adapted to biting off the grass or other vegetation. There are 24&nbsp;teeth adapted for chewing, the [[premolar]]s and [[Molar (tooth)|molars]], at the back of the mouth. Stallions and geldings have four additional teeth just behind the incisors, a type of [[Canine tooth|canine teeth]] called "tushes". Some horses, both male and female, will also develop one to four very small [[Vestigiality|vestigial]] teeth in front of the molars, known as "wolf" teeth, which are generally removed because they can interfere with the [[bit (horse)|bit]]. There is an empty interdental space between the incisors and the molars where the bit rests directly on the gums, or "bars" of the horse's mouth when the horse is [[bridle]]d.<ref>{{cite book |last=Kreling |first=Kai |title=Horses' Teeth and Their Problems: Prevention, Recognition, and Treatment |chapter-url=https://books.google.com/books?id=3q1LINmOkOIC&pg=PA117-IA1 |year=2005 |publisher=Globe Pequot |location=Guilford, CT |isbn=1-59228-696-8 |chapter=The Horse's Teeth |oclc=59163221 |pages=12–13 }}{{Dead link|date=April 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

An estimate of a horse's age can be made from looking at its teeth. The teeth continue to erupt throughout life and are worn down by grazing. Therefore, the incisors show changes as the horse ages; they develop a distinct wear pattern, changes in tooth shape, and changes in the angle at which the chewing surfaces meet. This allows a very rough estimate of a horse's age, although diet and veterinary care can also affect the rate of tooth wear.<ref name=Ensminger46>[[#Ensminger|Ensminger]], pp. 46–50</ref>

===Digestion===
{{Main|Equine anatomy#Digestive system|Equine nutrition|l1=Equine digestive system}}
Horses are [[herbivore]]s with a digestive system adapted to a [[forage]] diet of grasses and other plant material, consumed steadily throughout the day. Therefore, compared to humans, they have a relatively small stomach but very long intestines to facilitate a steady flow of nutrients. A {{convert|450|kg|adj=on}} horse will eat {{convert|7|to|11|kg}} of food per day and, under normal use, drink {{convert|38|to|45|L}} of [[water]]. Horses are not [[ruminant]]s, having only one stomach, like humans. But unlike humans, they can digest [[cellulose]], a major component of grass, through the process of  [[hindgut fermentation]]. Cellulose fermentation by symbiotic bacteria and other microbes occurs in the [[cecum]] and the [[large intestine]]. Horses cannot [[Vomiting|vomit]], so digestion problems can quickly cause [[horse colic|colic]], a leading cause of death.<ref>[[#Giffin|Giffin]], p. 175</ref> Although horses do not have a [[gallbladder]], they tolerate high amounts of fat in their diet.<ref>{{Cite journal |date=2001 |title=Role of dietary carbohydrate and fat in horses with equine polysaccharide storage myopathy |journal=Journal of the American Veterinary Medical Association |doi=10.2460/javma.2001.219.1537 |last1=Valentine |first1=Beth A. |last2=Van Saun |first2=Robert J. |last3=Thompson |first3=Kent N. |last4=Hintz |first4=Harold F. |volume=219 |issue=11 |pages=1537–1544 |pmid=11759989 |doi-access=free }}</ref><ref>{{Cite journal |date=2010 |title=The gall bladder and bile ducts |url=https://www.sciencedirect.com/science/article/abs/pii/S0263931910000542 |journal=Surgery (Oxford) |doi=10.1016/j.mpsur.2010.02.007 |last1=Ellis |first1=Harold |volume=28 |issue=5 |pages=218–221 |access-date=2021-05-11 |archive-date=2021-05-12 |archive-url=https://web.archive.org/web/20210512150222/https://www.sciencedirect.com/science/article/abs/pii/S0263931910000542 |url-status=live }}</ref>

===Senses===
{{See also|Equine vision}}
[[File:Pferdeauge.jpg|thumb|A horse's eye|alt=Close up of a horse eye, which is dark brown with lashes on the top eyelid]]

The horses' senses are based on their status as [[predation|prey animals]], where they must be aware of their surroundings at all times.<ref name=Ensminger309>[[#Ensminger|Ensminger]], pp. 309–310</ref> The equine eye is one of the largest of any land mammal.<ref name="Hartley 2016">{{cite book|last1=Hartley|first1=C|last2=Grundon|first2=RA|editor1-last=Gilger|editor1-first=BC|title=Equine Ophthalmology|date=2016|publisher=John Wiley & Sons|isbn=9781119047742|page=151|edition=3rd|chapter=Chapter 5: Diseases and surgery of the globe and orbit}}</ref> Horses are lateral-eyed, meaning that their eyes are positioned on the sides of their heads.<ref>{{cite journal |url = http://www.thehorse.com/ViewArticle.aspx?ID=15938 |title = Eye Position and Animal Agility Study Published |journal = The Horse |date = March 7, 2010 |access-date = 2010-03-11 |archive-date = 2015-07-23 |archive-url = https://web.archive.org/web/20150723214909/http://www.thehorse.com/articles/25141/eye-position-and-animal-agility-study-published |url-status = live }} Press Release, citing February 2010 Journal of Anatomy, Dr. Nathan Jeffery, co-author, University of Liverpool.</ref> This means that horses have a range of vision of more than 350°, with approximately 65° of this being [[binocular vision]] and the remaining 285° [[monocular vision]].<ref name="Sellnow">{{cite book |author=Sellnow, Les |url=https://archive.org/details/happytrailsyourc00sell/page/46 |title=Happy Trails: Your Complete Guide to Fun and Safe Trail Riding |publisher=Eclipse Press |year=2004 |isbn=1-58150-114-5 |page=[https://archive.org/details/happytrailsyourc00sell/page/46 46] |oclc=56493380}}</ref> Horses have excellent day and [[night vision]], but they have two-color, or [[Dichromacy|dichromatic vision]]; their [[color vision]] is somewhat like red-green [[color blindness]] in humans, where certain colors, especially red and related colors, appear as a shade of green.<ref>{{cite journal |url= http://www.thehorse.com/viewarticle.aspx?ID=9670 |title= In Living Color |url-access= registration |access-date= 2007-07-27 |author= McDonnell, Sue |journal= The Horse |date= June 1, 2007 |archive-date= 2007-09-27 |archive-url= https://web.archive.org/web/20070927223341/http://www.thehorse.com/viewarticle.aspx?ID=9670 |url-status= live }}</ref>

Their [[Olfaction|sense of smell]], while much better than that of humans, is not quite as good as that of a dog. It is believed to play a key role in the social interactions of horses as well as detecting other key scents in the environment. Horses have two olfactory centers. The first system is in the nostrils and nasal cavity, which analyze a wide range of odors. The second, located under the nasal cavity, are the [[vomeronasal organ]]s, also called Jacobson's organs. These have a separate nerve pathway to the brain and appear to primarily analyze [[pheromone]]s.<ref name="Briggs smell">{{cite web |last=Briggs |first=Karen |title=Equine Sense of Smell |url=http://www.thehorse.com/articles/10055/equine-sense-of-smell |publisher=The Horse |access-date=2013-12-15 |date=2013-12-11 |archive-date=2018-02-01 |archive-url=https://web.archive.org/web/20180201013608/http://www.thehorse.com/articles/10055/equine-sense-of-smell |url-status=live }}</ref>

A horse's hearing is good,<ref name=Ensminger309/> and the [[Auricle (anatomy)|pinna]] of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head.<ref>{{cite book |author=Myers, Jane |url=https://books.google.com/books?id=-gAgchKMXdkC |title=Horse Safe: A Complete Guide to Equine Safety |publisher=CSIRO Publishing |location=Collingwood, UK |year=2005 |isbn=0-643-09245-5 |oclc=65466652 |page=7 |access-date=2020-09-28 |archive-date=2023-03-20 |archive-url=https://web.archive.org/web/20230320223812/https://books.google.com/books?id=-gAgchKMXdkC |url-status=live }}</ref> Noise affects the behavior of horses and certain kinds of noise may contribute to stress—a 2013 study in the UK indicated that stabled horses were calmest in a quiet setting, or if listening to country or classical music, but displayed signs of nervousness when listening to jazz or rock music. This study also recommended keeping music under a volume of 21 [[decibel]]s.<ref name="Music">{{cite web |url=http://www.thehorse.com/articles/31229/music-genres-effect-on-horse-behavior-evaluated |title=Music Genre's Effect on Horse Behavior Evaluated |last=Lesté-Lasserre |first=Christa |date=January 18, 2013 |website=The Horse |publisher=Blood Horse Publications |access-date=23 January 2013 |archive-date=10 October 2017 |archive-url=https://web.archive.org/web/20171010200000/http://www.thehorse.com/articles/31229/music-genres-effect-on-horse-behavior-evaluated |url-status=live }}</ref> An Australian study found that stabled racehorses listening to talk radio had a higher rate of gastric ulcers than horses listening to music, and racehorses stabled where a radio was played had a higher overall rate of ulceration than horses stabled where there was no radio playing.<ref name="EquiNews">{{cite web |url=http://www.equinews.com/article/radios-causing-gastric-ulcers |title=Radios Causing Gastric Ulcers |author=Kentucky Equine Research Staff |date=February 15, 2010 |website=EquiNews |publisher=Kentucky Equine Research |access-date=23 January 2013 |archive-date=10 October 2017 |archive-url=https://web.archive.org/web/20171010200000/http://www.equinews.com/article/radios-causing-gastric-ulcers |url-status=live }}</ref>

Horses have a great sense of balance, due partly to their ability to feel their footing and partly to highly developed [[proprioception]]—the unconscious sense of where the body and limbs are at all times.<ref>{{cite web|url= http://www.thoroughbredtimes.com/horse-health/1998/October/17/True-horse-sense.aspx|title= True Horse Sense |access-date= 2008-07-08 |author= Thomas, Heather Smith |website= Thoroughbred Times |publisher= Thoroughbred Times Company |archive-date= 2012-11-02 |archive-url= https://web.archive.org/web/20121102020057/http://www.thoroughbredtimes.com/horse-health/1998/october/17/true-horse-sense.aspx |url-status= dead}}</ref> A horse's [[Somatosensory system|sense of touch]] is well-developed. The most sensitive areas are around the eyes, ears, and nose.<ref>{{cite web |url=http://www.unce.unr.edu/publications/files/ag/other/fs9829.pdf |title=Horse Handling and Riding Guidelines Part 1: Equine Senses |access-date=2008-07-09 |author1=Cirelli, Al Jr. |author2=Brenda Cloud |website=Cooperative Extension |page=4 |publisher=University of Nevada |archive-date=2015-09-08 |archive-url=https://web.archive.org/web/20150908031752/http://www.unce.unr.edu/publications/files/ag/other/fs9829.pdf |url-status=live }}</ref> Horses are able to sense contact as subtle as an insect landing anywhere on the body.<ref>{{cite book|author1= Hairston, Rachel|author2= Madelyn Larsen |title= The Essentials of Horsekeeping |publisher= Sterling Publishing Company, Inc. |year= 2004 |isbn= 0-8069-8817-7 |location= New York |url= https://archive.org/details/essentialsofhors00rach/page/77 |oclc= 53186526 |page= [https://archive.org/details/essentialsofhors00rach/page/77 77]}}</ref>

Horses have an advanced sense of taste, which allows them to sort through [[fodder]] and choose what they would most like to eat,<ref>[[#Miller|Miller]], p. 28</ref> and their [[Prehensility|prehensile]] lips can easily sort even small grains. Horses generally will not eat poisonous plants, however, there are exceptions; horses will occasionally eat toxic amounts of poisonous plants even when there is adequate healthy food.<ref>{{cite web |url= http://www.cvm.uiuc.edu/petcolumns/showarticle.cfm?id=16|title= Horse Pasture is No Place for Poisonous Plants |access-date=2008-07-09 |author= Gustavson, Carrie |website= Pet Column July 24, 2000 |publisher= University of Illinois |archive-url = https://web.archive.org/web/20070809051147/http://www.cvm.uiuc.edu/petcolumns/showarticle.cfm?id=16 |archive-date = August 9, 2007}}</ref>