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[[Image:Equine Guttural Pouch.jpg|thumb|right|250px|Equine Guttural Pouch - Copyright David Bainbridge]]

[[Image:Equine Guttural Pouch.jpg|thumb|right|250px|Equine Guttural Pouch - Copyright David Bainbridge]]

==Introduction==

==Introduction==

The Guttural Pouch is present only in members of the order Perissodactyla (nonruminant ungulates: horses; tapirs; rhinoceros) and another small band of small mammels including Hyraxes, certain bats and a South American mouse.  

The Guttural Pouch is present only in members of the order Perissodactyla (nonruminant ungulates: horses, tapirs, rhinoceros) and another small band of small mammals including Hyraxes, certain bats and a South American mouse.  

The guttural pouch, also known as the ''[[Ear - Anatomy & Physiology|Auditory]] Tube Diverticulum'' is a long, slitlike opening connecting the [[Nasal Cavity - Anatomy & Physiology|nasal cavity]] and [[Ear - Anatomy & Physiology#Middle Ear|middle ear]] which dilates to form pouches which can have a capacity of 300-500ml in the domestic horse.

The guttural pouches are paired ventral diverticulae of the eustachian (auditory) tubes, formed by escape of mucosal lining of the tube through a relatively long ventral slit in the supporting cartilages. The auditory tube connect the [[Nasal Cavity - Anatomy & Physiology|nasal cavity]] and [[Ear - Anatomy & Physiology#Middle Ear|middle ear]] and the diverticulum dilates to form pouches which can have a capacity of 300-500ml in the domestic horse. The pouches are normally air filled.

==Structure==

==Structure==

The Guttural Pouch is located below the cranial cavity, towards the caudal end of the [[Skull and Facial Muscles - Anatomy & Physiology#Bones of the Skull|skull]]. It is covered laterally by the Pterygoid muscles, [[Parotid Gland - Anatomy & Physiology|parotid]] and [[Mandibular Gland - Anatomy & Physiology|mandibular]] glands. The floor lies mainly on the pharynx. It is moulded to the Stylohyoid muscle which divides the medial and lateral compartments. The Guttural Pouch has close association with many major structures including several [[Cranial Nerves - Anatomy & Physiology|cranial nerves]] (Glossopharyngeal, Vagus, Accessory, Hypoglossal), the Sympathetic Trunk and the External and Internal Carotid Arteries. The pouch covers the temporohyoid joint. The pouch has an extremely thin wall which is lined by [[Respiratory Epithelium - Anatomy & Physiology|respiratory epithelium]] which secretes mucus. This normally drains into the pharynx when the horse is grazing.

The Guttural Pouch is located below the cranial cavity, towards the caudal end of the [[Skull and Facial Muscles - Anatomy & Physiology#Bones of the Skull|skull]]/wing of atlas. It is covered laterally by the Pterygoid muscles, [[Parotid Gland - Anatomy & Physiology|parotid]] and [[Mandibular Gland - Anatomy & Physiology|mandibular]] glands. The floor lies mainly on the [[Pharynx - Anatomy & Physiology|pharynx]] and beginning of the [[Oesophagus - Anatomy & Physiology|Oesophagus]]. The medial retropharyngeal lymph node lies between the pharynx and ventral wall of the pouches.

 

Right and left pouches are separated dorsomedially by rectus capitis ventralis and longus capitis muscles. Below this, by fused walls of the two pouches, the median septum is formed.

 

Each pouch is moulded to the stylohyoid bone which divides the medial and lateral compartments, the medial compartment being approximately double the size of the lateral one and extends further caudally and ventrally.

 

The guttural pouch has close association with many major structures including several [[Cranial Nerves - Anatomy & Physiology|cranial nerves]] (glossopharyngeal, vagus, accessory, hypoglossal), the sympathetic trunk and the external and internal carotid arteries. The pouch directly covers the temporohyoid joint. The pouch has an extremely thin wall which is lined by [[Respiratory Epithelium - Anatomy & Physiology|respiratory epithelium]] which secretes mucus. This normally drains into the pharynx when the horse is grazing.

 

 

 

 

 

 

 

==Function==

The function of the guttural pouches has been shown with experimental data to participate in the rapid cooling of arterial blood destined for the brain and surrounding structures. In other words, the horse's guttural pouches are 'brain-cooling devices', cooling blood within the internal carotid arteries during [[hyperthermia]] that occurs during heavy exercise.<ref>{{Cite journal|last=Baptiste|first=Keith E.|last2=Naylor|first2=Jonathan M.|last3=Bailey|first3=Jeremy|last4=Barber|first4=Ernest M.|last5=Post|first5=Klass|last6=Thornhill|first6=Jim|date=2000-02-01|title=Physiology - A function for guttural pouches in the horse|url=https://www.researchgate.net/publication/12647122|journal=Nature|volume=403|issue=6768|pages=382–383|doi=10.1038/35000284|pmid=10667779|issn=0028-0836|bibcode=2000Natur.403..382B}}</ref><ref>{{Cite journal|last=Baptiste|first=Keith E.|title=A preliminary study on the role of the equine guttural pouches in selective brain cooling|journal=The Veterinary Journal|url=https://www.sciencedirect.com/science/article/abs/pii/S1090023398800099?via%3Dihub|doi=10.1016/S1090-0233(98)80009-9}}</ref> However, this proven function has been called into question by another study that neither examined the guttural pouches nor the internal carotid arteries;<ref name=":4">{{Cite journal|last=Mitchell|first=Graham|last2=Fuller|first2=Andrea|last3=Maloney|first3=Shane K|last4=Rump|first4=Nicola|last5=Mitchell|first5=Duncan|date=2006-09-22|title=Guttural pouches, brain temperature and exercise in horses|journal=Biology Letters|doi=10.1098/rsbl.2006.0469|url=https://royalsocietypublishing.org/doi/10.1098/rsbl.2006.0469|pmc=1686210}}</ref> and others have argued that a cooling function would require an unattainable high rate of inspiratory air flow partly diverted into the guttural pouches.<ref>{{Cite journal |author=Shane K. Maloney |author2=Andrea Fuller |author3=Graham Mitchell |author4=Duncan Mitchell |title=On the guttural pouch and selective brain cooling in equids: research letter |date=March 2002 |journal=South African Journal of Science |volume=98 |issue=3 |hdl=10520/EJC97450 |hdl-access=free}}</ref>

The Guttural Pouch is a mechanism for cooling the cerebral blood supply after exercise, due to it's close connection with the Internal Carotid Artery.

The issue of necessary guttural pouch air flow rates, to provide rapid cooling of the internal carotid arteries, has been solved by further supporting evidence from microvascular studies of the guttural pouch mucosa.<ref>{{Cite journal |last=Ninomiya |first=Hiroyoshi |last2=Kuwano |first2=Atsutoshi |title=Microvasculature of the Guttural Pouch Mucosa and Relationship With the Internal Carotid Artery in Horses |date=March 2019 |journal=Journal of Equine Veterinary Science |volume=74 |pages=84–89 |doi=10.1016/j.jevs.2018.12.025 |url=https://www.sciencedirect.com/science/article/abs/pii/S0737080618306981?via%3Dihub}}</ref> Many of the guttural pouch mucosal superficial arterioles and capillaries extend outwards, forming two types of vascular plexuses surrounding the internal carotid arteries: one with capillary bundles parallel to the internal carotid arteries in the outer layer of the tunica adventitia (outer peri-arterial plexus) and the other with vein-artery-vein triads within the inner layer of the tunica adventitia (inner peri-arterial plexus). These guttural pouch microvascular plexuses, engulfing the internal carotid arteries, are typical of [[countercurrent exchange|countercurrent]] heat exchangers recognised in other animal species, supporting the data that guttural pouches participate in selective brain cooling, even at lower air flow rates.  

==Species Differences==

This completes the triad of internal carotid artery cooling that protects the horse's brain from [[hyperthermia]]. The triad including the function of guttural pouches achieving arterial cooling, via both by utilizing inspiratory air cooling as well as microvascular [[countercurrent exchange|countercurrent]] heat exchangers surrounding internal carotid arteries, and finally afterwards where the internal carotid arteries project upwards passing through the intracranial cavernous venous sinuses accepting cooled venous blood from the nasal sinuses.<ref>{{Cite journal |author=FF McConaghy |author2=JR Hales |author3=RJ Rose |author4=DR Hodgson |date=Dec 1995 |title=Selective brain cooling in the horse during exercise and environmental heat stress |journal=Journal of Applied Physiology |volume=79 |issue=6 |pages=1849-1854 |doi=10.1152/jappl.1995.79.6.1849 |url=https://journals.physiology.org/doi/abs/10.1152/jappl.1995.79.6.1849}}</ref>

==Links==

==Links==

'''Click here for information on [[Ear - Anatomy & Physiology#Equine Guttural Pouch|clinical conditions of the guttural pouch]]'''

[[:Category:Guttural Pouch - Pathology|pathology of guttural pouches]]

==References==

==References==

[[Category:Respiratory System - Anatomy & Physiology]]

[[Category:Respiratory System - Anatomy & Physiology]][[Category:Horse - Respiratory System]]

[[Category:To Do - AimeeHicks]]

[[Category:A&P Done]]