Changes

===Sweat Glands===

===Sweat Glands===

Sweat is considered to be the product of [[Necrosis - Pathology#Apoptosis|apoptosis]] releasing cell contents into the gland lumen, together with secretion where exocytosis, microapocrine blebbing, transcellular and perhaps paracellular processes involved in varying degrees.  Sweat is composed of secretions and cellular debris and includes inorganic ions, water, immunoglobulins, amino acids and waste products including urea and lactic acid.  Equine sweat is rich in proteins including glycoproteins, albumin and gammaglobulins, which cause the sweat to froth and the horse is said to "lather up".

Sweat is considered to be the product of [[Necrosis - Pathology#Apoptosis|apoptosis]] releasing cell contents into the gland lumen, together with secretion where exocytosis, microapocrine blebbing, transcellular and perhaps paracellular processes involved in varying degrees.  Sweat is composed of secretions and cellular debris and includes inorganic ions, water, immunoglobulins, amino acids and waste products including urea and lactic acid.  Equine sweat is rich in proteins including glycoproteins, albumin and gammaglobulins, which cause the sweat to froth and the horse is said to "lather up".

===Sebacious Glands===

===Sebaceous Glands===

One or more sebaceous glands surround each '''hair follicle''' at a point about 1/3 of its length beneath the skin's surface.  Each gland is composed of numerous ''acini'' attached to a common '''excretory duct''' through which they discharge their secretions ('''sebum''') onto the hair shaft and epidermis.  In the skin and eyelids such glands release the cell contents together with secretory products with holocrine secretion.  This secretion involves [[Necrosis - Pathology#Apoptosis|apoptosis]] in which the secretory products and cell debris are released into the gland lumen.  The secretory products vary from species to species and include various fatty acids, diglycerides, cholesterol, diesters, wax esters and giant-ring lactones. The function of sebum is that of an emolient of the hair and skin, preventing excessive dryness.  It may combine with '''sweat''' to form an emulsion within the stratum corneum and a form of cement around the hair pores preventing bacterial invasion of the epidermis.  Sebum is also released from the glands during piloerection, as the action of the [[Hair - Anatomy & Physiology#Hair Types|arrector pili]] muscle squeezes the gland as it raises the hair.

One or more sebaceous glands surround each '''hair follicle''' at a point about 1/3 of its length beneath the skin's surface.  Each gland is composed of numerous ''acini'' attached to a common '''excretory duct''' through which they discharge their secretions ('''sebum''') onto the hair shaft and epidermis.  In the skin and eyelids such glands release the cell contents together with secretory products with holocrine secretion.  This secretion involves [[Necrosis - Pathology#Apoptosis|apoptosis]] in which the secretory products and cell debris are released into the gland lumen.  The secretory products vary from species to species and include various fatty acids, diglycerides, cholesterol, diesters, wax esters and giant-ring lactones. The function of sebum is that of an emolient of the hair and skin, preventing excessive dryness.  It may combine with '''sweat''' to form an emulsion within the stratum corneum and a form of cement around the hair pores preventing bacterial invasion of the epidermis.  Sebum is also released from the glands during piloerection, as the action of the [[Hair - Anatomy & Physiology#Hair Types|arrector pili]] muscle squeezes the gland as it raises the hair.

===Specialised Glands===

===Specialised Glands===

Specialised glands in the horse exist in the frog.

Specialised glands in the horse exist in the [[Hoof - Horse Anatomy#Frog|frog]].

==Mammary Glands==

==Mammary Glands==

==Hoof==

==Hoof==

==Introduction==

===Introduction===

The hoof is defined from a physiologic perspective as the modified skin covering the tip of the digit and all enclosed structures. The hoof provides protection to the distal limb and is formed by keratinisation of the epithelial layer and modification of the underlying dermis. The keratin in the [[Skin - Anatomy & Physiology#Epidermis|epidermis]], when thickened and cornified, is referred to as [[Horn - Anatomy & Physiology|horn]].  Horn makes up the outer surface if the hoof and is particularly resistant to mechanical and chemical damage.

The hoof is defined from a physiologic perspective as the modified skin covering the tip of the digit and all enclosed structures. The hoof provides protection to the distal limb and is formed by keratinisation of the epithelial layer and modification of the underlying dermis. The keratin in the [[Skin - Anatomy & Physiology#Epidermis|epidermis]], when thickened and cornified, is referred to as [[Horn - Anatomy & Physiology|horn]].  Horn makes up the outer surface if the hoof and is particularly resistant to mechanical and chemical damage.

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The origin of the hoof is as a form of protection to the distal phalanx and stems from local modifications of the epidermis, [[Skin - Anatomy & Physiology#Dermis|dermis]] and subcutaneous layers.  The hoof also acts as a shock-absorber in that the hoof acts to reduce the concussion of foot impact on the structures of the distal limb. It has also been suggested that the elastic nature of components of the hoof may aid the return of blood to the heart.

The origin of the hoof is as a form of protection to the distal phalanx and stems from local modifications of the epidermis, [[Skin - Anatomy & Physiology#Dermis|dermis]] and subcutaneous layers.  The hoof also acts as a shock-absorber in that the hoof acts to reduce the concussion of foot impact on the structures of the distal limb. It has also been suggested that the elastic nature of components of the hoof may aid the return of blood to the heart.

==General Structure and Layering of the Hoof==

===General Structure and Layering of the Hoof===

===Wall===

====Wall====

The wall of the hoof is the element of the hoof that is visible. The wall of the hoof is widest at the distal aspect of the hoof, i.e. at the toe. The wall of the hoof decreases in width laterally and medially (around the quarters of the hoof). There is an '''outer layer''' of thin but dense horn that is shiny and acts to reduce evaporation from the horn and therefore prevent dehydration of the other layers of the horn. There is an '''intermediate layer''' which represents the main structure of the wall and is composed of amorphous horn reinforced with many tubular shaped horn rods. Finally, there is an '''inner lamellar layer''' where there are interdigitating lamellae of horn and dermal lamellae which ensure the hoof itself is firmly anchored to the distal phalanx. The bond between the horn and the underlying dermis is a living bond that gradually allows the wall of the hoof to slide in a distal direction towards the ground where the distal border is worn away via contact with the ground.

The wall of the hoof is the element of the hoof that is visible. The wall of the hoof is widest at the distal aspect of the hoof, i.e. at the toe. The wall of the hoof decreases in width laterally and medially (around the quarters of the hoof). There is an '''outer layer''' of thin but dense horn that is shiny and acts to reduce evaporation from the horn and therefore prevent dehydration of the other layers of the horn. There is an '''intermediate layer''' which represents the main structure of the wall and is composed of amorphous horn reinforced with many tubular shaped horn rods. Finally, there is an '''inner lamellar layer''' where there are interdigitating lamellae of horn and dermal lamellae which ensure the hoof itself is firmly anchored to the distal phalanx. The bond between the horn and the underlying dermis is a living bond that gradually allows the wall of the hoof to slide in a distal direction towards the ground where the distal border is worn away via contact with the ground.

===Periople===

====Periople====

The periople is a small band of soft tissue found over the proximal surface of the wall of the hoof. The periople represents the junction between the wall and the skin of the limb. The periople is responsible for the production of the outer layer of the wall (above). The periople segment extends around the proximal edge of the wall and in horses covers the bulb of the heel and part of the frog.  

The periople is a small band of soft tissue found over the proximal surface of the wall of the hoof. The periople represents the junction between the wall and the skin of the limb. The periople is responsible for the production of the outer layer of the wall (above). The periople segment extends around the proximal edge of the wall and in horses covers the bulb of the heel and part of the frog.  

===Coronary Dermis Segment===

====Coronary Dermis Segment====

The wall of the hoof is created at the coronary dermis and grows in a distal direction from the coronary dermis. The coronary dermis is studded with many papillae which are directed towards the ground in the direction of growth. The epidermis covering these papillae produce horn tubules which are embedded into amorphous inter-tubular horn. This inter-tubular horn is created by the spaces between the papillae within the coronary dermis. The combination of both of these horn types ensures the horn has sufficient strength.

The wall of the hoof is created at the coronary dermis and grows in a distal direction from the coronary dermis. The coronary dermis is studded with many papillae which are directed towards the ground in the direction of growth. The epidermis covering these papillae produce horn tubules which are embedded into amorphous inter-tubular horn. This inter-tubular horn is created by the spaces between the papillae within the coronary dermis. The combination of both of these horn types ensures the horn has sufficient strength.

   

   

The pigmentation of the hoof is derived from melanocytes found in the coronary epidermis. Any pigmentation in the hoof will be most pronounced in the outer part of the hoof wall as the deeper layers of the hoof usually contain fewer melanocytes. It is this unpigmented element of the hoof that forms the '''white line''' in the sole of hoofs and is particularly important in horses as a landmark for shoeing.

The pigmentation of the hoof is derived from melanocytes found in the coronary epidermis. Any pigmentation in the hoof will be most pronounced in the outer part of the hoof wall as the deeper layers of the hoof usually contain fewer melanocytes. It is this unpigmented element of the hoof that forms the '''white line''' in the sole of hoofs and is particularly important in horses as a landmark for shoeing.

===Sole Segment===

====Sole Segment====

The sole represents the part of the foot in contact with the ground. The keratin found in the sole is formed from the epidermis on the underside of the distal phalanx and can grow to a thickness of around 10mm. The keratin found on the sole is much more easily worn down or abraded than that of the wall of the hoof. The equine sole has a central '''frog'''.

The sole represents the part of the foot in contact with the ground. The keratin found in the sole is formed from the epidermis on the underside of the distal phalanx and can grow to a thickness of around 10mm. The keratin found on the sole is much more easily worn down or abraded than that of the wall of the hoof. The equine sole has a central '''frog'''.

===Frog-Bulb Segment===

====Frog-Bulb Segment====

The equine bulb is formed from keratin in the same manner as the sole and again this keratin is softer and more liable that the keratin found in the hoof wall. The frog ensures that the wall of the hoof is forced outwards when weight is put on the limb thus ensuring that the 'hoof mechanism' functions correctly and ensuring circulatory flow around the hoof and back towards the heart.

The equine bulb is formed from keratin in the same manner as the sole and again this keratin is softer and more liable that the keratin found in the hoof wall. The frog ensures that the wall of the hoof is forced outwards when weight is put on the limb thus ensuring that the 'hoof mechanism' functions correctly and ensuring circulatory flow around the hoof and back towards the heart.