Topical Anatomy and Nomenclature of the Foot
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The first step toward the understanding of the foot's anatomy is an appreciation of its external surfaces. This is necessary for orientation purposes, as the position of the internal components will most often be defined relative to surface structures. Many of the English terms (coronet, frog, bars, bulbs, etc.), used to identify surface structures originated with English, French and German horsemen and have been passed down through the years. Other terminology has a more recent scientific origin. The modern horseman must attend to both. In this presentation the common name will primarily be used, but the proper or scientific name will be included in parentheses following the first usage of the common name.
The external parts of the horse's foot are all by definition components of the hoof. Technically, the hoof is defined as the cornified epidermal covering of the distal end of the horse's digit.1 It is also frequently referred to as the hoof capsule which should give the picture that the hoof fits over the foot like a glove forming an outer protective covering. Older references refer to the hoof capsule as the coffin, again implying that it covers and protects the rest of the foot. The hoof is divided into six regions that consist of the coronet (corona ungulae), periople (limbus ungulae), wall (paries ungulae), sole (solae ungulae), frog (cuneus ungulae), and bulb (torus ungulae). 10 The term ungula (or ungulae if pleural) is the technical name for the hoof itself.
The foot is separated from the rest of the digit at the coronet. Most correctly the coronet, instead of being identifiable part, is really the single line traceable around the foot wherever the soft, haired skin gives way to harder structures of the hoof.11 The term itself is derived from the word that means an ornamental wreath, circlet, or band for the head and it has been suggested that its usage arose from the way that the hair of the lower leg forms a wreath around the upper edge of the hoof wall. The external surface at the front of the foot is its dorsal surface and that surface facing the ground is its solar surface. On the front leg the caudal aspect of the foot is its palmar surface, whereas on the back leg it is referred to as the plantar surface. Medial and lateral are directional terms that refer to the portions of the foot either nearest the foot on the opposite side (medial) or farther away (lateral) from the opposite foot. Likewise, the terms proximal and distal describe those structures nearest the leg (proximal) or nearer to the solar surface (distal). Last, the terms axial (toward the middle) and abaxial (toward the outside) describe structures within a single foot.
Just distal to
the coronet on the external surface of the foot is the periople, which
is the outer most layer of the hoof wall.12 It is a thin layer that
is fairly narrow on the dorsal surface and widens as it nears the palmar/plantar
surface where it spreads and becomes continuous with the outer covering
of the bulbs of the heels. The word periople apparently arose from a
combination of French word peri, meaning around, and houple, an older
German word for hoof. The technical phrase limbus ungulae is Latin designated
the fringe (limbus) of the hoof (ungula). The periople is most noticeable
near the coronet especially when the foot has been exposed to water.
Below the periople the hoof wall continues to the solar surface. For
descriptive purposes, the external wall is divided into dorsal, medial,
and lateral regions. The relatively short section of wall on the palmar/plantar
surface, has been
On all surfaces of the normal wall small, thin, generally parallel lines can be seen reaching from the coronet to the solar surface. These lines are the tubules of the wall that lie just under its outer surface. In addition to these lines, the external wall surface may have a variable number of ridges reaching from heel to heel, running perpendicular to the tubules. These ridges represent variations in the nutritional status, environmental changes, or pathologic conditions to which the foot has been exposed and can be considered to be similar to growth rings in trees.14 This second type of lines may not be present in all horses.
The bearing, or solar, surface of the wall is generally flat except as its contour is shaped by ground contact or trimming, and is grossly divided into regions that include the toe, quarters, heels, and bars. The toe, quarter, and heel regions are not discrete but blend into each other, allowing descriptions of gray areas such as the lateral or medial toe, or the dorsal part of the medial quarter, and so on.
The heels of the hoof form the most caudal part of the bearing surface of the wall; it is here that the wall makes an acute change in direction turning back toward the toe. Because of this change in direction this region of the bearing wall is also called the "angles" of the wall.2 Those portions of the wall that travel back toward the toe are the bars of the foot.
The sole of the hoof is the hard epidermal structure that lies between and is joined to the bearing surface of the wall and the bars. The term sole is derived form the Latin word solea or sandal. The sole is regionally described as having a body and medial and lateral cruses, or angles. The angles of the sole are located between the bars and bearing wall at the heels. The body of the sole fills the space between the bearing portions of the wall on the dorsal half of the solar surface.
Two distinct structures can been seen where the bearing surface of the wall joins to the sole. The outer of these is a region of the wall that is nonpigmented and thus appears white: This is the zona alba, or white zone, of the wall.15 The zona alba is most easily seen on horses with dark external hooves. Just inside, or axial to, the zona alba is a yellowish, striated line. These striations, or lines, are generally perpendicular to the wall's external edge. This striated area is formed as the most internal part of the wall (the laminar layer) grows down and off the foot and are referred to as the terminal laminae. Thus, everything including the striated part outward is hoof wall.
The term "white
line" has been used to describe a landmark for nail placement during
shoeing. The most correct "technical" definition for the white
line is disputed. Schummer defines the white line as the zona alba.10
Others state the yellow striated laminar region is the white line,1,16
and still others use a combination of the terminal lamina and zona alba
as the definition of white line.17-19 At this time, the NAV,15 uses
the zona alba as the most correct definition.
How the horse's frog acquired its name is some what obscure. It seems doubtful that the term was derived from a resemblance to the back of the amphibian. Older English texts refer to the frog as the frush, which is confusing as it indicates that the word may be of German origin where the word for frog is frosch. Alternatively, earlier references indicate that the word for frog is derived from the French word forchetta and/or forchette which are best translated as a "little fork" which, in turn, is used to refer to a fork with only tines. This suggests that the two limbs, or crus, of the frog were thought to resemble the tines of a fork. Technically, the Latin cuneus ungulae describes the concept that the frog is a wedge (cuneus) in the hoof (ungulae).
Between the heels and dorsal to the frog on the palmar/plantar surface are the bulbs of the foot. The bulbs of the hoof are hairless and are separated from the skin on the leg by the continuation of the coronet. Proximally, as the bulbs near the quarters of the wall, the bulbs are continuous with the periople. On its most palmar or plantar surface the bulbs are separated by a groove that becomes more pronounced as it nears the frog. The name itself is derived from the comparative appearance of the smooth, bilateral swellings or protuberances that resemble a plant bulb such as the onion.