The skin is thinner than the ox. Sweat glands are well developed. The dorsal border of wither, neck and forehead presents long hairs. This is the mane of the horse. The tail bears long hairs, typical to the species.
The hoof is a horny box, which encloses a number of sensitive cartilaginous and bony structures, etc., It consists of three parts wall, sole and frog.
The wall is that part of the hoof, which is visible when the foot is on the ground. It covers the front and sides and is reflected inwards at its posterior aspect to form the bars. The bars appear on the ground surface as convergent bands, which subside in front and fused to the sole. They are united with each other by the frog. The anterior part of the wall is termed the toe, the sides as the quarters and the posterior part, the heel.
It presents for description two surfaces and two borders. The external surface is convex from side to side and slopes from edge to edge. This slope is variable. It is more in the hind limb than in the forelimb. It is steeper at the sides than in front and more on the inside than outside. This surface presents striae from edge to edge indicating the direction of horn tubules of which it is composed. The internal surface is concave and bears about 600 horny laminae extending between the borders. Each bears a number of secondary laminae and these dovetail with those of the sensory laminae. The upper or coronary border is covered by layer of soft light coloured band, periople, which appears as a ring like prominence at the junction of skin and hoof. It is narrower in front but widens out laterally and completely caps the heels and blends below with the frog. The internal aspect of this border shows two grooves, an upper one for the perioplic band and a lower one for the coronary band, designated respectively as the perioplic and coronary grooves. The perioplic groove is narrow at the toe but becomes very wide at the heels. Conversely the coronary groove is wide in front and becomes narrow behind and blends with the perioplic groove. The grooves are perforated by small openings for the lodgment of the papillae of the sensory structures of the periople and coronary band. The basal or ground border is convex. It is thicker in front and thinner at the sides. Its inner face is united to the periphery of the sole by horn of lighter colour, which appears on the ground surface of the hoof as the white line.
The sole constitutes the greater part of the ground surface of the hoof. It is crescentric in outline and presents two surfaces and two borders. The internal surface is convex and presents numerous fine openings as in the coronary groove. The external or ground surface is concave. The anterior border is convex and is joined to the wall. The posterior border is deeply notched and the bars and the apex of the frog are situated in these angles. The part between the bars and wall is the angle of the sole.
The frog is a wedge shaped mass, which occupies the angle between the bars. It presents four surfaces, a base and an apex. The internal surface presents a central ridge the frog stay, which is high behind and subsides in front. On either side of this is a deep depression. This surface presents fine striae or openings for the corium. The ground surface presents a central sulcus bordered by two ridges the crura. The internal and external surfaces are united at the upper part with the bars and sole but are free below and form the collateral sulci bordered outwardly by the bars. The base is depressed centrally and prominent at the sides where it unites with the angles of the wall. The apex occupies the central angle of the concave border of the sole and is blunt.
Structure
The hoof is made up of epithelial cells, which are more or less keratinized except at the deepest part, stratum germinativum. Here, the cells are not keratinized but by their division maintain growth of the hoof. The cells are in part arranged to form horn tubes united by intertubular horn. The wall consists of three layers: (1) an external layer of periople (above) and stratum tectorium. The periople is made of soft nonpigmented tubular horn, which becomes white when the foot is soaked in water. It is continuous with the epidermis of the skin above. The stratum tectorium is a thin layer of horny scales which gives the outer face of the wall below the periople a smooth glossy appearance. (2) The middle layer which forms the bulk of the wall consists of horn tubes which run between the borders and (3) The deep layer of horny laminae composed of non-tubular horn.
The sole is made up of tubular and non-tubular horn
The frog is composed of softer horn, which is not fully keratinized.
The hoof is a vascular and receives its nutrition from the corium. It has no nerves
Corium
The corium is the specially and highly modified part of the dermis of the skin and it furnishes nutrition to the hoof. It is composed of the following parts:
The perioplic corium is situated in the perioplic groove. It is continuous with the dermis above and is marked off from the coronary corium below and blends with it laterally. It completely blends with the corium of the frog and bears fine papillae which are received into depreciations in the horny part and it supplies nutrition to the periople.
The coronary corium occupies the coronary groove and furnishes nutrition to the bulk of the wall -middle layer. It has numerous fine papillae. It is wide in front but becomes very narrow posteriorly. It is attached by its deep face to the extensor tendons and cartilages of third phalanx.
The laminar corium bears primary and secondary laminae, which are interlaced with the corresponding horny laminae. It is attached to the laminar face of the os pedis by modified periosteum and supplies nutrition to the horny laminae.
The corium of sole nourishes the horny sole. It bears papillae and is continuous with the corium of bars and frog. The deep face is attached to the solar face of the os pedis.
The corium of the frog is moulded on the deep face of the frog. Its deep face is blended with the plantar cushion and it nourishes the horny frog.
The plantar cushion is a wedge shaped mass, which lies over the frog. It presents four faces, a base and apex. The deep face is connected to the fibrous sheath of the deep flexor tendon. The superficial face is covered by the corium of the frog. The lateral faces are related to the lateral cartilages of the third phalanx. The base is posterior, subcutaneous and is divided by a central depression into two rounded prominences bulbs of the cushion. The apex is adherent to the terminal part of deep flexor tendon. The blood supply is very poor. It consists of a meshwork of white and elastic fibres containing fat.
The lateral cartilages are attached to the angles of the third phalanx. The external face is convex and the internal face is concave. The upper border extends higher up and hence easily palpable. The lower border is thicker and is partly attached to the angle. The posterior end curves towards its fellow.
Tendons: The tendon of extensor pedis with the reinforcing branches of the suspensory ligament runs down over the joint and is inserted to the pyramidal process of the os pedis. The deep flexor tendon is behind.
Bones: The lower end of the second phalanx, the distal sesamoid or navicular bone and third phalanx.
The ergot is a small piece of horn situated in the tuft of hair at the flexor face of the fetlock vestige of the second and fourth digits.
The chestnut is a mass of horn on the medial face of the forearm about a handbreadth above the carpus and on the distal part of the medial face of the tarsus, vestige of the first digit.
Dog
The skin is loose with variable thickness. The palmar surfaces of the carpus, metacarpus and digits are hairless and bear pads The pad is composed of fibro elastic tissue, fat and a thick keratinised epidermal covering. The size of the hairs is highly variable from breed to breed. The claws are five in a limb, one on each digit. The horn of each claw covers the concerned third phalanx and presents a body and sole. They curved downward.
Pig
The skin of the pig is thick due to accumulation of subcutaneous fat. The hairs on the skin are sparse. The hoof resembles that of the ox. The hooves of the accessory digits are well developed.
Rabbit
The skin is very soft due to the presence of moderate amount of subcutis fat. The hairs are long and cover most of the skin except muzzle. The tail is covered by thick short hairs. Palmar surface of foot bears pads. The claws are elongated and curved down wards. The tips are blunt .The claws are five in number one for each digit.
Fowl
The skin appendages of the epidermis in fowl are comb, wattles, earlobes, feathers, scales and claws. Beaks are the horny covering of the upper jaws. The distal end of the metatarsus presents the metatarsal spur. It is capped by the horny covering.
