FIG. 14.—THE ARTERIES OF THE FOOT. The digital; 2, the perpendicular—(a) its ascending branch, (b) its descending branch; 3, circumflex artery of coronary cushion; 4, the preplantar (ungual) artery—this is seen issuing from the preplantar foramen, and distributing numerous ascending (c) and descending (d) branches (the latter concur in forming the circumflex artery of the toe); 5, the circumflex artery of the toe; 6, at the point marked (*) the terminal branch of the digital—namely, the plantar ungual—is hidden behind the lateral cartilage; 7, the lateral cartilage.
5.Under the Lateral Cartilagetwo transverse branches, an anterior and a posterior, to form theCoronary Circle. The numerous ramifications of these branches anastomose both anteriorly and posteriorly with their corresponding branches of the artery of the opposite side. This circle closely embraces the os coronæ. Among the larger branches given off from its anterior portion are two descending, one on each side of the extensor pedis, to assist in the formation of theCircumflex Artery of the Coronary Cushion. The formation of this last-named artery is completed posteriorly by the before-mentioned branch from the artery of the plantar cushion.
THE PREPLANTAR (UNGUAL[A]) ARTERY.—This, the smaller of the two terminal branches of the digital, is situated inside the basilar process of the os pedis. It turns round this to gain the fissure between the basilar and retrossal processes, and becomes lodged in the preplantar fissure. Here it terminates in several divisions which bury themselves in the os pedis. Before leaving the inner aspect of the pedal wing it supplies a deep branch to the heel and the villous tissue. Gaining the outer aspect of the wing, it distributes a further backward branch, which passes behind the circumflex artery of the pedal bone, and, during its passage in the preplantar fissure, gives off ascending and descending branches, which ramify in the laminal tissue.
THE PLANTAR (UNGUAL[A]) ARTERY.—This, the larger of the two terminals of the digital, may be looked upon as a continuation of the main vessel. Running along the plantar groove, it gains the plantar foramen. Here it enters the interior of the bone (the semilunar sinus) and anastomoses with the corresponding artery of the opposite side. The circle of vessels so formed is called thePlantar Archor theSemilunar Anastomosis.
[Footnote A: The epithet 'ungual' is added by Chauveau to distinguish these arteries from the properly so-called plantar arteries—the terminal divisions of the posterior tibial artery.]
From the semilunar anastomosis radiate two main groups of arterial branches, an ascending group and a descending one. Theascendingbranches penetrate the substance of the os pedis, and emerge by the numerous foraminæ on its laminal surface. Thedescendingbranches, larger in size, also penetrate the substance of the pedal bone, and emerge in turn from the foraminæ cribbling its outer surface—in this case the set of larger foraminæ opening on its inferior edge. Having gained exit from the bone, their frequent anastomosis, right and left, with their fellows forms a large vessel following the contour of the inferior edge of the os pedis. This constitutes theCircumflex Artery of the Toe.
These commence at the foot with a series of plexuses, which may be described as forming (1) AN INTERNAL OR INTRA-OSSEOUS VENOUS SYSTEM, and (2) AN EXTERNAL OR EXTRA-OSSEOUS VENOUS SYSTEM.
1. THE INTRA-OSSEOUS VENOUS SYSTEM.—This is a venous system within the structure of, and occupying the semilunar sinus of the os pedis. It follows in every respect the arrangement of the arteries as before described in the same region. Efferent vessels emerge from the plantar foraminæ, follow the plantar fissures, and ascend within the basilar processes of the os pedis. Here they lie under shelter of the lateral cartilages, and assist in the formation of the deep layer of the coronary plexus of the extra-osseous system.
2. THE EXTRA-OSSEOUS VENOUS SYSTEM.—This may be regarded as a close-meshed network enveloping the whole of the foot. Although a continuous system, it is best described by recognising in it three distinct parts:
(a) The Solar Plexus.(b) The Podophyllous Plexus.(c) The Coronary Plexus.
(a) The Solar Plexus.—The veins of this plexus discharge themselves in two directions: (1)By a central canalor canals running along the bottom of the lateral lacunæ of the plantar cushion to gain the deep layer of the coronary plexus. (2)By the Circumflex or Peripheral Vein of the Toe, a canal formed by ramifications from the solar and the podophyllous plexuses, and following the direction of the artery of the same name. The circumflex vein terminates by forwarding branches to concur in the formation of the superficial coronary plexus.
(b) The Podophyllous or Laminal Plexus.—The podophyllous veins anastomose below with the circumflex vein of the solar plexus, and above with the veins of the coronary plexus.
(c) The Coronary Plexus.—This proceeds from the podophyllous, the intra-osseous, and the solar networks, and consists of acentralandtwo lateral parts.
Thecentralportion lies between the lateral cartilages and immediately under the coronary cushion. Thelateral portionsare ramifications on both surfaces of the lateral cartilages. The ramifications on the lateral cartilages may be again distinguished assuperficialanddeep. The superficial layer is distributed over the external face of the cartilage, forming thereon a dense network, and finally converges towards the superior limit of the plexus to form ten or twelve principal branches, which again unite to form two large vessels. These vessels, by their final fusion at the lower end of the first phalanx, constitute the digital vein. The deep layer is formed, as before described, by ascending branches from the posterior parts of the podophyllous and solar plexuses, and by branches from the intra-osseous system of the pedal bone. The veins of this deep layer finally drain into the two vessels proceeding from the superficial layer, which go to the formation of the digital vein.
THE DIGITAL VEINS—These arise from the network formed on the surfaces of the lateral cartilages, and ascend in front of the digital arteries to unite above the fetlock, where they form an arch between the deep flexor and the suspensory ligament. From this arch (named theSesamoidean)proceed the Metacarpal Veins.
THE METACARPAL VEINS.—Three in number, they are distinguished as anInternaland anExternal Metacarpal, and aDeeporInterosseous Metacarpal. As we shall be concerned with these in the higher operation of neurectomy, we may give them brief mention.
THE INTERNAL METACARPAL VEIN, the largest of the three, has relations with the internal metacarpal artery and the internal plantar nerve. These relations were shortly discussed under the section devoted to the arteries, to which the reader may refer.
THE EXTERNAL METACARPAL VEIN.—This ascends on the external side of the flexor tendons in company with the external plantar nerve.
The Interosseous Vein.—This is an irregular vessel running up between the suspensory ligament and the posterior face of the large metacarpal bone.
THE PLANTAR NERVES.—These are two in number, and are distinguished as Internal and External.
THE INTERNAL PLANTAR NERVE lies behind and in close contact with the great metacarpal artery during that vessel's course down the region of the cannon. A point of interest is that it gives off at about the middle of the cannon a branch which bends obliquely downwards and behind the flexor tendons to join its fellow of the opposite side—namely, the external plantar. This it joins an inch or more above the bottom of the splint bone. Measured in a straight line, this is about 2-1/2 inches below its point of origin. Near the fetlock, at the level of the sesamoids, the internal plantar nerve ends in several digital branches.
THE EXTERNAL PLANTAR NERVE.—This holds a position to the outside of the metacarpal region, analogous to that of the internal plantar nerve on the inside of the limb, running down on the external edge of the flexor tendons. Unlike the internal nerve, it is accompanied by a single vessel only, the external metacarpal vein, behind which it lies. At the level of the sesamoid bones it divides, as does theinternalnerve, into three main branches—the digital nerves.
THE VEINS AND NERVES OF THE FOOT
FIG. 15.—THE VEINS AND NERVES OF THE FOOT. 1, The digital vein; 2, its main tributaries, draining the podophyllous plexus, and concurring to form the digital; 3. the digital artery (the main trunk only of this is shown, in order to show its relationship with the vein and nerve); 4, the plantar nerve, with its three branches—(a) the anterior digital, (b) the middle digital, (c) the posterior digital; 5, the podophyllous plexus; 6, superficial portion of the coronary plexus; 7, the peripheral or circumflex vein of the toe.
THE DIGITAL NERVES.—These are distinguished as Anterior, Middle, and Posterior.
The Anterior Branchdescends in front of the vein, distributing cutaneous branches to the front of the digit, and terminating in the coronary cushion.
The Middle Branchdescends between the artery and the vein, and freely anastomoses with the two other branches. It terminates in the coronary cushion and the sensitive laminæ.
The Posterior Branch.—This is the largest of the three, and may be regarded as the direct continuation of the plantar. At the fetlock it is placed immediately above the digital artery, but afterwards takes up a position directly behind that vessel. Together with the digital artery it descends to near the basilar process of the os pedis. Here it passes with the plantar artery into the interior of the os pedis, and continues its main branch, with the preplantar artery, in the fissure of the same name, to finally furnish supply to the os pedis and the sensitive laminæ. It is this nerve which is divided in the low operation of neurectomy.
Beyond the fact of this branch descending, in the region of the pastern, 1 inch behind the digital artery, a further point of interest presents itself to the surgeon, and one to which attention must be paid. This is the presence in close proximity to the nerve of the Ligament of the Pad (Percival), or the Ligament of the Ergot (McFadyean). This is a subcutaneous glistening cord originating in the ergot of the fetlock, passing in an oblique direction downwards and forwards, and crossing over on its way both the digital artery and the posterior branch of the digital nerve.
In the foregoing description of the anatomy, we have taken the fore-limb as our guide. In the hind-limb, where they reach the foot, the counterparts of the tendons, arteries, veins, and nerves differ in no great essential from their fellows in the fore. They will therefore need no special mention.
This consists of two lateral pieces, the LATERAL CARTILAGES orFibro-cartilagesof the pedal bone, united behind and below by thePlantar Cushion.
1. THE LATERAL CARTILAGES.—Each is a flattened plate of cartilage, possessing two faces and four borders separated by four angles.
The external face is convex, covered by a plexus of veins, and slightly overhangs the pedal bone. The internal face is concave, and covers in front the pedal articulation and the synovial sac, already mentioned as protruding between the antero- and postero-lateral ligaments of that joint. We have already remarked that this is a point of interest to be remembered in connection with the operation for quittor. Below and behind, the internal face of the cartilage is united to the plantar cushion.
EXTERNAL FACE OF THE OUTER LATERAL CARTILAGE
FIG. 16.—EXTERNAL FACE OF THE OUTER LATERAL CARTILAGE. 1, External face of cartilage—(a) its upper border, (b) its posterior border, (c) its anterior border, (d) its inferior border; 2, the os pedis; 3, wing of os pedis.
The upper border, sometimes convex, sometimes straight, is thin and bevelled, and may easily be felt in the living animal. It is this border that the digital vessels cross to gain the foot, and the border is often broke by a deep notch to accommodate them. The inferior border is attached in front to the basilar and retrossal processes, behind which it blends with the plantar cushion. The posterior border is oblique from before to behind, and above to below, and joins the preceding two. The anterior border is oblique in the same direction, and is intimately attached to the antero-lateral ligament of the pedal articulation. The cartilages of the fore-feet are thicker and more extensive than those of the hind.
2. THE PLANTAR CUSHION on FIBRO-FATTY FROG.—Composed of a fibrous meshwork, in the interstices of which are lodged fine elastic and connective fibres and fat cells, this wedge-shaped body occupies the space between the two lateral cartilages, the extremity of the perforans tendon, and the horny frog. It offers for consideration an antero-superior and an infero-posterior face, a base, an apex, and two borders.
The antero-superior face is in contact with the terminal expansion of the perforans tendon. The infero-posterior face is covered by the keratogenous membrane, and follows closely the shape of the horny frog, on whose inner surface it is moulded. It presents, therefore, at its centre a single conical prolongation, thePyramidal Body, which is continued behind, as is the horny frog, in the shape of two lateral ridges divided by a median cleft. Thebaseof the cushion lies behind, and consists of two lateral masses,the Bulbs of the Plantar Cushion. In front these are continuous with the ridges of the pyramidal body, while behind they become confounded with the lateral cartilages and the coronary cushion. Theapexis fixed into the plantar surface of the os pedis, in front of its semilunar ridge. Theborders, right and left, are wider behind than before, and are in relation with the inner faces of the lateral cartilages.
THE KERATOGENOUS, OR HORN-PRODUCING MEMBRANE, is in reality an extension of the dermis of the digit. It covers the extremity of the digit as a sock covers the foot, spreading over the insertion of the extensor pedis, the lower half of the external face of the lateral cartilages, the bulbs of the plantar cushion, the pyramidal body, the anterior portion of the plantar surface of the os pedis, and over the anterior face of the same bone. In turn, as the human foot with its sock is covered by the boot, this is encased by the hoof, the formation of which we shall study later.
To expose the membrane for study the hoof must be removed. This may be done in two ways. By roasting in a fire, and afterwards dragging off the horny structures with a pair of pincers, a knife having first been passed round the superior edge of the horny box. Or by maceration in water for several days, when the hoof will become loosened by the process of decomposition, and may be easily removed by the hands. The latter method is less likely to injure the sensitive structures, and will expose them with a fresh appearance for observation.
For purposes of description the keratogenous membrane is divided into three regions:
1. The Coronary Cushion.2. The Velvety Tissue.3. The Podophyllous Tissue, or the Sensitive Laminæ.
1. THE CORONARY CUSHION. In the foot stripped of the hoof the coronary cushion is seen as a rounded structure overhanging the sensitive laminæ after the manner of a cornice. It extends from the inner to the outer bulbs of the plantar cushion, and is bounded above by the perioplic ring, and below by the laminæ.
Whenin situit is accommodated by theCutigeral Groove, a cavity produced by the bevelling out of the superior portion of the inner face of the wall of the hoof. Its superior surface is covered by numerous elongated papillæ, set so closely as to give the appearance of the 'pile' of velvet. This is observed to the best advantage with the foot immersed in water.
The Superior Borderof the cushion is bounded by thePerioplic Ring, the cells of which have as their function the secreting of thePeriople, a layer of thin horn to be noted afterwards as covering the external face of the wall. From the perioplic ring the cushion is separated by a narrow and shallow, though well-marked, groove.
The inferior border is bounded by the sensitive laminæ.
THE KERATOGENOUS MEMBRANE
FIG. 17.—THE KERATOGENOUS MEMBRANE (VIEWED FROM THE SIDE). (THE HOOF REMOVED BY MACERATION.) 1. The sensitive laminæ, or podophyllous tissue; 2, the coronary cushion; 3, the perioplic ring; 4, portion of plantar cushion; 5, groove separating perioplic ring from coronary cushion; 6. the sensitive sole.
The upper portions of the laminæ, those in contact with the cushion, are pale in contrast with the portions immediately below, and thus there is given the appearance of a white zone adjoining the inferior border of the cushion.
Widest at its centre, the cushion narrows towards its extremities, which, arriving at the bulbs of the plantar cushion, bend downwards into the lateral lacunæ of the pyramidal body, where they merge into the velvety tissue of the sole and frog.
The papillæ of the coronary cushion secrete the horn tubules forming the wall, and the papillæ of the perioplic ring secrete the varnish-like veneer of thin horn covering the outside surface of the hoof.
THE KERATOGENOUS MEMBRANE
FIG. 18.—THE KERATOGENOUS MEMBRANE (VIEWED FROM BELOW). (THE HOOF REMOVED BY MACERATION.) 1, The sensitive sole; 2, the sensitive frog[A]—(a) its median lacuna, (6) its lateral lacuna; 3. V-shaped depression accommodating the toe-stay; 4, the sensitive laminæ which interleave with the horny laminæ of the bar.
[Footnote A: The sensitive frog thinly invests the plantar cushion or fibre-fatty frog, the outline of which is here indicated.]
2. THE VELVETY TISSUE.—This is the portion of the keratogenous membrane covering the plantar surface of the os pedis and the plantar cushion. To the irregularities of the latter body—its bulbs, pyramidal body, and its lacunæ—it is closely adapted. Its surface may, therefore, be divided into(a) The Sensitive Frog, and(b) The Sensitive Sole.
(a) The Sensitive Frogis that part of the velvety tissue moulded on the lower surface of the plantar cushion. The shape of the plantar cushion has already been described as identical with that of the horny frog. It only remains to state that, like the coronary cushion, the surface of the sensitive frog is closely studded with papillæ. The cells clothing the papillæ are instrumental in forming the horny frog.
(b) The Sensitive Sole.—As its name indicates, this is the portion of the keratogenous membrane that covers the plantar surface of the os pedis. It also is clothed with papillæ, which again give rise to the formation of that part of the horny box to which they are adapted—namely, the sole.
3. THE PODOPHYLLOUS TISSUE, OR SENSITIVE LAMINÆ.—This portion of the keratogenous membrane is spread over the anterior face and sides of the os pedis, limited above by the coronary cushion, and below by the inferior edge of the bone. It presents the appearance of fine longitudinal streaks, which, when closely examined with a needle, are found to consist of numerous fine leaves. These extend downwards from the lower border of the coronary cushion to the inferior margin of the os pedis. At this point each terminates in several large villous prolongations, which extend into the horny tubes at the circumference of the sole. At the point of the toe this membrane sometimes shows a V-shaped depression, into which fits a inverted V-shaped prominence on the inner surface of the wall at this point.
The sensitive laminæ increase in width from above to below. Their free margin is finely denticulated, while their sides are traversed from top to bottom by several folds (about sixty), which, examined microscopically, are seen to consist of secondary leaves, orlaminellæ.
Examined on the foot, deprived of its horny covering, the sensitive laminæ are, the majority of them, in close contact with each other. In the normal state this is not so. The interstices between the leaves are then occupied by the horny leaves, to be afterwards described as existing on the inner surface of the wall.
Reaching and rounding the heels, the sensitive laminæ extend forward for a short distance, where they interleave with the horny laminæ of the bars.
Much discussion has centred round the point as to whether or no the cells of the sensitive laminæ take any share in the formation of the horn of the wall. This will be alluded to in a future chapter.
Removed from the foot by maceration a well-shaped hoof is cylindro-conical in form, and appears to the ordinary observer to consist of a box or case cast in one single piece of horn. Prolonged maceration, however, will show that the apparently single piece is divisible into three. These are known as (1) THE WALL, (2) THE SOLE, and (3) THE FROG. In addition to these, we have also an appendage or circular continuation of the frog named (4) THE PERIOPLE, or CORONARY FROG BAND. These various divisions we will study separately.
1. THE WALL is that portion of the hoof seen in front and laterally when the horse's foot is on the ground. Posteriorly, instead of being continued round the heels to complete the circle, its extremities become suddenly inflected downwards, forwards, and inwards. These inflections can only be seen with the foot lifted from the floor, and form the so-calledBars. It will be noticed, too, with the foot lifted, that the wall projects beyond the level of the other structures of the plantar surface, taking upon itself the bearing of the greatest part of the animal's weight.
The horn of the wall, viewed immediately from the front, is known as theToe, which again is distinguished asOutside ToeorInside Toe, according as the horn to its inner or outer aspect is indicated. The remainder of the external face of the wall, that running back to the heels, is designated theQuarters.
In the middle region of the toe, the wall following the angle of the bones is greatly oblique. This obliquity decreases as the quarters are reached, until on reaching the heels the wall is nearly upright.
THE WALL OF THE HOOF
FIG. 19.—THE WALL OF THE HOOF. 1, The toe; 2, inner toe; 3, outside toe; 4, the quarter; 5, entigeral groove; 6, horny laminæ.
For observation the wall offers two faces, two borders, and two extremities.
The External Faceis convex from side to side, but straight from the upper to the lower border. Examined closely, it is seen to be made up of closely-arranged parallel fibres running in a straight line from the upper to the lower border, and giving the surface of the foot a finely striated appearance. In addition to these lines, which are really the horn tubules, the external face is marked by a series of rings which run horizontally from heel to heel. These are due to varying influences of food, climate, and slight or severe disease. This will be noted again in a later page. In a young and healthy horse the whole of the external face of the wall is smooth and shining. This appearance is due to a thin layer of horn, secreted independently of the wall proper, termed the periople.
INTERNAL FEATURES OF THE WALL, FROG, AND SOLE
FIG. 20.—INTERNAL FEATURES OF THE WALL, FROG, AND SOLE (MESIAN SECTION OF HOOF). 1, Horny laminæ covering internal face of wall; 2, superior border of wall; 3, junction of wall with horny sole; 4, the cutigeral groove; 5, the horny sole; 6, the horny frog (that portion of it known as the 'frog-stay'); 7, inverted V-shaped ridge on wall and sole (known as the 'toe-stay'); 8, anterior face of wall; 9, inferior border of wall.
The Internal Faceof the wall, that adapted to the sensitive laminæ, is closely covered over its entire surface with white parallel leaves(Keraphyllæ, or horn leaves, to distinguish them from thePodophyllæ, or sensitive leaves). These keraphyllæ dovetail intimately with the sensitive laminæ, covering the os pedis. Running along the superior portion of the inner face is theCutigeral Groove. This cavity has been mentioned before as accommodating the coronary cushion, whose shape and general contour it closely follows, being widest and deepest in front, and gradually decreasing as it proceeds backwards. It is hollowed out at the expense of the wall, and shows on its surface numberless minute openings which receive the papillæ of the coronary cushion.
At the bottom of the internal face, at the point where the toe joins the sole, will be noted the before-mentioned inverted V-shaped prominence. Its position will be clearly understood when we say that it gives the appearance of having been forced there by the pressure of the toe-clip of the shoe. This will be noted again when dealing with the sole.
The Inferior Borderof the wall offers little to note. It is that portion in contact with the ground, and subject to wear. A point of interest is its union with the sole. This will be noticed in a foot which has just been pared as a narrow white or faint yellow line on the inner or concave face of the wall at its lower portion. It marks the point where the horny leaves of the wall terminate and become locked with corresponding leaves of the circumference of the sole.
The Superior Borderfollows closely the line marked by the perioplic ring and the groove separating the latter from the coronary cushion.
The Extremitiesof the wall are formed by the abruptly reflected portions of the wall at the heels. Termed by some the 'Inflexural Nodes,' they are better known to us as the 'Points of the Heels.'
2. THE SOLE.—The sole is a thick plate of horn which, in conjunction with the bars and the frog, forms the floor of the foot. In shape it is irregularly crescentic, its posterior portion, that between the horns of the crescent, being deeply indented in a V-shaped manner to receive the frog. Its upper surface is convex, its lower concave. It may be recognised as possessing two faces and two borders.
The Superior or Internal Faceis adapted to the sole of the os pedis. Its highest point, therefore, is at the point of its V-shaped indentation. From this point it slopes in every direction downwards and outwards until near the circumference. Here it curves up to form a kind of a groove in which is lodged the inferior edge of the os pedis. In the centre of its anterior portion—that is to say, at the toe—will be seen a small inverted V-shaped ridge, which is a direct continuation of the same shaped prominence before mentioned on the internal face of the wall. This Fleming has termed the toe-stay, from a notion that it serves to maintain the position of the os pedis. The whole of the superior face of the sole is covered with numerous fine punctures which receive the papillæ of the sensitive sole.
The Inferior Faceis more or less concave according to circumstances, its deepest part being at the point of the frog. Sloping from this point to its circumference, it becomes suddenly flat just before joining the wall. Its horn in appearance is flaky.
INFERIOR ASPECT OF HOOF
FIG. 21.—INFERIOR ASPECT OF HOOF.aThe inferior face of horny sole;b, inferior border of the wall;c, body or cushion of the frog;d, median lacuna of the frog;e, lateral lacuna of the frog;f, the bar;g, the quarter;h, the point of the frog;ithe heel.
The External Borderor Circumference is intimately dovetailed with the horny laminæ of the wall. At its circumference the sole, if unpared, is ordinarily as thick as the wall. This thickness is maintained for a short distance towards its centre, after which it becomes gradually more thin.
The Internal Borderhas the shape of an elongated V with the apex pointing forwards. It is much thinner than the external border, and, like it, is dovetailed into the horny laminæ of the inflections of the wall—namely, the bars. In front of the termination of the bars it is dovetailed into the sides and point of the frog. Where unworn by contact with the ground, the horn of the sole is shed by a process of exfoliation.
3. THE FROG.—Triangular or pyramidal in shape, the frog bears a close resemblance to the form of the plantar cushion, upon the lower surface of which body it is moulded. It offers for consideration two faces, two sides, a base, and a point or summit.
HOOF WITH THE SENSITIVE STRUCTURES REMOVED
FIG. 22.—HOOF WITH THE SENSITIVE STRUCTURES REMOVED. 1, Superior face of horny frog; 2, the frog-stay; 3, the lateral ridges of the frog's superior surface; 4, the horny laminæ at the inflections of the wall.
The Superior Faceis an exact cast of the lower surface of the plantar cushion. It shows in the centre, therefore, a triangular depression, with the base of the triangle directed backwards. Posteriorly, the depression is continued as two lateral channels divided by a median ridge. The median ridge widens out as it passes backwards, forming the larger part of the posterior portion of the frog. This median ridge fits into the cleft of the plantar cushion. It serves to prevent displacement of the sensitive from the horny frog, and has been rather aptly termed the 'Frog-stay.'
The Inferior Surfaceis an exact reverse of the superior. The triangular depression of the superior surface is represented in the inferior surface by a triangular projection, and the ridge-like frog-stay of the upper surface is represented below by a median cleft, theMedian Lacunaof the frog. The triangular projection in front of the median lacuna is the body or cushion of the frog. It is continued backwards as two ridge-like branches, which, at the points of the heels, form acute angles with the bars. On the outer side of each lateral ridge is a fissure. These are known as the Lateral Lacunæ.
The Sidesof the frog are flat and slightly oblique. They are closely united to the bars and to the triangular indentation in the posterior border of the sole.
The Baseof the frog is formed by the extremities of its branches, which, becoming wider and more convex as they pass backwards, form two rounded, flexible, and elastic masses separated from each other by the median lacuna. These constitute the 'glomes' of the frog. They are continuous with the periople.
The Point of the Frogis situated, wedge-like, within the triangular notch in the posterior border of the sole.
4. THE PERIOPLE, OR CORONARY FROG BAND.—This is a continuation of the substance of the frog around the extreme upper surface of the hoof. It is widest at the heels over the bulbs or glomes of the frog, and gradually narrows as it reaches the front of the hoof. It is, in reality, a thin pellicle of semi-transparent horn secreted by the cells of the perioplic ring. When left untouched by the farrier's rasp it serves the purpose, by acting as a natural varnish, of protecting the horn of the wall from the effects of undue heat or moisture.
The matter embraced by the heading of this chapter will offer for discussion many subjects of great interest to the veterinary surgeon. Around some of them debate has for many years waxed more than keen. Of the points in dispute, some of them may be regarded as satisfactorily settled, while others offer still further room for investigation.
In this volume we can only hope to deal with them in brief, and must select such as appear to have the greatest bearing on the veterinarian's everyday practice.
Always prolific of heated discussion has been one question: 'Are the horny laminæ secreted by the sensitive?' To answer this satisfactorily, it will be best to give a short account of the mode of production of the hoof in general.
Starting with the statement that it is epidermal in origin, we will first consider the structure of the skin, and follow that with a brief description of the structure and mode of growth of the human nail, a short study of which will greatly assist us when we come to investigate the manner of growth of the horse's hoof.
THE SKIN is composed of two portions, the EPIDERMIS and the CORIUM.
THE EPIDERMIS is a stratified epithelium. The superficial layers of the cells composing it are hard and horny, while the deeper layers are soft and protoplasmic. These latter form the so-calledRete Mucosumof Malpighi.
VERTICAL SECTION OF EPIDERMIS
FIG. 23.—VERTICAL SECTION OF EPIDERMIS (HUMAN). (AFTER RANVIER)A, The horny layer of the epidermis;B, the rete mucosum;a, the columnar pigment-containing cells of the rete;b, the polyhedral cells;c, the stratum granulosum;d, the stratum lucidum;e, swollen horny cells;fthe stratum squamosum.
Commencing from below and proceeding upwards, we find that the lowermost cells of the rete mucosum, those that are set immediately on the corium, are columnar in shape. In animals that have a coloured skin these cells contain pigment granules. Directly superposed to these we find cells which in shape are polyhedral. Above them, and forming the most superficial layer of the rete mucosum, is a series of flattened, granular-looking cells known as thestratum granulosum.
Immediately above the stratum granulosum the horny portion of the epidermis commences. In the human skin this is formed of three distinct layers. Undermost a layer of clear compressed cells, thestratum lucidum. Next above it a layer of swollen cells, the nuclei of which are indistinguishable. Finally, a surface layer of thin, horny scales, thestratum squamosum, which become detached and thrown off in the form of scurf or dandruff. In the skin of the horse, except where it is thickest, these layers are not clearly defined.
It is the Malpighian layer of the epidermis that is most active in cell division. As they are formed the new cells push upwards those already there, and the latter in their progress to the surface undergo a chemical change in which their protoplasm is converted into horny material. This change, as we have already indicated, takes place above the stratum granulosum.
In addition to its constant formation of cells to replace those cast off from the surface, the active proliferation of the elements of the Malpighian layer is responsible for the development of the various appendages of the skin, the hairs with their sebaceous glands, the sweat glands, horny growths and the hoof, and, in the human subject, the nail. These occur as thickenings and down-growths of the epithelium into the corium.
The epidermis is devoid of bloodvessels, but is provided with fine nerve fibrils which ramify between the cells of the rete mucosum.
THE CORIUM is composed of dense connective tissue, the superficial layer of which bears minute papillæ. These project into the epidermis, which is moulded on them. For the most part the papillæ contain looped capillary vessels, rendering the superficial layer of the corium extremely vascular. Why this must be a moment's reflection will show. The epidermis, as we have already said, is devoid of bloodvessels. It therefore depends entirely for its nourishment upon the indirect supply it receives from the vessels of the corium. The need for extreme vascularity of the corium is further explained when we call to mind the constant proliferation and casting off of the cells of the epidermis, the growth of the hairs, the production of the horn of the hoof, and the work performed by the numerous sweat and other glands.
Others of the papillæ contain nerves, ending here in tactile corpuscles, or continuing, as we have mentioned before, to ramify as fine fibrils in the rete mucosum of the epidermis.
THE HAIRS are growths of the epidermis extending downwards into the deeper part of the corium. Each is developed in a small pit, theHair Follicle, from the bottom of which it grows, the part lying within the follicle being known as theRoot. It is important to note their structure, as it will be seen later that they bear an extremely close relation to the horn of the hoof.
Under a high power of the microscope, and in optical section, the central portion of a hair is tube-like. In some cases the cavity of the tube is occupied by a dark looking substance formed of angular cells, and known as theMedulla. The walls of the tube, or the main substance of the hair, is made up of a pigmented,horny, fibrous material. This fibrous structure is covered by a delicate layer of finely imbricated scales, and is termed theHair Cuticle.
The root of the hair, that portion within the follicle, has exactly the same formation save at its extreme end. Here it becomes enlarged into a knob-like formation composed of soft, growing cells, which knob-like formation fits over a vascular papilla projecting up in the bottom of the follicle.
We have already stated that the hairs are down-growths of the epidermis. It follows, therefore, that the hair follicles, really depressions or cul-de-sacs of the skin itself, are lined by epithelial cells and connective tissue. So closely does the epidermal portion of the follicle invest the hair root that it is often dragged out with it, and is known as theRoot Sheath. This is made up of an outer layer of columnar cells (the outer root sheath) corresponding to the Malpighian layer of the epidermis, and of an inner horny layer, next to the hair, corresponding to the more superficial layer of the epidermis, and known as theinner root sheath.
The hair grows from the bottom of the follicle by a multiplication of the cells covering the papilla upon which its root is moulded. When a hair is cast off a new one is produced from the cells covering the papilla, or, in case of the death or degeneration of the original papilla, the new hair is produced from a second papilla formed in place of the first at the bottom of the follicle.
SECTION OF SKIN WITH HAIR FOLLICLE AND HAIR
FIG. 24.—SECTION OF SKIN WITH HAIR FOLLICLE AND HAIR.a, The hair follicle;b, the hair root;c, the medulla;d, the hair cuticle;e, the outer root sheath;f, the inner root sheath;g, the papilla from which the hair is growing;h, a sebaceous gland;i, a sudoriferous gland.
THE SEBACEOUS GLANDS are small saccular glands with their ducts opening into the mouths of the hair follicles. They furnish a natural lubricant to the hairs and the skin.
THE SUDORIFEROUS OR SWEAT GLANDS are composed of coiled tubes which lie in the deeper portion of the skin, and send up a corkscrew-like duct to open on the surface of the epidermis. They are numerous over the whole of the body.
LONGITUDINAL SECTION THROUGH NAIL AND NAIL-BED OF A HUMAN FOETAL FINGER
FIG. 25.—LONGITUDINAL SECTION THROUGH NAIL AND NAIL-BED OF A HUMAN FOETAL FINGER.[A]a, The nail;b, the rete mucosum;c, the longitudinal ridges of the corium.
[Footnote A: Seeing that the section is a longitudinal one, it would appear from the way the ridges cut that they are running transversely beneath the nail. Their extreme delicacy, however, prevents a single one showing itself along the length of the section, and their constant accidental cutting makes themappearto run transversely (H.C.R.).]
THE HUMAN NAILS are thickenings of the lowermost layer of the horny portion of the epidermis, the stratum lucidum. They are developed over a modified portion of the corium known as the nail-bed. The horny substance of the nail is composed of clear horny cells, and rests immediately upon a Malpighian layer similar to that found in the epidermis generally. Instead of the papillæ present elsewhere in the skin, the corium of the nail-bed is marked by longitudinal ridges, a similar, though less distinct, arrangement to that found in the laminæ of the horse's foot.
Having thus paved the way, we are now in a better position to discuss our original question (Are the horny laminæ secreted by the sensitive?), and better able to appreciate the work that has been done towards the elucidation of the problem.
A most valuable contribution to this study is an article published in 1896 by Professor Mettam.[A] Here the question is dealt with in a manner that must effectually silence all other views save such as are based upon similar methods of investigation—namely, histological examination of sections of equine hoofs in various stages of foetal development.
[Footnote A: TheVeterinarian, vol. lxix., p.1.]
Professor Mettam commences by drawing attention to the error that has been made in this connection by studying the soft structures of the foot separated by ordinary putrefactive changes from the horny covering. "In this way," the writer points out, "a wholly erroneous idea has crept in as to the relation of the one to the other, and the two parts have been treated as two anatomical items, when, indeed, they are portions of one and the same thing. As an illustration, and one very much to the point at issue, the soft structures of the foot are to the horny covering what the corium of the skin and the rete Malpighii are to the superficial portions of the epidermis. Indeed, the point where solution of continuity occurs in macerating is along the line of the soft protoplasmic cells of the rete."
In the foregoing description of the skin we have seen that the corium is not aplanesurface, but that it is studded by numerous papillary projections, and that these projections, with the depressions between them, are covered by the cells of the epidermis.
The corium of the horse's foot, however, although possessed of papillæ in certain positions (as, for example, the papillæ of the coronary cushion, and those of the sensitive frog and sole), has also most pronounced ridges (laminæ) which run down the whole depth of the os pedis. Each lamina again carries ridges (laminellæ) on its lateral aspects, giving a section of a lamina the appearance of being studded with papillæ. We have already pointed out the ridge-like formation of the human nail-bed, and noted that, with the exception that the secondary ridges are not so pronounced, it is an exact prototype of the laminal formation of the corium of the horse's foot.
The distribution of the laminæ over the foot we have discussed in the chapter devoted to the grosser anatomy. In a macerated foot the sensitive laminæ of the corium interdigitate with the horny laminæ of the hoof; that is to say, there is no union between the two, for the simple reason that it has been destroyed; they simply interlock like theungluedjunction of a finely dovetailed piece of joinery. But no further, however, than the irregularities of the underneath surface of the epidermis of the skin can be said to interlock with the papillæ of the corium does interlocking of the horny and sensitive laminæ occur. It is only apparent. The horny laminæ are simply beautifully regular epidermal ingrowths cutting up the corium into minute leaf-like projections.
In a macerated specimen, then, the exposed sensitive structures of the foot exhibit the corium as (1) theCoronary Cushion, fitting into the cutigeral groove; (2) theSensitive Laminæ, clothing the outer surface of the terminal phalanx, and extending to the bars; (3) thePlantar Cushion, or sensitive frog; and (4) theSensitive Sole.
The main portion of the wall is developed from the numerous papillæ covering the corium of the coronary cushion. We have in this way numberless down-growing tubes of horn. Professor Mettam describes their formation in a singularly happy fashion: "Let the human fingers represent the coronary papillæ, the tips of the fingers the summits of the papillæ, and the folds of skin passing from finger to finger in the metacarpo-phalangeal region the depressions between the papillæ. Imagine that all have a continuous covering of a proliferating epithelium. Then we shall have a more or less continuous column of cells growing from the tip of the finger or papilla (a hollow tube of cells gradually moving from off the surface of the finger or papilla like a cast), and similar casts are passing from off all the fingers or papillæ."
From this description it will be noticed that each down-growing tube of horn bears a striking resemblance to the growth of a hair, described on p. 47. In fact, the horn tube may be regarded as what it really is, a modified hair.
We next continue Professor Mettam's illustration, and note how the modified hairs or horn tubes become as it were matted together to form the hoof wall. The cells lining the depressions are also proliferating, and their progeny serve to cement together the hollow casts of the papillæ, thus giving theinter-tubular substance. We have thus produced hollow tubes, united together by cells, all arising from the rete Malpighii of the coronary corium. Section of the lower part of the horn tubes shows them to contain a cellular debris.
Thus, in all, in the horn of the wall we find a tubular, an intertubular, and intratubular substance. In fact, hairs matted together by intertubular material, and only differing from ordinary hairs in their development in that they arise, not from papillæ sunk in the corium, but from papillæ projecting from its surface.
Although this disposes of the wall proper, there still confronts us the question of the development of the horny laminæ. To accurately determine this point it is absolutely essential to examine, histologically, the feet from embryos.
In the foot of any young ungulate in the early stages of intra-uterine life horizontal sections will show a covering of epidermis of varying thickness.[A] This may be only two or three cells thick, or may consist of several layers. Lowermost we find the cells of the rete Malpighii. As some criterion of the activity with which these are acting, it may be noted that with the ordinary stains their nuclei take the dye intensely. The cells of this layer rest upon a basement membrane separating the epidermis from the corium. At this stagethe corium has a perfectly plane surface.
[Footnote A: Equine foetus, seventy-seven days old.]