THE PASSIVE STAY-APPARATUS

It is well-known that horses can remain on their feet for much longer than other domestic animals. In fact, they are thought by many to sleep while standing. This is not quite true: they may rest or doze standing, but for a refreshing sleep they lie down, often only at night when unobserved.

The bony column of the forelimb supports the cranial end of the trunk at the attachment of the serratus ven­tralis muscle to the medial surface of the scapula (Figure 23-38, A/1). A vertical line dropped from the center of this attachment passes caudal to the shoulder, through the elbow, through or slightly cranial to the carpal joint, and cranial to the fetlock and pastern joints. If unsup­ported, the column would collapse by flexion of the shoulder and elbow joints, by overextension (or possibly flexion [buckling forward]) of the carpal joint, and by overextension of the fetlock and pastern joints. (The coffin joint actually flexes when the fetlock sinks under weight and can be disregarded in this discussion.)

The shoulder joint is prevented from flexion by the strong internal biceps tendon (Figure 23-38/2) that con­nects the supraglenoid tubercle of the scapula with the radius. The latter attachment can be regarded as fixed because it is very close to the axis of rotation of the elbow joint (Figure 23-38/5), which is stabilized by the weight on the limb. Tension in the wide biceps tendon puts great pressure on the intertubercular groove of the humerus. Indeed, some believe that the molding of the tendon to the intermediate tubercle actually causes the joint to lock. At its other end, the pull of the biceps is transmitted via the lacertus and extensor carpi radia­lis (Figure 23-38/6,10) to a second fixed point at the upper end of the large metacarpal bone.

The fetlock joint is prevented from overextension principally by the suspensory apparatus (comprising the interosseus, proximal sesamoid bones, and distal sesa- moidean ligaments), which is tensed under load (Figure

Figure 23-34 A, Dermis exposed by removal of the hoof. B, Hoof shoe removed from specimen A. C, Changes in the form of the hoof during locomotion. D, Shoe showing the heel part polished by movement of the hoof heel. 1, Perioplic dermis; 2, coronary dermis; 3, laminar dermis; 4, terminal papillae on the ends of the dermal laminae; 5, cut edge of skin.

23-38/13,14,16-18). The effect is reinforced by tension in the accessory (check) ligaments and distal parts of the superficial and deep flexor tendons (Figure 23-38/9,11).

Tension in the deep flexor tendon tends to flex the coffin joint, which causes the toe of the hoof to dig into the ground. The extensor branches of the interosseus (Figure 23-38/15), pulling on the extensor process of the bone at impact, counteract this and keep the hoof level.

Overextension of the pastern joint is opposed by the axial and abaxial palmar and straight sesamoidean liga­ments (Figure 23-38/18,19), which span its palmar aspect. The taut deep flexor tendon gives additional support. (Buckling forward is prevented by the superfi­cial flexor that attaches on the palmar aspect of the joint.)

With the shoulder joint fixed (by the biceps tendon), the weight of the trunk rests on the upper end of the nearly vertical radius. Therefore, unless the horse sways

Figure 23-35 A, Transverse section of the digit at the level of the navicular bone, proximal surface.

Figure 23-36 A, In former days horses at pasture were hobbled with a "pastern"; this is why the narrow part of the limb above the hoof is known today as the pastern. B, Palmar (plantar) view of the foot; the lateral (L) angle of the wall (with the ground) is more acute than the medial (M). C and D, The angle at the toe is more acute in the forelimb than in the hindlimb.

Figure 23-37 Hoof of a newborn foal. 1, Mass of soft, primary horn covering the ground surface and distal half of the hard, permanent hoof wall; 2, pigmented permanent hoof wall.

Figure 23-38 A, The stay-apparatus of the left forelimb; lateral view. B, Detail of digit; lateral view. 1, Weight of trunk; 2, internal biceps tendon; 3, triceps; 4, brachiocephalicus and brachial fascia to elbow joint; 5, axis of elbow rotation, next to eccentric collateral ligament; 6, lacertus fibrosus; 7, ulnaris lateralis; 8, flexor carpi ulnaris; 9, superficial digital flexor and accessory (check) ligament; 10, extensor carpi radialis; 11, deep digital flexor and accessory (check) ligament; 12, common digital extensor; 13, interosseus; 14, proximal sesamoid bones; 15, extensor branch of interosseus; 16, 17, 18, cruciate, oblique, and straight sesamoi- dean ligaments; 19, axial palmar ligament; 20, navicular bone.

markedly forward, only small forces are required to prevent the elbow joint from flexing. These are mainly supplied by passive tension of the tendinous compo­nents of the carpal and digital flexors (the superficial digital flexor especially) and the eccentrically placed collateral ligaments (Figure 23—38/5,7-9). Recent infor­mation indicates that because of their muscle fiber composition—characteristic of postural muscles—the anconeus and the medial head of the triceps may also oppose flexion of the elbow joint. The large mass of the long and lateral heads of the triceps—the principal extensor of the elbow joint—remains flaccid even when the other forelimb is picked up to make the horse stand three-legged (see Figure 23—2/1, and the effects of radial paralysis on page 621).