Apparatus of Motions and Support of the Horse (Functional Analysis)

On the basis of a comparative-anatomical and functional analysis of the apparatus of motions and support in the horse, carried out by studying the motions in the various joints of the limbs and by comparing tht muscles as to their strength development, topography and character of function, the author comes to the following conclusions.

1. In the evolutionary process of the horse (increase of body mass, lengthening of lim|bs, transition to the phalangal progression, acquirement of one-toed condition, etc.) some considerable changes have taken place in the organization of the joints, muscles and socalled "soft" coimectivetissue skeleton, taking the form of specialization.

2. The process of specialization af the joints, even in the joints connecting the free limb skeleton with the girdle (coxofemoral and scapulo-humeral articulations), has led to the loss of the formorfreedom of motion, and the coxafemoral articulation (of these 2 joints) has undergone an especially high dogree of specialization.

3. The vertical flattening of the head of the femur and some slight difference in the area of the joint surfaces of their head and of the acetabulum led to a close dependence of the rotatory and lateral motions of the femur on the motions around the transverse axis.

With the forward- and backward-motions of the limb, the lower end of the femur describes an inward convex curve having a centre that lies beyond the body. Adduction in this joint is possible only in the intermediate phase (vertical positions of the femur), abduction accompanies the phases of the front- and backward-motions of the limb, i. e., is bound with flexion and extension in the coxofemoral joint. The outward-rotation of the knee (supinatio) is connected with the front-motion of the limb (flexio) ; and the inward rotation (pronatio), with the backward motion (extensio).

This combination is so specific that it cannot be repeated in any other phases of motions.

Frequent luxations in the coxofemoral joint in the horse is adequately explained by the absence of adduction in joint in the phases of the forward- and backward-motion.

4. The strengthening of the ligamental apparatus of the limbs was brought about not only because of an increase in the deep fascial layers reinforcing the ligamental apparatus, but is also due to the transformation of many muscles into ligaments (m. m. interosseus medius, pronator teres, pronator quadratus).

5. The specialization of muscles is manifested:

a. in a considerable or complete loss of the active muscular elements,

b. in the strengthening of the connective tissue muscle stroma, proper rascias and tendons,

c. in a partial or complete inosculation of the adjoining muscles,

d. in the inosculation of the ligamental (connective tissue-) portions of the limb muscles situated above and below.

e. in the acquirement of additional (interjacent) points of fixation on the skeleton by the formation of ligamental heads or the fixation on the osseous protuberances by means of the muscle.

6. Contrary to the fore (drawing and supporting) limbs, the hind limbs are active-locomotive. This condition is confirmed by the comparison of the homologous joints of the two limbs and of the topography of these joint tops and also of the strength of development of the corresponding groups of muscles.

7. The muscle may work independently (separate function). The same muscle, or musclar group, may accomplish a directly opposite function, if the limb position changes (the transition from the phase of suspension to the phase of support and vice versa).

8. This "autoantagonism" is especially pronounced in the volar (plantar) limb muscles, which prevail very much in the fore- as in the hind limb over the fore (dorsal) muscles.

The "autoantagonism" of the carpal and toe flexor in the fore limb and of the toe flexor in the hind limb should be considered as a function of these muscles attending the active support of the limb straightened out in these joints (the strain synchronous with the extensors of these same joints aiding the active planting of the hoof in the soil). The contraction of these muscles result in the pawing motions of the limb which has been freed from the weight of the body.

9. In relation to the individual joints, comparing the amount of work done, we may distinguish muscles of "great" and of "little" work.

The formed work with the loaded; the second; with the limb that is free of the body weight.

These muscles form pairs of direct antagonists and the first group greatly predominates over the second in the strength of development, for instance:

the posterior-femoral group as a group extensor and pronator predominates over m. iliopsoas and m. pectineus as a group flexor and supinator; the gluteal group (except the superficial glutial muscle), as a group pronator and extensor over the obturators, and the transverse miuscle of the femur, as a group supinator and flexor.

10. The trunk in relation to the limbs may be looked upon as a two-armed lever which acts independently in relation to each pair of limbs, namely:

a. in relation to the pectoral limbs - the head and .neck play the part of a short and very mobile arm (the arm of power) and the of the body, together mith the mobile neck-head portion, the part

b. in relation to the pelvic limbs - the sacrum together with the basal portion of the tail play the part of the short arm and the rest of the body, together with the mobile neck-head portion, the part of the long arm (the arm of gravity). The transverse axis for the first lever is the axis connecting the scapula - trunk angles of both fore-limbs The transverse axis for the second lever is the axis passing through the coxofemoral joints.

11. The head and neck acts as a natural active distributor of the weight among the four limbs. The action of this lever ocours in following manner:

a. a pendulum-like oscillation connected with the alternating bilateral bending down and raising up of the head, - the motions that accompany the successive change of the stepping limb (for instance, by an loaded step);

b. a bending of the head and neck which sets free the hind legs (for instance, for an defensive kick) and loads the forelegs;

c. a raising np of the head and neck which sets free the fore limbs and loads the hindlegs (for instance, on taking a barrier or on prancing).

12. Of a great importance for the establishment of close interde- pendency between the two pairs of limbs is the connection of the leg muscles with the muscles or skeleton of the trunk:

a. an immediate connection between the muscles of the limbs and the trunk skeleton (for instance, m. semitendinosus, biceps fermoris, brachiocephalics),

b. a direct connection between the limb muscles and the trunk muscles (for instance, m. glutaeus medius - m. longissimus dorsi; m. pectineus - m. rectus abdominis);

c. an indirect connection between the muscles of the fore and hindlimbs (for instance, humeral joint - m. latissimus dorsi - fascia lumbodorsalis - m. longissimus dorsi - m. glutaeus medius - coxofemoral joint);

d. an immediate connection of the body muscles with the limb joints (for instance, m. rectus abdominus - ligam. accessorium - the head of the femur; m. obi. abd. externals - its femoral tendon - knee joint).

13. In the process of specialization of the limbs in the horse, their tendo-ligamental apparatus becomes autonomous to a considerable extent.

This apparatus is composed of the connective tissue elements of muscles, of their proper fascias, of muscles transformed into a tendinous band or in the ligaments, and, lastly, of specific and also very specialized ligaments of separate joints. The action of this autonomous apparatus is facilitated by

a. the determination of the additional points of fixation on the skeleton by the tendonous portions of the muscles;

b. the establishment of a close interconnection between the tendons of the muscles situated above and below;

c. the transformation of many joints into the so-called "clicking", the collateral ligaments acquire the role of flexor brakes (locking joints with unequal curvature of their joint surfaces, which helps of the joints in the extended state).

14. The tendo-ligamentous apparatus of the limbs is set in motion by two contrary forces which play the part of a peculiar "winding mechanism" for this apparatus - namely,

a. for the fore limbs, the part of this "mechanism" is played above, toy the body weight taken by the back angle of the scapula by means of the fascia dentata, and from below, by the tension of the distal tendo-ligamental apparatus, caused by the supporting position of the digit;

b. for the hind limbs the same part is played above, by the 4-headed muscle moving the knee-cap on the highest point of the medial patellar crest, and from below, by the tension of the distal tendo-ligamentous apparatus, caused by the supporting position of the digit.

The hind limb is automatically braced in the extension condition by the tendo-osseous parallelogram, the elastic sides of which form the shank portion of the superficial digital flexor and the third fibular muscle.

15. Both pairs of limbs possess theree peculiar amortisors (proximal, median and distal) protecting the body, and the organs in it from shaking and securing the elastic support of the body weight on the distal links of the limbs.