The anterior ulnar arises above from the epitrochlea and the olecranon; thence it is directed towards the carpus, to be inserted into the pisiform bone. It proceeds therefore from the inner side of the elbow to the outer side of the upper part of the hand; it consequently crosses the posterior surface of the forearm obliquely. This is why, as we have pointed out above, it receives the name of the oblique flexor of the metacarpus.
It is not unprofitable to recall in this connection that there is an internal flexor of the metacarpus, which is the flexor carpi radialis; and an external flexor of the metacarpus, which is the posterior ulnar (in human anatomy, extensor carpi ulnaris). It is between these two muscles that we find the oblique flexor--the anterior ulnar which we have just been studying.
This muscle flexes the hand on the forearm.
=Superficial Flexor of the Digits= (_Flexor digitorum sublimis_) (Fig.
76, 12; Fig. 77, 10, 10).--This muscle arises from the epitrochlea; thence it pa.s.ses towards the hand, becomes tendinous, pa.s.ses in a groove on the posterior aspect of the carpus, and terminates on the palmar surface of the phalanges in furnishing a number of tendons proportioned to the digital division of the hand. Whatever the number, to which we will again refer, each tendon is attached to the second phalanx, after bifurcating at the level of the first, so as to form a sort of ring, destined to give pa.s.sage to the corresponding tendon of the deep flexor.
This ring and this pa.s.sage have gained for the muscle the name of _perforated flexor_.
In the dog and the cat the princ.i.p.al tendon is divided into four parts, which go to the four last digits.
In the ox it is divided into two parts only; as, moreover, in the pig, whose superficial flexor is destined for the two large digits only, the lateral digits receiving no part of it.
Finally, in the horse the tendon is single.
We have previously pointed out that in the carnivora this muscle is visible on the internal and posterior aspects of the forearm, in the interval which is limited in front by the flexor carpi radialis and behind and outside by the anterior ulnar.
Certain details are still to be added to the description of this muscle. We will enter on an a.n.a.lysis of them after we have given some indications relative to the following muscle:
=Deep Flexor of the Digits= (_Flexor digitorum profundus_) (Fig. 75, 21, 22; Fig. 76, 12; Fig. 77, 11, 11).--This muscle is covered by the superficial flexor. It arises from the epitrochlea, from the radius, and from the ulna, either from the olecranon process--as in the ox, pig, and horse--or from almost the whole extent of the shaft of the same bone, as in the cat and dog.
[Ill.u.s.tration: FIG. 78.--LEFT ANTERIOR LIMB OF THE HORSE: INTERNAL ASPECT.
1, Internal flexor of the metacarpus or great palmar; 2, inferior part of the biceps; 3, inferior part of the brachialis anticus; 4, internal lateral ligament of the elbow; 5, p.r.o.nato teres muscle.]
The radial fasciculus represents in the domestic quadrupeds the long proper flexor muscle of the thumb in man. For this reason we shall describe the muscle afresh in the following paragraph:
The fleshy bundles of which we have just spoken terminate in a tendon which afterwards divides into slips, the number of which is in proportion to the digital division of the hand. These slips then pa.s.s through the slit or _b.u.t.tonhole_ in the tendon of the superficial flexor, and proceed to terminate on the third phalanx; hence the name of _perforating_, which is also given to the deep flexor of the digits.
In the dog and the cat the tendon is divided into five portions, each of which proceeds to one of the digits. The internal tendon, which is destined for the thumb, terminates on the second phalanx of this digit.
In the pig the tendon divides into four tendons destined for the four digits.
In the ox there are but two tendons.
In the horse the tendon is single.
As their names indicate, these muscles, both superficial and deep, flex the digits. In addition to this, they flex the hand on the forearm.
We mentioned above that certain details relative to the superficial flexor must be a.n.a.lyzed in a special way. We now add that this should also be done with regard to the deep flexor. The point in question is the arrangement which the tendons of these muscles present at the level of the palmar region of the hand.
It is easy, in the case of the dog or the cat, to picture to one"s self this arrangement, especially if we recollect that which exists in the human species. The tendons of the flexors are placed on a kind of muscular bed formed by the union of the muscles of the region, but, moreover, from the point of view of external form, these tendons are not of very great importance.
But in the ox and the horse it is quite otherwise. From the simplification of the skeleton of the hand, and the reduction of the number of movements which the bones that form it are able to execute, there naturally results a diminution of its muscular apparatus. Apart from the existence of muscular vestiges of but little importance, we can say that, in reality, the hand does not possess any muscles. On its palmar aspect are found only the tendons of the flexors of the digits, and as these tendons are large, and the hand long, they give origin to external forms which it is necessary to examine.
[Ill.u.s.tration: FIG. 79.--LEFT ANTERIOR LIMB OF THE HORSE: EXTERNAL ASPECT.
1, Ulna of abnormal length.]
[Ill.u.s.tration: FIG. 80.--LEFT ANTERIOR LIMB OF THE HORSE: EXTERNAL ASPECT.
1, Normal ulna.]
In the horse, which we take as a type, the tendons of the flexors, after being retained in position at the carpus by a fibrous band, the _carpal sheath_, which recalls the anterior annular ligament of the human carpus, and having pa.s.sed this region, descend vertically, remaining separated from the posterior surface of the metacarpus, so that the skin sinks slightly on the lateral parts in front of the thick cord which these tendons form. This cord is known by the name of _tendon_.
The flexors then reach the fetlock, and occupy the groove formed by the peculiar arrangement of the two large sesamoid bones. They are retained in position at this level by a fibrous structure, which forms the metacarpo-phalangeal sheath. They then reach the phalanges, being directed obliquely downwards and forwards, as, moreover, the latter are also inclined. Then the tendon of the superficial flexor divides into two slips, which are inserted into the second phalanx, between which slips pa.s.ses the tendon of the deep flexor, which in its turn goes to be inserted, in the form of an expansion, into the semilunar crest, by which the inferior surface of the third phalanx is divided into two parts.[30]
[30] See, as regards this crest, in the paragraph relative to the hoof of the solipeds, the figures which represent the third phalanx, viewed on its inferior surface (Figs. 101 and 102, p. 258).
The part which these tendons play is of great importance in the large quadrupeds.
These tendons, in fact, in addition to the action determined by the contraction of the fleshy fibres to which they succeed, maintain the angle formed by the canon-bone and the phalangeal portion of the hand, and prevent its effacement under the weight of the body during the time of standing. Their strong development, and the position they occupy, make this understood, without it being necessary to insist on it further.
We mentioned above that the "tendon" descends vertically from the carpus towards the fetlocks. This is as it should be. But, in some horses, it is oblique downwards and backwards, so that the canon, instead of being of equal depth from before backwards in its whole length, is a little narrower in its upper part.
This results from the fact that the tendons of the flexors, too firmly bound by the carpal sheath, gradually separate as they pa.s.s from the metacarpus, going to join the fetlock; hence the obliquity pointed out above. This abnormality producing a deleterious result, in the sense that the tendinous apparatus acts with less strength as an organ of support, it const.i.tutes a defect of conformation which is expressed by saying that the tendon has "failed."
=Long Proper Flexor of the Thumb= (_Flexor longus pollicis_) (Fig. 76, 14).--As we have already pointed out, this muscle is represented in quadrupeds by the radial bundle of the deep flexor of the digits, so that the two muscles are in reality blended the one to the other. This union is sometimes found, but only as an abnormality, in the human species. We have met some examples of this in the course of our dissections.
=p.r.o.nator Quadratus.=--This muscle conforms to the general law which we have already pointed out in connection with those which have for their action the rotation of the radius around the ulna. We remember, indeed, that when the bones of the forearm are fused with one another, the muscles which are destined to produce a mobility which has then become impossible disappear at the same blow.
[Ill.u.s.tration: FIG. 81.--DIAGRAM OF THE POSTERIOR PART OF A TRANSVERSE SECTION Pa.s.sING THROUGH THE MIDDLE OF THE LEFT FORE-LIMB OF THE DOG: SURFACE OF THE INFERIOR SEGMENT OF THE SECTION.
1, Radius; 2, ulna; 3, posterior ulnar; 4, anterior ulnar; 5, great palmar (_flexor carpi radialis_); 6, flexors of the digits.]
[Ill.u.s.tration: FIG. 82.--DIAGRAM OF A HORIZONTAL SECTION OF THE MIDDLE OF THE FOREARM OF THE LEFT LEG OF THE HORSE: SURFACE OF THE INFERIOR SEGMENT OF THE SECTION.
1, Radius; 2, ulna; 3, posterior ulnar; 4, anterior ulnar; 5, great palmar (_flexor carpi radialis_); 6, flexors of the digits.]
For this cause we do not find the square p.r.o.nator in either the ox or the horse, but can demonstrate its presence in the dog and the cat.
It is very deeply situated. This is why, and also on account of the plan which we have traced for ourselves, we will simply say that it is situated on the postero-internal aspect of the skeleton of the forearm, and that it extends from the ulna to the radius.
It seems to us, however, sufficiently interesting to add that, instead of occupying, as in the human species, the inferior fourth of the two bones, it extends, particularly in the dog, over their whole length, with the exception of their superior and inferior extremities.
Muscles of the Hand
We will first recall that, in man, the palm of the hand is divided into three regions: a median palmar region, which is occupied by the tendons of the flexors of the digits, the lumbricales, and, deeply, by the interosseous muscles; an external region, or thenar eminence, formed by the muscles destined for the movements of the thumb; an internal region, or hypothenar eminence, which contains the muscles proper to the small digit and the palmar cutaneous muscle.
These muscles are found, more or less developed, in the dog and the cat.
In the ox and the horse we meet with no vestige of the muscles of the thenar or hypothenar eminences. Nevertheless, in these animals we find the muscles which belong to the central region of the palm. We refer to the lumbricales and the interosseous.
Although this fact has no relation to the object of our study, it appears to us interesting to announce that there are traces of the lumbricales found in the solipeds. These muscles are represented by two fleshy bundles, situated one on each side of the tendon of the deep flexor, above the ring of the tendon of the superficial flexor. These small muscles are continued as slender tendons, which become lost in the fibrous tissue of the _spur_, which is the h.o.r.n.y process situated at the posterior part of the fetlock, and which is covered by the hairs, more or less abundant, which const.i.tute the _wisp_.
As for the interosseous muscles, they are represented by the _suspensory ligament of the fetlock_, and by two other small muscles, tendinous throughout, which are situated between the princ.i.p.al metacarpal and the rudimentary ones.
The suspensory ligament of the fetlock is considered an interosseous muscle, on account of the red fleshy striations which it contains, and from certain relations which it forms with the tendon of the common extensor of the digits or anterior extensor of the phalanges. This ligament (Fig. 75, 26; Fig. 77, 13), which plays an important part in the standing position as a support of the foot, is a fibrous band situated between the tendons of the flexors of the digits and the princ.i.p.al metacarpal. It arises above, from the second row of the carpals, descends towards the fetlock, where it divides into two branches, which are inserted into the large sesamoid bones. At the same level, this ligament gives off two fibrous bands which, pa.s.sing downwards and forwards, join the tendon of the anterior extensor of the phalanges, blending with it, after having each crossed one of the lateral aspects of the pastern. We have already referred to these bands (p. 183).