The tendons have been removed to show the canals in which they lie.
The joint cavity is black. By an oversight the carpal canal for the
two flexor tendons behind the posterior ligament of the carpus is
not marked. _Cr_, Radial, _Ci_, intermediate, _Cu_, ulnar, _Ca_,
accessory carpal bones.
]
_Relations._—Superficially, the skin, fascia, and cutaneous branches of
the ulnar nerve; deeply, the elbow joint, the ulna, and the flexors of
the digit; in front, the lateral extensor of the digit; behind, the
middle flexor of the carpus, the ulnar head of the deep flexor, and the
ulnar vessels and nerve.
_Blood-supply_.—Interosseous, ulnar, and median arteries.
_Nerve-supply._—Radial nerve.
4. =Superficial digital flexor= or =flexor perforatus= (M. flexor
digitalis sublimis; superficial flexor of the phalanges).—This muscle is
situated in the middle of the flexor group, chiefly under cover of the
middle flexor of the carpus.
_Origin._—(1) The flexor epicondyle of the humerus, between the flexor
carpi internus and the deep flexor of the digit; (2) a ridge on the
posterior surface of the radius, below its middle and near the internal
border.
_Insertion._—(1) The eminences on the proximal extremity of the second
phalanx behind the lateral ligaments; (2) the distal extremity of the
first phalanx, also behind the lateral ligaments.
_Action._—To flex the digit and carpus, and to extend the elbow.
[Illustration:
FIG. 195.—SYNOVIAL SHEATHS AND BURSÆ OF LOWER PART OF RIGHT FORE LIMB
OF HORSE, INTERNAL VIEW.
_a_, Sheath of extensor carpi obliquus; _b_, sheath of flexor carpi
internus; _c_, carpal sheath; _d_, _d′_ _d″_, _d‴_, digital sheath;
_e_, bursa under anterior extensor tendon; _f_, capsule of fetlock
joint; _1_, extensor carpi radialis; _2_, tendon of extensor carpi
obliquus; _3_, flexor carpi internus; _4_, flexor carpi medius; _5_,
superficial flexor tendon; _6_, deep flexor tendon; _7_, suspensory
ligament; _8_, small metacarpal bone; _9_, large metacarpal bone;
_10_, posterior annular ligament of fetlock; _11_, proximal digital
annular ligament; _12_, radius; _13_, radio-carpal joint; _14_,
fetlock joint; _15_, lateral cartilage; _16_, band from first
phalanx to lateral cartilage. (After Ellenberger, in Leisering’s
Atlas.)
The synovial sheaths (colored yellow) and the joint capsules (colored
pink) are injected.
]
[Illustration:
FIG. 196.—SYNOVIAL SHEATHS AND BURSÆ OF LOWER PART OF RIGHT FORE LIMB
OF HORSE, EXTERNAL VIEW.
_a_, Sheath of extensor carpi radialis; _b_, sheath of anterior
extensor; _c_, sheath of lateral extensor; _d_, sheath of outer
tendon of flexor carpi externus; _e_, _e′_, carpal sheath; _f_, _f′_
_f″_, digital sheath; _g_, bursa under anterior extensor tendon;
_h_, bursa under lateral extensor tendon; _i_, capsule of fetlock
joint; _1_, extensor carpi radialis; _2_, anterior digital extensor;
_3_, lateral digital extensor; _4_, flexor carpi externus; _4′_
_4″_, tendons of _4_; _5_, superficial flexor tendon; _6_, deep
flexor tendon; _7_, suspensory ligament; _8_, external metacarpal
bone; _9_, large metacarpal bone; _10_, posterior annular ligament
of fetlock; _11_, digital annular ligament; _12_, fetlock joint;
_13_, lateral cartilage; _14_, band from first phalanx to lateral
cartilage. (After Ellenberger, in Leisering’s Atlas.)
]
_Structure._—The fleshy portion of the muscle or =humeral head= takes
origin from the humerus. The =radial head= (Caput tendineum) consists of
a strong fibrous band, usually termed the =radial= or =superior check
ligament=, which fuses with the tendon near the carpus. The belly of the
muscle is intersected by tendinous strands, and fuses more or less with
that of the deep flexor. Near the carpus it is succeeded by a strong,
thick tendon which passes down through the carpal canal, formed by the
posterior ligament of the carpus, the accessory carpal bone, and the
posterior annular ligament (Ligamentum carpi transversum). Here it is
enveloped by a synovial sheath, in common with the deep flexor. This
=carpal sheath= (Vagina carpea) begins three or four inches (8 to 10
cm.) above the carpus, and extends downward nearly to the middle of the
metacarpus. Below the carpus the tendon becomes flattened and broader.
Near the fetlock it forms a ring through which the tendon of the deep
flexor passes (Fig. 150). Here the two tendons are bound down in the
sesamoid groove by the posterior annular ligament, which fuses more or
less with the superficial flexor tendon. At the distal end of the first
phalanx the tendon divides into two branches which diverge to reach
their points of insertion, and between these branches the tendon of the
deep flexor emerges (Fig. 189). A second =synovial sheath=, the
=digital= or =sesamoidean= (Vagina digitalis), begins at the distal
third of the metacarpus, three to four inches (8 to 10 cm.) above the
fetlock, and extends to the middle of the second phalanx.
[Illustration:
FIG. 197.—CROSS-SECTION OF DISTAL PART OF LEFT METACARPUS OF HORSE,
JUST ABOVE SESAMOIDS.
]
_Relations._—The belly of the muscle is related superficially to the
ulnar head of the deep flexor, the flexor carpi medius, and, at its
origin, to the ulnar vessels and nerve; deeply to the humeral head of
the deep flexor. The tendon is related superficially to the skin and
fascia; deeply to the deep flexor tendon.
_Blood-supply._—Posterior radial artery.
_Nerve-supply._—Median nerve.
5. =Deep digital flexor= or =flexor perforans= (M. flexor digitalis
profundus; deep flexor of the phalanges).—The fleshy part of this muscle
lies on the posterior surface of the radius, under cover of the
preceding muscles. It is the largest muscle of the flexor group.
_Origin._—(1) The flexor (internal) epicondyle of the humerus; (2) the
inner surface of the olecranon; (3) the middle of the posterior surface
of the radius and a small adjacent area of the ulna.
_Insertion._—The semilunar crest of the third phalanx, and the adjacent
surface of the lateral cartilage.
_Action._—To flex the digit and carpus, and to extend the elbow.
_Structure._—This muscle consists of three principal heads. The =humeral
head= (Caput humerale) constitutes the bulk of the muscle. It is marked
by tendinous intersections, and is separable into three secondary heads.
The =ulnar head= (Caput ulnare, ulnaris accessorius) is much smaller,
and is at first superficially situated between the outer and middle
flexors of the carpus, the =radial head= (Caput radiale, radialis
accessorius) is the smallest, and is not always present; it is situated
on the distal two-thirds of the posterior surface of the radius, under
the humeral head. Each of these heads is provided with a tendon. The
principal tendon—that of the humeral head—begins about three or four
inches (8 to 10 cm.) above the carpus. It is soon joined by the small
tendon of the ulnar head, which begins about the middle of the forearm.
The tendon of the radial head fuses with the principal tendon close to
the carpus. The conjoined tendon passes downward through the carpal
canal, being included in the =carpal synovial sheath= with the
superficial flexor tendon, as previously described. Continuing downward,
it is joined about the middle of the metacarpus by a strong fibrous
band, the so-called =inferior= or =subcarpal check ligament= (Caput
tendineum). This is a direct continuation of the posterior ligament of
the carpus. Below this the tendon passes through the ring formed by the
perforatus, then in succession over the sesamoid groove, the inferior
sesamoidean ligaments, and the tendon surface of the third sesamoid, to
its insertion (Figs. 150 and 151). Its terminal part is much widened.
From the distal third of the metacarpus to the distal end of the second
phalanx it is inclosed in the =digital synovial sheath= described in
connection with the perforatus. The =navicular bursa= or =bursa
podotrochlearis= is found between the tendon and the third sesamoid or
navicular bone. The terminal part of the tendon is bound down by the
fibrous sheet described with the fascia.
[Illustration: