Chapter XII: Introduction (10)

The lower border of the cartilage is covered externally by fibers which attach it to the wing of the third phalanx.

THE ARTICULATIONS OF THE PELVIC LIMB

THE SACRO-ILIAC ARTICULATION

This joint (Articulatio sacro-iliaca) is a diarthrosis formed between the auricular surfaces of the sacrum and ilium. These surfaces are not smooth in the adult, but are marked by eminences and depressions, and are covered by a thin layer of cartilage. The joint cavity is a mere cleft, and is often crossed by fibrous bands.

The =capsule= is very close fitting, and is attached around the margins of the articular surfaces. It is reinforced by the =ventral sacro-iliac ligament= (Ligamentum sacro-iliacum ventrale), which surrounds the joint, and is exceedingly strong above.

The =movements= are inappreciable in the adult—stability, not mobility, being the chief desideratum.

The following ligaments may be regarded as accessory to the joint, although not directly connected with it.

The =dorsal sacro-iliac ligament= (Ligamentum sacro-iliacum dorsale breve) is a strong band which is attached to the internal angle (Tuber sacrale) of the ilium and the summits of the sacral spines.

The =lateral sacro-iliac ligament= (Ligamentum sacro-iliacum dorsale longum) is a triangular, thick sheet which is attached in front to the internal angle and border of the ilium above the great sciatic notch, and below to the lateral border of the sacrum. It blends above with the dorsal sacro-iliac ligament, below with the sacro-sciatic ligament, and behind with the coccygeal fascia.

The =sacro-sciatic ligament= (Ligamentum sacrospinosum et tuberosum) is a quadrilateral sheet which completes the lateral pelvic wall. Its upper border is attached to the border of the sacrum and the transverse processes of the first and second coccygeal vertebræ. Its lower border is attached to the superior ischiatic spine and tuber ischii. Between these it bridges over the external border of the ischium and completes the lesser sciatic foramen. The anterior border is concave, and completes the greater sciatic foramen. The posterior border is fused with the vertebral head of the semimembranosus muscle.

The =ilio-lumbar ligament= (Ligamentum ilio-lumbale) is a triangular sheet which attaches the ends of the lumbar transverse processes to the ventral surface of the ilium below the attachment of the longissimus muscle (Fig. 184).

FIG. 153.—LEFT OS COXÆ AND ADJACENT PARTS OF SACRUM AND FEMUR OF
HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

SYMPHYSIS PELVIS

The =symphysis pelvis= is formed by the junction of the two ossa coxarum at the ventral median line. In the young subject the bones are united by a layer of cartilage; in the adult the latter is gradually replaced by bone, the process beginning in the pubic portion and extending backward. The union is strengthened by white fibrous tissue above and below, and a transverse band also covers the anterior border of the pubis (pecten). No appreciable movement occurs even before synostosis takes place.

OBTURATOR MEMBRANE

This (Membrana obturatoria) is a thin layer of fibrous tissue which covers the obturator foramen, leaving, however, a passage (Canalis obturatorius) for the obturator vessels and nerve.

THE HIP JOINT

This joint (Articulatio coxæ) is an enarthrosis formed by the proximal end of the femur and the acetabulum.

=Articular Surfaces.=—The head of the femur presents an almost hemispherical articular surface, which is continued a short distance on the upper surface of the neck. It is more extensive than the socket which receives it. Internally it is cut into by a deep notch for the attachment of the round and pubo-femoral ligaments. The acetabulum is a typical cotyloid cavity. Its articular surface is somewhat crescentic, being deeply cut into internally by the acetabular notch and fossa. It is increased and deepened by a ring of fibro-cartilage, the =cotyloid ligament= (Labrum glenoidale), which is attached to the bony margin; that part of the ligament which crosses the notch is called the =transverse ligament= (Fig. 456).

FIG. 154.—PELVIC LIGAMENTS AND HIP JOINT.

_1_, Dorsal sacro-iliac ligament; _2_, lateral sacro-iliac ligament;
_3_, sacro-sciatic ligament; _4_, greater sciatic foramen; _5_,
lesser sciatic foramen; _6_, line of attachment of intermuscular
septum between biceps femoris and semitendinosus; _7_, capsule of
hip joint; _8_, rectus parvus or capsularis muscle; _9_, outer
tendon of origin of biceps femoris; _10_, internal, _11_, external,
angle of ilium; _12_, shaft of ilium; _13_, superior ischiatic
spine; _14_, pubis; _15_, tuber ischii; _16_, trochanter major;
_17_, semimembranosus; _18_, fifth lumbar spine; _19_, _20_, first
and second coccygeal vertebræ.
]

The =joint capsule= is roomy. It is attached around the margin of the acetabulum and the neck of the femur. It is thickest externally.

The attachment on the femur is about 1 cm. from the margin of the
articular surface, except above, where 2 to 3 cm. of the neck is
intracapsular. A thin oblique band corresponding in direction with the
rectus parvus muscle reinforces the antero-external part of the
capsule; this appears to be the feeble homologue of the very strong
ilio-femoral ligament of man. The capsule is very thin under the
ilio-psoas, and is adherent to the muscle. Internally, its fibrous
part is perforated by the pubo-femoral and round ligaments and the
articular vessels.

The =round ligament= (Ligamentum teres) is a strong hand which is attached in the subpubic groove close to the acetabular notch, passes outward, and ends in the notch on the head of the femur (Fig. 456).

The =pubo-femoral ligament= (Ligamentum accessorium) does not occur in the domestic animals other than the equidæ. It is a strong band detached from the prepubic tendon of the abdominal muscles (Fig. 456). It is directed outward, backward, and upward, passes through the acetabular notch above the transverse ligament, and ends behind the round ligament in the notch on the head of the femur. The origin of the pectineus muscle is perforated by the ligament, which furnishes attachment to the greater part of the fibers of the muscle.

FIG. 155.—LEFT STIFLE JOINT OF HORSE, EXTERNAL VIEW, IN PARTIAL
FLEXION.

The capsules have been removed. _18_, Femur; _20_, patella; _21_,
tibia; _a″_, internal patellar ligament; _b_, external
femoro-patellar ligament; _d_, external semilunar cartilage. (After
Ellenberger-Baum, Anat. für Künstler.)
]

The =synovial membrane= is reflected over the intracapsular parts of these ligaments and covers the fossa acetabuli. A pouch also extends from the acetabular notch for a variable distance along the subpubic groove above the pubo-femoral ligament.

=Movements.=—This joint is capable of all the movements of a ball-and-socket joint, viz., flexion, extension, abduction, adduction, rotation, and circumduction. The greatest range of movement is displayed in flexion and extension. When standing at rest, the joint is partially flexed, the articular angle (in front) being about 100° to 115°. The other movements occur to a very limited extent in normal action.

Abduction appears to be checked by tension of the round ligament. The
pubo-femoral ligament is tensed most promptly by inward rotation of
the thigh.

THE STIFLE JOINT

This joint (Articulatio genu), which corresponds to the knee-joint of man, is the largest and most elaborate of all the articulations. Taken as a whole, it may be classed as a ginglymus, although it is not a typical example of the group. In reality it consists of two joints—the =femoro-patellar= and the =femoro-tibial=.

The =femoro-patellar articulation= (Articulatio femoro-patellaris) is formed between the trochlea of the femur and the articular surface of the patella.

FIG. 156.—LEFT STIFLE JOINT OF HORSE, LATERAL VIEW.

The capsules are distended and the external patellar ligament is
removed.
]

=Articular Surfaces.=—The trochlea consists of two slightly oblique ridges, with a wide and deep groove between them. The inner ridge is much the larger of the two, especially at its upper part, which is wide and rounded. The outer ridge is much narrower, and is more regularly curved; its upper part lies about an inch behind a frontal plane tangent to the inner ridge. The articular surface of the patella is much smaller than that of the trochlea. It is completed internally by a supplementary plate of fibro-cartilage (Fibrocartilago patellæ), which curves over the internal surface of the inner lip of the trochlea. A narrow strip of cartilage is found along the outer border also. The articular cartilage on the trochlea completely covers both surfaces of the inner ridge, but only a narrow marginal area on the external surface of the outer ridge.

=Joint Capsule.=—This is thin and is very capacious. On the patella it is attached around the margin of the articular surface, but on the femur the line of attachment is at a varying distance from the articular surface. On the inner side it is an inch or more from the articular cartilage; on the outer side and above, about half an inch. It pouches upward under the quadriceps femoris for a distance of two or three inches, a pad of fat separating the capsule from the muscle. Below the patella it is separated from the straight ligaments by a thick pad of fat, but inferiorly it is in contact with the femoro-patellar capsules. The joint cavity is the most extensive in the body. It usually communicates with the inner femoro-tibial joint cavity by a slit-like opening situated at the lowest part of the inner ridge of the trochlea. A similar, but smaller, communication with the outer femoro-patellar capsule is often found at the lowest part of the outer ridge.

The inner communication appears to be constant in adult horses, but is
liable to be overlooked on account of the fact that it is covered by a
valvular fold of the synovial membrane. It is about half an inch wide,
and lies under the narrow articular area which connects the trochlea
and internal condyle. The outer communication occurs in 18 to 25 per
cent. of cases, according to Baum. It is instructive to distend this
capsule and thus obtain an idea of its potential capacity and
relations ([Fig. 156]).

FIG. 157.—STIFLE JOINT OF HORSE, FRONT VIEW, IN EXTENSION.

The capsules are removed. _18_, Femur; _20_, patella; _21_, tibia.
(After Ellenberger-Baum, Anat. für Künstler.)
]

=Ligaments.=—The =lateral femoro-patellar ligaments=, external and internal (Ligamentum femoro-patellare fibulare, tibiale), are two thin bands which reinforce the capsule on either side. The external ligament is fairly distinct; it arises from the external epicondyle of the femur just above the lateral femoro-tibial ligament, and ends on the external border of the patella. The internal ligament is thinner and is not distinct from the capsule; it arises above the internal epicondyle, and ends on the patellar fibro-cartilage.

The =patellar ligaments= (Ligamenta patellæ), also called the straight ligaments of the patella, are three very strong bands which attach the patella to the tuberosity of the tibia. The =external patellar ligament= is attached above to the outer part of the anterior surface of the patella, and below to the outer part of the tuberosity. It receives a strong tendon from the biceps femoris muscle. The =middle patellar ligament= extends from the front of the apex of the patella to the lower part of the groove on the tuberosity of the tibia, a bursa being interposed between the ligament and the upper part of the groove. The =internal patellar ligament= is attached above to the patellar fibro-cartilage, and ends on the tuberosity of the tibia, internally to the groove. It is joined by the common aponeurosis of the gracilis and sartorius. These so-called ligaments are, in reality, the tendons of insertion of the quadriceps femoris muscle, and transmit the action of the latter to the tibia; they also function similarly for the other muscles attached to them as noted above.

It will be noticed that the upper attachments are further apart than
the lower ones, so that the ligaments converge below. The inner
ligament is especially oblique. The middle ligament is more deeply
placed than the others, and therefore cannot usually be felt
distinctly in the living animal.

FIG. 158.—SEMILUNAR CARTILAGES AND CRUCIAL LIGAMENTS OF RIGHT STIFLE
OF HORSE.

_F_, Femoral ligament of external cartilage. (After Schmaltz, Atlas d.
Anat. d. Pferdes.)
]

The =femoro-tibial articulation= (Articulatio femoro-tibialis) is formed between the condyles of the femur, the proximal end of the tibia, and the interposed semilunar cartilages.

=Articular Surfaces.=—The condyles of the femur are slightly oblique in direction. The articular surface of the outer one is more strongly curved than that of the inner one; the latter is confluent below with the inner ridge of the trochlea, while the narrow ridge which connects the external condyle with the trochlea is usually non-articular. The saddle-shaped surfaces of the condyles of the tibia are not adapted to the femoral condyles, and are in contact with only a small part of them.

FIG. 159.—PROXIMAL ENDS OF RIGHT TIBIA AND FIBULA OF HORSE, WITH
SEMILUNAR CARTILAGES AND CRUCIAL LIGAMENTS, EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =semilunar cartilages= (Meniscus lateralis, medialis) are two C-shaped or crescentic discs of fibro-cartilage which produce congruence in the articular surfaces. Each has an upper concave surface adapted to the condyle of the femur, and a lower surface which fits the corresponding condyle of the tibia. The external cartilage does not cover the outer and posterior part of the condyle, over which the tendon of origin of the popliteus muscle plays. The peripheral border is thick and convex, the central one very thin and concave. The fibrous ends or =cornua= are attached to the tibia in front of and behind the spine. The external cartilage has a third attachment by means of an oblique band (Ligamentum femorale menisci lateralis) which passes from the posterior cornu to the posterior part of the intercondyloid fossa of the femur.

The cornua of the internal cartilage (Ligamenta tibiæ anterius et
posterius menisci medialis) are attached in front of and behind the
inner eminence of the spine. The anterior cornu of the external
cartilage (Ligamentum tibiæ anterius menisci lateralis) is attached in
front of the outer eminence of the spine. The posterior cornu
bifurcates; the lower branch (Ligamentum tibiæ posterius menisci
lateralis) is inserted at the popliteal notch, the upper (Ligamentum
femorale menisci lateralis) in a small fossa in the extreme posterior
part of the intercondyloid fossa.

The =joint capsule= is attached to the margin of the tibial articular surface, but on the femur the line of attachment is for the greater part about half an inch from the articular margin. It is also attached to the convex borders of the semilunar cartilages and to the crucial ligaments. It is strong posteriorly, but in front it practically consists only of the synovial layer. There are two synovial sacs, corresponding to the double nature of the articular surfaces; they do not usually communicate, and each is partially divided into an upper and a lower compartment by the semilunar cartilage. The inner sac pouches upward about half an inch over the condyle of the femur. The external sac invests the tendon of origin of the popliteus muscle, and also pouches downward about three or four inches (ca. 8 to 10 cm.) beneath the peroneus tertius and long extensor muscles (Fig. 156). As stated above, the outer sac sometimes communicates with the femoro-patellar joint cavity, and the inner sac usually, if not always, does so in the adult.

FIG. 160.—LEFT STIFLE JOINT OF HORSE, INTERNAL VIEW IN EXTREME
EXTENSION.[38]

_18_, Femur; _20_, patella (base); _21_, tibia; _d_, internal
semilunar cartilage. (After Ellenberger-Baum, Anat. für Künstler.)
]

=Ligaments.=—There are four of these—two lateral and two crucial.

The =internal lateral ligament= (Ligamentum collaterale tibiale) is attached above to the prominent internal epicondyle of the femur, and below to a rough area below the margin of the internal condyle of the tibia.

The =external lateral ligament= (Ligamentum collaterale fibulare) is somewhat thicker; it arises from the upper depression on the external epicondyle, and ends on the head of the fibula. It covers the tendon of the origin of the popliteus muscle, and a bursa is interposed between the lower part of the ligament and the margin of the external condyle of the tibia.

The =crucial ligaments= are two strong rounded bands situated mainly in the intercondyloid fossa of the femur, between the two synovial sacs. They cross each other somewhat in the form of an X, and are named according to their tibial attachments. The =anterior crucial ligament= (Ligamentum cruciatum anterius) arises in the central fossa on the tibial spine, extends upward and backward, and ends on the upper part of the inner surface of the external condyle of the femur. The =posterior crucial ligament= (Ligamentum cruciatum posterius) is internal to the preceding, and is somewhat larger. It is attached to an eminence at the popliteal notch of the tibia, is directed upward and forward, and ends in the anterior part of the intercondyloid fossa of the femur.

FIG. 161.—RIGHT STIFLE JOINT OF HORSE; SAGITTAL SECTION PASSING
THROUGH OUTER PART OF INNER RIDGE OF TROCHLEA AND INTERCONDYLOID
FOSSA.

_1_, Anterior crucial ligament; _2_, posterior crucial ligament; _3_,
posterior cornu of internal semilunar cartilage; _4_, femoral
ligament of external semilunar cartilage; _5_, articular artery;
_6_, part of vastus internus.
]

It may be added that these ligaments do not lie in a sagittal plane,
but are somewhat twisted across each other; outward rotation of the
leg untwists and slackens them.

=Movements.=—The principal movements of the stifle joint as a whole are flexion and extension. In the ordinary standing position the articular angle (behind) is about 140° to 150°. Flexion is limited only by contact of the leg with the thigh if the hock is also flexed. Extension is incomplete, _i. e._, the femur and tibia cannot be brought into the same straight line. Rotation is limited, and is freest during semiflexion. The patella glides on the femoral trochlea upward in extension, downward in flexion.

Extension is checked mainly by tension of the crucial and lateral
ligaments. In extreme extension, which is accompanied by slight
outward rotation of the leg, the patella can be pushed upward and
inward so that its fibro-cartilage hooks over the upper end of the
inner ridge of the trochlea, but it will not remain there unless held
in position. When pressure is removed, the base of the patella tips
forward and the cartilage lies upon the most prominent part of the
trochlear ridge. During flexion, which is accompanied by slight inward
rotation of the leg, the condyles of the femur and the semilunar
cartilages glide backward on the tibia; the movement of the external
condyle and cartilage is greater than that of the inner one. In
extreme flexion the patellar and posterior crucial ligaments are
tense; the other ligaments are relaxed. The movement of the patella is
gliding with coaptation, _i. e._, different parts of the opposing
articular surfaces come into contact successively. Only a narrow
transverse strip (ca. 1.5 to 2 cm. wide) of the patella is in contact
with the trochlea at a time.

TIBIO-FIBULAR ARTICULATION

The head of the fibula articulates with a crescentic facet just below the outer margin of the external condyle of the tibia. The =joint capsule= is strong and close. The shaft of the fibula is attached to the external border of the tibia by the =interosseous membrane= of the leg (Membrana interossea cruris); this is perforated about an inch from its proximal end by an opening which transmits the anterior tibial vessels to the front of the tibia. A fibrous cord usually extends from the distal end of the shaft of the fibula to the external malleolus. The latter is the distal end of the fibula which has fused with the tibia. No appreciable movement occurs in this joint.

FIG. 162.—LEFT HOCK JOINT OF HORSE, EXTERNAL VIEW.
]

FIG. 163.—LEFT HOCK JOINT OF HORSE, INTERNAL VIEW.

_21_, Tibia; _24_, tuber calcis; _25_, large metatarsal bone; _47_,
long external lateral ligament; _51_, long internal lateral
ligament. (After Ellenberger-Baum, Anat. für Künstler.)
]

THE HOCK JOINT

This is a composite joint made up of a number of articulations (Articulationes tarsi). These are: (1) The tibio-tarsal articulation; (2) the intertarsal articulations; (3) the tarso-metatarsal articulation.

The =tibio-tarsal articulation= (Articulatio talo-cruralis) is a typical ginglymus formed by the trochlea of the tibial tarsal bone (astragalus or talus) and the corresponding surface of the distal end of the tibia. The ridges and grooves of these surfaces are directed obliquely forward and outward at an angle of about 12° to 15°, with a sagittal plane. The trochlear surface is about twice as extensive as that on the tibia, and its ridges have a spiral curvature. The other articulations are arthrodia, which have joint surfaces and ligaments of such a nature as to allow only a minimal amount of gliding motion.

As in the case of the carpal joints, it is convenient to describe first the common capsule and ligaments, which are the more important practically, and then to consider very briefly the special ligaments.

The fibrous part of the =joint capsule= is attached around the margin of the tibial articular surface above and the metatarsal surfaces below; it is also attached in part to the free surface of the bones which it covers, and blends with the lateral ligaments. Its anterior part (anterior ligament) is rather thin; in distention of the capsule, as in “bog-spavin,” its antero-internal part, which is not bound down by the tendons passing over the joint, forms a fluctuating swelling over the inner ridge of the trochlea. The posterior part (posterior and tarso-metatarsal ligaments) is very thick, and is intimately attached to the tarsal bones. Its superficial face is in part cartilaginous, and forms a smooth surface for the perforans tendon. Superiorly, it pouches upward behind the distal end of the tibia for a distance of about two inches (ca. 5 cm.). Interiorly, it is continued downward, forming the =subtarsal= or =check ligament=, which unites with the perforans tendon about the middle of the metatarsus.

FIG. 164.—LEFT HOCK JOINT OF HORSE, ANTERIOR VIEW.

_21_, Tibia; _22_, trochlea of tibial tarsal bone; _25_, large
metatarsal bone; _50_, dorsal or oblique ligament. (After
Ellenberger-Baum, Anat. für Künstler.)
]

There are four =synovial sacs=: 1. The =tibio-tarsal sac= lubricates the proximal joint, and is much the largest and most important. It is chiefly involved in the swelling produced by excess of fluid in the joint cavity, when the capsule bulges antero-internally and postero-superiorly. 2. The =first intertarsal sac= lines the joints formed by the tibial and fibular tarsal bones above, and the central and fourth tarsals below; it communicates in front with the proximal synovial capsule. 3. The =second intertarsal sac= lubricates the joints formed between the central tarsal and the bones below and on either side. 4. The =tarso-metatarsal sac= lubricates the joints formed between the tarsal and metatarsal bones, those between the proximal ends of the metatarsal bones, and those formed by the third tarsal with the bones on either side.

=Common Ligaments.=—The =external lateral ligament= (Ligamentum collaterale fibulare longum et breve) consists of two distinct bands which cross each other. The =long= (superficial) =ligament= arises on the posterior part of the external malleolus, is directed almost straight downward, and is attached to the fibular and fourth tarsal bones and the large and external small metatarsal bones. It forms a canal for the lateral extensor tendon. The =short= (deep) =ligament= arises on the anterior part of the external malleolus, is directed chiefly backward, and ends on the rough excavation on the external surface of the tibial tarsal and the adjacent surface of the fibular tarsal bone.

FIG. 165.—LEFT HOCK JOINT OF HORSE, POSTERIOR VIEW.

_21_, Tibia; _24_, tuber calcis. (After Ellenberger-Baum, Anat. für
Künstler.)
]

The =internal lateral ligament= (Ligamentum collaterale tibiale longum et breve) also consists of two parts which cross each other. The =long ligament= arises on the posterior part of the internal malleolus, becomes wider below, and is attached on the lower tuberosity of the tibial tarsal, the large and outer small metatarsal bones, and the inner surface of the lower tarsal bones which it covers. The =short ligament= lies largely under cover of the long one. It extends from the anterior part of the internal malleolus, runs backward and somewhat downward, and divides into two branches; one of these ends on the upper tuberosity on the inner surface of the tibial tarsal bone, the other on the sustentaculum tali.

The =plantar= or =calcaneo-metatarsal ligament= (Ligamentum tarsi plantare) is a very strong flat band which covers the outer part of the posterior surface of the tarsus. It is attached to the posterior surface of the fibular and fourth tarsal bones and the proximal end of the external metatarsal bone.

The =dorsal= or =oblique ligament= (Ligamentum tarsi dorsale) is a triangular sheet which is attached above to the lower tuberosity on the inner face of the tibial tarsal bone, and spreads out below on the central and third tarsal bones, and the proximal ends of the large and inner small metatarsal bones, to all of which it is attached.

=Special Ligaments.=—A considerable number of short bands which connect adjacent bones of the tarsus and metatarsus are described by various authors; some of these are quite distinct; others are difficult to isolate. Most of them are not of sufficient importance to justify detailed description.

(1) The =tibial= and =fibular tarsal= bones are united by four bands
(astragalo-calcaneal ligaments). The =internal ligament= extends from
the sustentaculum tali to the adjacent part of the tibia tarsal,
blending with the short lateral ligament. The =external ligament=
extends from the anterior process of the fibular tarsal to the
adjacent part of the external ridge of the trochlea. The =superior
ligament= extends from the posterior margin of the trochlea to the
fibular tarsal. The =interosseous ligament= is deeply placed in the
sinus tarsi between the two bones, and is attached in the rough areas
of the opposed surfaces.

(2) The smaller bones are attached to each other as follows: The
central and third tarsal are united by an interosseous and an oblique
anterior ligament (scaphoido-cunean ligaments). The central and fourth
tarsal are united by an interosseous and a transverse external
ligament (cuboido-scaphoid ligament). The third and fourth tarsals are
similarly connected (cuboido-cunean ligaments). The third tarsal is
joined by an interosseous (intercunean) ligament to the (fused) first
and second tarsals; the latter are connected with the fourth tarsal by
a transverse posterior ligament.

(3) The smaller bones are connected with the upper row as follows: The
central is attached to the tibial tarsal by posterior and interosseous
(astragalo-seaphoid) ligaments, and to the fibular tarsal by a short
oblique (calcaneo-scaphoid) band. The fourth is attached to the
fibular tarsal by interosseous and posterior (calcaneo-cuboid)
ligaments. The (fused) first and second tarsals are connected with the
fibular tarsal by a posterior (calcaneo-cunean) ligament.

(4) The lower tarsal bones are connected with the metatarsus by
tarso-metatarsal ligaments, which are not distinct from the common
ligaments, except in the case of the interosseous ligament between the
third tarsal and metatarsal bones.

FIG. 166.—SAGITTAL SECTION OF HOCK OF HORSE.
]

=Movements.=—These are flexion and extension, which take place at the tibio-tarsal joint. The movements between the tarsal bones, and between the latter and the metatarsus, are so limited as to be negligible so far as the action of the joint as a whole is concerned. In the standing position the articular angle (in front) is about 150° to 160°. Complete extension is prevented by tension of the lateral ligaments. Flexion is checked only by contact of the metatarsus with the leg, provided the stifle joint is also flexed. Owing to the fact that the axis of motion is slightly oblique, the lower part of the limb deviates somewhat outward during flexion.

The movements of the hock joint must correspond with those of the
stifle on account of the tendinous bands in front and behind (peroneus
tertius and flexor perforatus), which extend from the lower part of
the femur to the tarsus and metatarsus.

The remaining joints differ in no material respect from those of the thoracic limb.

COMPARATIVE ARTHROLOGY[39]

JOINTS AND LIGAMENTS OF THE VERTEBRÆ

=Ox.=—The ligamentum nuchæ is better developed than in the horse. The funicular part is clearly divided into two lateral halves, which are round at their occipital attachment, but from the axis backward become rapidly wider and flat. This wide portion is almost sagittal, lies on either side of the vertebral spines, and is covered by the trapezius and rhomboideus muscles. From the highest part of the withers (third thoracic spine) it gradually diminishes in size and fades out in the lumbar region. The lamellar part is thick, and consists of anterior and posterior parts. The anterior part is double; its fibers proceed from the funicular part to the second, third, and fourth cervical spines. The posterior part is single; its fibers extend from the first thoracic spine to the fifth, sixth, and seventh cervical spines.

FIG. 167.—LIGAMENTUM NUCHÆ OF OX.

_a_, Funicular part; _b_, wide portion; _c_, _d_, lamellar part; _e_,
interspinous ligaments; _1_, spinous process of first thoracic
vertebra; _2_, axis. (Ellenberger-Baum, Anat. d. Haustiere.)
]

The inferior common ligament is very strong in the lumbar region.

The intervertebral fibro-cartilages are thicker than in the horse.

The interspinous ligaments of the back and loins consist largely of elastic tissue.

There are no intertransverse joints in the lumbar region.

=Pig.=—The ligamentum nuchæ is represented by a fibrous raphé and thin layers of elastic tissue which extend between the cervical spines.

The atlanto-occipital and atlanto-axial joints resemble those of the dog.

The interspinous ligaments of the neck are elastic.

=Dog.=—The ligamentum nuchæ consists of a small fibrous band which extends from the spine of the axis to the anterior thoracic spines; it may be regarded as a mere fibrous raphé between the right and left muscles.

There are interspinous muscles instead of ligaments in the neck.

There are three ligaments in connection with the odontoid process. The two alar ligaments (Ligamenta alaria) arise on either side of the odontoid process, diverge, and end on either side of the foramen magnum. The transverse ligament of the atlas (Ligamentum transversum atlantis) stretches across the dorsal surface of the odontoid process and binds it down on the ventral arch of the atlas, a bursa being interposed. It is attached on either side to the lateral masses of the atlas.

The two capsules of the atlanto-occipital joint communicate with each other, and usually with the capsule of the atlanto-axial joint also.

ARTICULATIONS OF THE THORAX

=Ox.=—The second to the eleventh costo-chondral joints inclusive are diarthroses with close capsules, reinforced externally. (They are synchondroses in the sheep.) The upper parts of the cartilages are attached to each other by distinct elastic ligaments (Ligamenta intercostalia).

The first pair of chondro-sternal joints are separate from each other; intercrossing fibers unite the costo-chondral junctions above the joints.

The first segment of the sternum forms a diarthrosis with the body. The anterior joint surface is concave, the posterior convex. The joint (Articulatio intersternalis) is surrounded by a close capsule, and the joint surfaces are attached to each by a small intraarticular ligament. Limited lateral movement is possible. (In the sheep the joint is a synchondrosis.)

Both surfaces of the sternum are covered by a layer of fibrous tissue.

=Pig.=—The second to the fifth or sixth costo-chondral joints are diarthroses. The intersternal articulation and the sternal ligaments resemble those of the ox.

=Dog.=—The first chondro-sternal joints do not coalesce.

The internal sternal ligament divides into three bands.

TEMPORO-MANDIBULAR ARTICULATION

=Ox.=—The articular surfaces are of such a character as to permit more extensive lateral movement than in the horse.

The condyle of the mandible is relatively small and is concave
transversely. The temporal articular surface is extensive and is
convex in both directions. The postglenoid process is small.

=Pig.=—The considerable longitudinal diameter of the temporal articular surfaces and the very small size of the postglenoid process allow great freedom of protraction and retraction of the lower jaw. Lateral movement is limited. The posterior ligament is absent.

=Dog.=—The articular surfaces allow extremely little lateral or gliding movement. They are cylindrical in curvature, and the interarticular disc is very thin. The posterior ligament is absent.

The other articulations of the skull are sufficiently described in the Osteology.

Articulations of the Thoracic Limb

SHOULDER JOINT

=Ox.=—The articular angle is about 100°.

=Pig and Dog.=—The joint capsule communicates freely with the bicipital bursa. There is a rudimentary marginal cartilage around the rim of the glenoid cavity. In the dog there is usually a strong band extending from the acromion to the outer part of the capsule; another band (Ligamentum coraco-acromiale) often stretches between the scapular tuberosity and the acromion.

ELBOW JOINT

=Ox.=—No important differences exist. The upper part of the interosseous radio-ulnar ligament is commonly ossified in the adult.

=Pig.=—There are no important differences. The radius and ulna are so firmly united by the interosseous ligament as to prevent any appreciable movement between them.

=Dog.=—The joint capsule is reinforced in front by an oblique ligament which arises on the front of the external condyle of the humerus above the joint surface, and joins the terminal part of the biceps and brachialis below. The external lateral ligament is thick and divides into two parts; the anterior part is attached to the radius and blends with the annular ligament; the posterior part widens below, forming a sort of cap, and is attached to the ulna. The internal lateral ligament also divides into two branches; the anterior branch ends on the inner surface of the neck of the radius; the posterior enters the interosseous space and is attached to both bones. An elastic band (Ligamentum olecrani) extends from the outer surface of the internal epicondyle to the anterior border of the ulna.

There are two radio-ulnar joints. The proximal radio-ulnar joint is included in the capsule of the elbow, but is provided with an annular ligament which extends from the lower part of the external ligament across the front of the proximal end of the radius to the ulna, blending with the biceps and brachialis tendons. The distal joint is formed by a concave facet on the radius and a convex one on the radius, and is surrounded by a tight capsule. The interosseous membrane unites the shafts of the two bones. The movements consist of limited rotation of the radius (ca. 20°), carrying the paw with it. The ordinary position is termed pronation; outward rotation is supination.[40]

THE CARPAL JOINTS

These have the same general arrangement as in the horse. Numerous minor differences naturally exist, but must be excluded from this brief account, which contains only important special features.

The lateral movements are freer, especially in the dog, but flexion is not so complete: the anatomical explanation of these facts lies in the nature of the articular surfaces and certain ligamentous differences. The lateral ligaments are much weaker, the long external one being especially small in the ox. Two oblique, somewhat elastic, bands cross the front of the radio-carpal and intercarpal joints. The proximal one is attached to the distal end of the radius and passes downward and outward to the ulnar carpal bone; the other one connects the radial and fourth carpal bones in a similar fashion.

In the ox the short lateral ligaments are well defined, a ligament connects the accessory carpal with the distal end of the ulna, and strong bands connect the distal bones with the metacarpus.

The interosseous and interordinal ligaments vary with the number of carpal bones present in the different species.

INTERMETACARPAL JOINTS

In the ox the small (fifth) metacarpal bone articulates with the large metacarpal, but not with the carpus. The joint cavity is connected with that of the carpo-metacarpal sac. The proximal end of the small metacarpal bone is attached by a ligament to the fourth carpal, and another band extends from its distal part to the side of the large metacarpal. There is also an interosseous ligament, which is permanent and allows a small amount of movement.

The chief metacarpal bones of the pig, and the second to the fifth of the dog, articulate with each other at their proximal ends, and are connected by interosseous ligaments, which do not, however, unite them closely, as in the horse.

FIG. 168.—DISTAL PART OF LIMB OF OX, SHOWING LIGAMENTS AND TENDONS.
ONE DIGIT AND CORRESPONDING ARTICULAR PART OF METACARPAL BONE ARE
REMOVED.

_a_, Suspensory ligament; _a′_, branch of _a_ to superficial flexor
tendon; _a″_, _a‴_, lateral and central branches of _a_; _b_, deep
flexor tendon; _b′_, branch of _b_ to digit removed; _c_, _c′_,
superficial flexor tendon; _d_, _d′_, intersesamoid ligament (cut);
_e_, interdigital lateral ligament of fetlock joint; _f_, tendon of
common extensor; _g_, proximal interdigital ligament; _h_, digital
annular ligament; _i_, posterior annular ligament of fetlock; _k_,
lateral ligament of pastern joint; _l_, distal interdigital
ligament; _m_, crucial interdigital ligament (cut); _m′_, _m″_,
attachments of _m_ to second phalanx and distal sesamoid bone; _n_,
suspensory ligament of distal sesamoid; _o_, anterior elastic
ligament; _p_, lateral volar ligament of pastern joint; _1_,
metacarpus, sawn off at _1′_; _2_, first phalanx; _3_, second
phalanx; _4_, third phalanx. (Ellenberger-Baum, Anat. d. Haustiere.)
]

METACARPO-PHALANGEAL JOINTS

=Ox.=—There are two joints, one for each digit. The two capsules communicate posteriorly. The two interdigital lateral ligaments (Ligamenta collateralia interdigitalia) result from the bifurcation of a band which arises in the furrow between the divisions of the distal end of the large metacarpal bone; they spread out and end on the proximal ends of the first phalanges. A strong =superior interdigital ligament= (Ligamentum interdigitale), consisting of short intercrossing fibers, unites the middles of the interdigital surfaces of the first phalanges.

Crucial ligaments (Ligamenta phalango-sesamoideæ) connect the sesamoids with the proximal end of the opposite first phalanx.

The intersesamoid ligament connects all four sesamoids, and extends upward much less than in the horse.

The lateral sesamoidean ligaments end almost entirely on the first phalanges.

The superficial inferior sesamoidean ligament is absent. The middle inferior sesamoidean ligaments of each digit are two short strong bands which extend from the distal margins of the sesamoids to the proximal ends of the first phalanges. The deep inferior sesamoidean ligaments are strong and distinctly crucial.

FIG. 169.—LIGAMENTS AND TENDONS OF DIGITS OF PIG, VOLAR VIEW.

_a_, Superficial flexor tendon; _b_, deep flexor tendon; _b′_,
branches of _b_ to accessory digits; _c_, _c′_, annular ligaments;
_d-d‴_, ligaments of accessory digits; _e_, cruciate interdigital
ligaments; _f_, _f′_, spiral band around the flexor tendons of the
accessory digits; _g_, abductor of accessory digit.
(Ellenberger-Baum, Anat. d. Haustiere.)
]

FIG. 170.—LIGAMENTS AND TENDONS OF PAW OF DOG, HIND LIMB, VOLAR VIEW.

_a_, _a′_, Superficial flexor tendon; _b_, tendon to large pad; _c_,
lumbricales muscles; _d_, interossei muscles; _e_, _f_, annular
ligaments at metatarsophalangeal joints; _g_, suspensory ligament of
large pad; _h_, digital annular ligaments; _i_, deep flexor tendon;
_k_, distal sesamoid; _l_, suspensory ligament of _k_; _m_,
suspensory ligament of digital pad; _n_, digital pads.
(Ellenberger-Baum, Anat. d. Haustiere.)
]

The suspensory or superior sesamoidean ligament is more distinctly muscular than in the horse—indeed in the young animal it consists almost entirely of muscular tissue. At the distal third of the metacarpus it divides into three branches. These give rise to five subdivisions, either by bifurcation of the lateral branches or trifurcation of the middle branch. The four lateral bands end on the sesamoid bones and the distal end of the large metacarpal bone, and detach slips to the extensor tendons. The middle band passes through the groove between the two divisions of the distal end of the metacarpus, and divides into two branches which join the tendons of the proper extensors of the digits; it sends fibers also to the interdigital lateral ligaments and to the central sesamoids. About the middle of the metacarpus the suspensory ligament detaches a band which unites lower down with the superficial flexor tendon, thus inclosing the tendon of the deep flexor of the digit; it also blends with the thick fascia of the region. The latter gives off a band on either side to the accessory digits, and a tendinous band descends from each accessory digit to the third phalanx and sesamoid bone, blending with the tendon of the corresponding proper extensor.

=Pig.=—There are four metacarpo-phalangeal joints, each of which has a capsule, lateral, intersesamoidean, and crucial sesamoidean ligaments. Since distinct interosseous muscles are present, there are, of course, no suspensory ligaments.

=Dog.=—There are five metacarpo-phalangeal joints, each having its own capsule and indistinct lateral ligaments. A small sesamoid bone occurs in the anterior part of each capsule, over which the corresponding extensor tendon plays. The intersesamoidean ligaments do not extend above the sesamoids. The cruciate ligaments are present, as well as a fibrous layer which attaches the distal margins of the sesamoids to the posterior surface of the proximal end of the first phalanx.

INTERPHALANGEAL JOINTS

=Ox.=—The two proximal joints have separate capsules, and broad, but rather indistinct, lateral ligaments. Each joint has also two central and two lateral volar ligaments. The central ligaments are largely fused to form a strong band. The lateral ones extend from the borders of the first phalanx to the proximal end of the second phalanx.

The distal interphalangeal joints have, in addition to the capsules and lateral ligaments, bands which reinforce them on either side. The central or interdigital pair arise in the depressions on the distal ends of the first phalanges, receive fibers from the second phalanges, and end on the interdigital surfaces of the third phalanges at the margin of the articular surface. The lateral pair have a similar course, but are thinner, and end on the corresponding third sesamoid. An elastic band crosses the front of the second phalanx obliquely, from the distal end of the first phalanx to the extensor process of the third phalanx.

The =crucial= or =inferior interdigital ligaments= (Ligamenta cruciata interdigitalia) are two strong bands which limit the separation of the digits. They are attached above to the lateral (or abaxial) eminences on the proximal ends of the second phalanges (blending with the lateral ligaments), cross the deep flexor tendon obliquely, and reach the interdigital space, where they intercross and blend. Most of the fibers end on the third sesamoid of the opposite side, but some are attached to the interdigital aspect of the second phalanx and the third sesamoid of the same side. In the sheep there is, instead of the foregoing, a transverse ligament which is attached on either side to the interdigital surfaces of the second and third phalanges and the third sesamoid bone. It is related below to the skin, above to a pad of fat.

=Pig.=—The interphalangeal joints of the chief digits resemble in general those of the ox. The inferior interdigital ligament resembles, however, that of the sheep, and is intimately adherent to the skin. There is, besides, a remarkable arrangement of ligaments which connect the small digits with each other and with the chief digits.

This apparatus is somewhat complex, but its chief features are as
follows: A superior interdigital ligament is attached on either side
to the third phalanges of the small digits, while centrally it blends
with the annular ligaments of the flexor tendons behind the
metacarpo-phalangeal joints of the chief digits. Two bands (central
longitudinal interdigital ligaments) arise on the bases of the small
digits, cross the flexor tendons obliquely downward and inward, pass
through the superior interdigital ligament, and blend below with the
inferior interdigital ligament. Two lateral bands (lateral
longitudinal interdigital ligaments) are attached in common with the
superior interdigital ligaments to the third phalanges of the small
digits, and blend below with the outer part of the inferior
interdigital ligament.

=Dog.=—Each joint has a capsule and two lateral ligaments. The distal joints have also two elastic =dorsal ligaments= (Ligamenta dorsalia), which extend from the proximal end of the second phalanx to the ridge at the base of the third phalanx (Fig. 133). They produce dorsal flexion of the joint, and thus raise or retract the claws when the flexor muscles relax. The distal sesamoids are represented by complementary cartilages attached to the volar margins of the articular surfaces of the third phalanges.

Three interdigital ligaments restrict the spreading apart of the digits. Two of these cross the volar surface of the proximal parts of the chief digits, _i. e._, one for the second and third, the other for the fourth and fifth; they blend with the annular ligaments on either side. The third ligament is attached on either side to the foregoing ligaments and the annular ligaments of the third and fourth digits, and curves downward centrally, ending in the large pad on the paw.

ARTICULATIONS OF THE PELVIC LIMB

SACRO-ILIAC JOINT

This joint and the pelvic ligaments present no very striking differences in the other animals. The sacro-sciatic ligament in the dog is a narrow but strong band which extends from the posterior part of the lateral margin of the sacrum to the tuber ischii; it is the homologue of the ligamentum sacro-tuberosum of man.

HIP JOINT

=Ox.=—The shallowness of the acetabulum is compensated by the greater size of the marginal cartilage. The head of the femur has a smaller radius of curvature than that of the horse, and the articular surface extends a considerable distance outward on the upper surface of the neck. The round ligament is entirely intraarticular; it is small, and sometimes absent. The pubo-femoral or accessory ligament is absent.

There are no important differences in the other animals.

STIFLE JOINT

=Ox.=—There is a considerable communication between the femoro-patellar and inner femoro-tibial joint cavities; this is situated as in the horse, but is wider. A small communication with the external femoro-tibial capsule sometimes occurs. The two femoro-tibial capsules usually communicate. The middle patellar ligament is not sunken, as there is no groove on the tuberosity of the tibia where it is attached. The external patellar ligament fuses completely with the tendon of insertion of the biceps femoris, and a synovial bursa is interposed between them and the external condyle of the femur.

In the other animals there is a single ligamentum patellæ, and the synovial sacs communicate so freely as to constitute a common joint cavity. In the dog the semilunar cartilages are united anteriorly by a transverse ligament, and the posterior part of the capsule contains the two Vesalian sesamoids, which articulate with the condyles of the femur.

TIBIO-FIBULAR JOINTS

=Ox.=—The proximal end of the fibula fuses with the external condyle of the tibia. The distal end remains separate, and forms an arthrosis with the distal end of the tibia; the movement here is imperceptible, as the two bones are closely united by strong peripheral fibers.

=Pig.=—The superior joint is provided with a capsule which is reinforced in front and behind by fibrous tissue.

The interosseous ligament attaches the shaft of the fibula to the outer border of the tibia.

The inferior joint is included in the capsule of the hock joint, and is strengthened by oblique anterior and posterior ligaments. There is also an interosseus ligament.

=Dog.=—The arrangement is essentially the same as in the pig, but there is no interosseous ligament in the inferior joint.

HOCK JOINT

=Ox.=—There is very considerable mobility at the first intertarsal joint, the capsule of which is correspondingly roomy. The short external lateral ligament is attached below on the tibial tarsal only. A strong transverse ligament attaches the external malleolus (distal end of the fibula) to the back of the tibial tarsal bone. The dorsal or oblique ligament is narrow and thin.

=Pig.=—The arrangement in general resembles that of the ox.

=Dog.=—The long lateral ligaments are very small, and the short ones double. The plantar ligament is weak, and ends on the fourth metacarpal bone. No distinct dorsal (or oblique) ligament is present. Movement occurs almost exclusively at the tibio-tarsal joint.

The remaining joints resemble those of the thoracic limb.

THE MUSCULAR SYSTEM

MYOLOGY

The =muscles= (Musculi) are the active organs of motion. They are characterized by their property of contracting when stimulated. Muscular tissue is of three kinds: (_a_) =Striated= or striped; (_b_) =non-striated=, unstriped, or smooth; and (_c_) =cardiac=. Only the first of these varieties will be considered in this section. The striped muscles, being for the most part directly or indirectly connected with the skeleton, are often termed =skeletal= or somatic, while unstriped muscle may be spoken of as =visceral= or splanchnic. The former cover the greater part of the skeleton, and thus in a large measure determine the form of the animal. They are red in color, the shade varying in different muscles and under various conditions.

Muscles vary greatly in =form=, and may be classified as—(_a_) =Long=; (_b_) =short=; (_c_) =flat=; (_d_) =ring-like= or =orbicular=. Long muscles are found chiefly in the limbs, while the flat or broad muscles occur principally in the trunk, where they assist in forming the walls of the body cavities. The ring-like or orbicular muscles circumscribe orifices which they close, and are hence termed =sphincters=.

=Attachments.=—The muscles are attached to =bones=, =cartilages=, =ligaments=, =fasciæ=, or the =skin=. In all cases the attachment is by means of fibrous tissue, the muscle-fibers not coming into direct relation with the bone or cartilage. The perimysium of the muscle may fuse directly with the periosteum or perichondrium (fleshy attachment), or the union may be by means of intermediary fibrous structures called =tendons= or =aponeuroses= (tendinous attachment). Tendons may be funicular, ribbon-like, or in the form of membranous sheets; to the latter the term aponeurosis is commonly applied.

In certain positions, especially where tendons play over joints or are subjected to great pressure, =sesamoid bones= develop in the original tendon tissue. Some of these are large and constant, as the patella and the great sesamoids of the fetlock.

The accessory structures connected with the muscles are the =synovial membranes= and the =fasciæ=.

FIG. 171.—DIAGRAMS OF CROSS-SECTIONS OF TENDON SHEATH (A) AND BURSA
(B); T, TENDON.
]

The =synovial membranes= are arranged in two principal forms: (_a_) =Bursal=; (_b_) =vaginal=. A =bursa= (Bursa mucosa) is a simple sac interposed between the tendon or muscle and some deeper seated structure—most commonly a bony prominence. A =vagina tendinis= or =tendon sheath= differs from a bursa in the fact that the synovial sac is folded around the tendon more or less completely, so that two layers can be distinguished; of these, the inner one adheres closely to the tendon, while the outer lines the groove or canal in which the tendon lies. The two layers are continuous along a fold termed the =mesotendon=. The arrangement is shown in Fig. 171.[41] The articular synovial membranes in some places form extra-articular pouches, which facilitate the play of tendons.

The =fasciæ= are sheets of connective tissue, mainly of the white fibrous variety, with a greater or less admixture of elastic fibers in certain cases. Two layers may usually be recognized. Of these, the =superficial fascia= (Fascia subcutanea) is composed of loose connective tissue which may contain more or less fat and is subcutaneous. The =deep fascia= is composed of one or more layers of dense fibrous tissue spread over the surface of the muscles chiefly. Its deep face may be very loosely attached to the underlying structures or may fuse with the epimysium, tendons, bones, or ligaments. In some parts, especially the limbs, septal plates pass between the muscles and are attached to the bones or ligaments; these are termed =intermuscular septa=. In this way many muscles are inclosed in fibrous sheaths which hold them in position. Not uncommonly special bands stretch across the grooves in which tendons play, converting these into canals. Such bands are termed =vaginal= or =annular ligaments=. The deep fascia is often so distinctly tendinous in structure, furnishing attachment to special tensor muscles, as to render the distinction between fascia and aponeurosis quite arbitrary. Bursæ occur in certain situations between the fascia and underlying structures (subfascial bursæ), or between the fascia and the skin (subcutaneous bursæ).

It is convenient to divide the description of a muscle into seven heads, viz.: (1) Name, followed by important synonyms; (2) position and form; (3) attachments; (4) action; (5) structure; (6) relations; (7) blood and nerve supply.

1. The =name= is determined by various factors, viz.: (_a_) The action, _e. g._, extensor, adductor, etc.; (_b_) the shape, _e. g._, quadratus, triangularis; (_c_) the direction, _e. g._, rectus, obliquus; (_d_) the position, _e. g._, the subscapularis, iliacus; (_e_) the division (into heads, etc.), _e. g._, biceps, triceps, etc.; (_f_) the size, _e. g._, major, minor, etc.; (_g_) the attachments, _e. g._, sterno-cephalicus, mastoido-humeralis; (_h_) the structure, _e. g._, semitendinosus. In most cases two or more of these factors have combined to produce the name, _e. g._, adductor magnus, longus colli, obliquus externus abdominis.

2. The =shape= is, in many cases, sufficiently definite to allow the use of such terms as triangular, quadrilateral, fan-shaped, long, flat, fusiform, ring-like, etc.

3. The =attachments= are in most cases to bone, but many muscles are attached to cartilage, ligaments, fascia, the skin, etc. It is usual to apply the term =origin= to the attachment which always or more commonly remains fixed when the muscle contracts. The term =insertion= designates the movable attachment. Such a distinction cannot always be made, as the action may be reversible, or both attachments may be freely movable.

4. The =action= belongs rather to physiological study, but is briefly indicated in anatomical descriptions.