Part 28

=The Inguinal Canal.=—This term (Canalis inguinalis) is applied to an oblique passage through the posterior part of the abdominal wall.[53] It begins at the internal inguinal or abdominal ring, and extends obliquely downward, inward, and somewhat forward, to end at the external inguinal or subcutaneous ring. Its anterior wall is formed by the fleshy posterior part of the internal oblique muscle, and the posterior wall by the strong tendinous inguinal (Poupart’s) ligament. The average length of the canal, measured along the spermatic cord, is about four inches (ca. 10 cm.). The =internal inguinal ring= (Annulus inguinalis abdominalis) is bounded in front by the thin margin of the internal oblique muscle, and behind by the inguinal ligament. It is directed approximately from the edge of the prepubic tendon toward the external angle of the ilium. Its length is about four or five inches (ca. 10 to 12 cm.). The edge of the muscle is attached to the surface of the ligament here by delicate connective tissue, except where structures intervene between the walls of the canal. Consequently the limits of the ring are not very clearly defined. The =external inguinal ring= (Annulus inguinalis subcutaneus) is a well defined slit in the aponeurosis of the external oblique muscle, situated lateral to the prepubic tendon. Its long axis is directed from the edge of the prepubic tendon outward and forward, and its average length is about four inches (ca. 10 cm.). The canal contains in the male the spermatic cord, the tunica vaginalis, the external cremaster muscle, the external pudic artery and a small satellite vein, and the inguinal lymph vessels and nerves. In the female it contains the mammary vessels and nerves; in the bitch it also lodges the round ligament of the uterus, inclosed in a tubular process of peritoneum.

The two rings do not correspond in direction, so that the length of the canal varies greatly when measured at different points. The inner angle of the internal ring lies almost immediately above that of the external ring, but the outer angle is situated five to six inches (ca. 12 to 15 cm.) from that of the external ring. The inner angles of the external rings are well defined and distinctly palpable in the living subject; they are about three to four inches (ca. 8 to 10 cm.) apart.

=The Prepubic Tendon.=—The =prepubic tendon= is essentially the tendon of insertion of the two recti abdominis, but also furnishes attachment to the obliqui, the graciles, and the pectinei. It is attached to the anterior borders of the pubic bones, including the ilio-pectineal eminences. It has the form of a very strong thick band, with concave lateral borders which form the inner boundaries of the external inguinal rings. Its direction is oblique upward and backward.[54] Its structure is somewhat complex. Most of the fibers of the posterior part extend from one ilio-pectineal eminence to the other. The fibers which belong to the recti curve in to the median line. The aponeuroses of the internal oblique muscles are inserted into its abdominal surface, and the inguinal ligaments are attached to and continue across it in arciform fashion. The anterior part of the tendon of origin of the gracilis is fused with it ventrally, and many of the fibers of the pectineus arise from it. It gives off on either side a strong round band, the so-called =pubo-femoral= or =accessory ligament=, which is inserted into the fossa of the head of the femur with the round ligament (_vide_ hip joint).

MUSCLES OF THE THORACIC LIMB

I. THE MUSCLES OF THE SHOULDER GIRDLE (Figs. 177, 178, 179, 186)

This group consists of those muscles which connect the thoracic limb with the head, neck, and trunk. The group naturally falls into two divisions—dorsal and ventral.[55]

A. DORSAL DIVISION

This division consists of two layers which overlie the proper muscles of the neck and back.

FIRST LAYER

1. =Trapezius.=—This is a flat, triangular muscle, the base of the triangle corresponding with the spine. It is divided by an aponeurotic portion into two divisions:

(=a=) =Trapezius cervicalis.=—_Origin._—The funicular portion of the ligamentum nuchæ, from the second cervical to the third thoracic vertebra.

_Insertion._—The spine of the scapula and the fascia of the shoulder and arm.

(=b=) =Trapezius thoracalis s. dorsalis.=—_Origin._—The supraspinous ligament, from the third to the tenth thoracic vertebra.

_Insertion._—The tubercle of the spine of the scapula.

_Action._—Acting as a whole, to elevate the shoulder; the cervical portion draws the scapula forward and upward and the thoracic portion draws it backward and upward.

_Structure._—The muscle arises by a short, thin aponeurosis, from which the fibers of the flat fleshy portion converge to the spine of the scapula and the aponeurosis which separates the two portions. The cervical fascia joins the ventral edge of the cervical portion to the mastoido-humeralis, or the two muscles may unite here.

_Relations._—Superficially, the skin and fascia; deeply, the rhomboideus, latissimus dorsi, supraspinatus, infraspinatus, deltoid, splenius, serratus magnus, and anterior deep pectoral muscles, and the cartilage of the scapula.

_Blood-supply._—Deep cervical and intercostal arteries.

_Nerve-supply._—Spinal accessory nerve.

SECOND LAYER

This consists of two muscles—the rhomboideus and the latissimus dorsi.

2. =Rhomboideus.=—This consists of two portions:

(=a=) =Rhomboideus cervicalis s. cervicis.=—_Origin._—The funicular portion of the ligamentum nuchæ, from the second cervical to the second thoracic vertebra.

_Insertion._—The internal surface of the cartilage of the scapula.

(=b=) =Rhomboideus thoracalis s. dorsalis.=—_Origin._—The spinous processes of the second to the seventh thoracic vertebra by means of the dorso-scapular ligament.

_Insertion._—The inner surface of the cartilage of the scapula.

_Action._—To draw the scapula upward and forward. When the limb is fixed the cervical portion will elevate the neck.

_Structure._—The cervical portion is narrow, pointed at its anterior extremity, and lies along the funicular part of the ligamentum nuchæ, to which it is attached by short tendon bundles. The fibers are directed for the most part longitudinally. The thoracic portion is quadrilateral in shape, and its fibers are nearly vertical. Its deep face is intimately attached to the dorso-scapular ligament.

_Relations._—Superficially, the skin and fascia (over a small area), the trapezius, and the cartilage of the scapula; deeply, the dorso-scapular ligament, the splenius, complexus, longissimus dorsi, and serratus anticus.

_Blood-supply._—Dorsal and superior cervical arteries.

_Nerve-supply._—Sixth cervical nerve.

3. =Latissimus Dorsi.=—This is a wide muscle which has the form of a right-angled triangle. It lies for the most part under the skin and panniculus, on the lateral wall of the thorax, from the spine to the arm.

_Origin._—The lumbo-dorsal fascia—and by this means from the lumbar and thoracic spines as far forward as the highest point of the withers.

_Insertion._—The internal tubercle of the humerus, in common with the teres major.

_Action._—To draw the humerus upward and backward and flex the shoulder joint. If the limb is advanced and fixed, it draws the trunk forward.

_Structure._—The muscle arises by a wide aponeurosis, which fuses with that of the serratus posticus and with the lumbo-dorsal fascia. The muscular portion is at first rather thin, but by the convergence of its fibers becomes thicker as it approaches the arm. The anterior fibers pass almost vertically downward over the dorsal angle of the scapula and its cartilage. The posterior fibers are directed downward and forward. The thick belly formed by the convergence of these passes under the triceps to end on the flat tendon of insertion, which is common to this muscle and the teres major.

_Blood-supply._—Subscapular, intercostal, and lumbar arteries.

_Nerve-supply._—Brachial plexus (eighth cervical and dorsal roots).

B. VENTRAL DIVISION

1. =Mastoido-humeralis= (M. brachiocephalicus; levator humeri).—This muscle extends along the side of the neck from the head to the arm. It is incompletely divisible into two portions.

_Origin._—(1) The mastoid process of the petrous temporal bone and the occipital crest; (2) the wing of the atlas and the transverse processes of the second, third, and fourth cervical vertebræ.

_Insertion._—The deltoid tuberosity and the curved rough line which extends from this to the distal extremity of the humerus.

_Action._—When the head and neck are fixed, to draw the limb forward, extending the shoulder joint. When the limb is fixed, to extend the head and neck, if the muscles act together; acting separately, to incline the head and neck to the same side.

_Structure._—As already mentioned, the muscle is capable of incomplete division into two parts, the line of division being indicated by the emergence of superficial branches of the ventral divisions of the cervical nerves. The mastoid portion (M. cleido-mastoideus) partly overlaps the other portion (M. cleido-transversarius), which lies dorsal to it. The former is attached to the mastoid process and the occipital bone by a broad tendon which fuses with that of the splenius and trachelo-mastoideus; it is also attached to the tendon of insertion of the sterno-cephalicus by aponeurosis. The dorsal portion is attached to the transverse processes by four fleshy digitations. The belly of the muscle is adherent superficially to the cervical fascia and the panniculus, and deeply to the subscapulo-hyoideus. In front of the shoulder its deep face is marked by a tendinous intersection of variable development.[56] Here the muscle becomes wider, covers the shoulder joint, passes between the brachialis and biceps, and is inserted by means of a wide tendon which it shares with the superficial pectoral muscle.

_Relations._—Superficially, the skin, cervical fascia, the parotid gland, the panniculus, brachialis, and branches of the cervical nerves; deeply, the splenius, trachelo-mastoideus, rectus capitis anterior major, omo-hyoideus, serratus magnus, anterior deep pectoral and biceps muscles, the inferior cervical artery, the prescapular lymph glands, and branches of the cervical nerves. The ventral edge of the muscle forms the dorsal boundary of the jugular furrow. The dorsal border may be in contact with the cervical trapezius, or separated from it by a variable interval.

_Blood-supply._—Inferior cervical, carotid, and vertebral arteries.

_Nerve-supply._—Spinal accessory and cervical nerves.

The =pectoral fascia= is a thin membrane covering the surface of the pectoral muscles, to which it is, for the most part, pretty intimately attached. It detaches a layer which passes between the superficial and deep pectorals. At the posterior edge of the triceps another layer is given off, which passes on the outer surface of this muscle to blend with the scapular fascia; the deeper layer becomes continuous with the subscapular and cervical fasciæ.

The =pectoral muscles= form a large fleshy mass which occupies the space between the ventral part of the chest-wall and the shoulder and arm. They are clearly divisible into a superficial and a deep layer. The superficial layer may be subdivided into two portions by careful dissection; the deep layer is clearly made up of two muscles.

2. =Superficial pectoral= (M. pectoralis superficialis).

(=a=) =Anterior superficial pectoral= (portio clavicularis s. descendens; pectoralis anticus).—This is a short, thick, somewhat rounded muscle, which extends between the anterior part of the sternum and the front of the arm. It forms a distinct prominence on the front of the breast, which is easily recognized in the living animal.

_Origin._—The cariniform cartilage of the sternum.

_Insertion._—(1) The curved line of the humerus with the mastoido-humeralis; (2) the fascia of the arm.

_Action._—To adduct and advance the limb.

_Structure._—The belly of the muscle is convex on its superficial face, but deeply it is flattened where it overlaps the posterior superficial pectoral. Here the two muscles are usually pretty intimately attached to each other, and care must be exercised in making the separation. The tendon of insertion blends with that of the mastoido-humeralis and with the fascia of the arm. At the middle line of the breast a furrow occurs between the two muscles; laterally, another furrow, containing the cephalic vein, lies between the muscle and the mastoido-humeralis.

_Relations._—Superficially, the skin, fascia, and panniculus; deeply, the posterior division, the deep pectoral, and the biceps. The cephalic vein lies in the groove between this muscle and the mastoido-humeralis.

(=b=) =Posterior superficial pectoral= (portio sternocostalis; pectoralis transversus).—This is a wide muscular sheet which extends from the ventral edge of the sternum to the fascia on the inner surface of the forearm.

_Origin._—(1) The ventral edge of the sternum as far back as the sixth cartilage; (2) a fibrous raphé common to the two muscles.

_Insertion._—(1) The fascia on the proximal third of the forearm; (2) the curved line of the humerus with the preceding muscle.

_Action._—To adduct the limb and to tense the fascia of the forearm.

_Structure._—It is thin and pale, and mixed with a good deal of fibrous tissue. The right and left muscles fuse at a median fibrous raphé. The tendon of insertion unites with the fascia on the inner side of the forearm for the most part; only a small part in front, about an inch in width, is attached to the humerus.

_Relations._—Superficially, the skin, fascia, and the preceding muscle; deeply, the deep pectoral, the biceps, and the brachialis; at the elbow, the posterior radial vessels, the median nerve, and the inner and middle flexors of the carpus.

3. =Deep pectoral= (M. pectoralis profundus).—This muscle is much thicker and more extensive in the horse than the superficial pectoral. It consists of two distinct portions.

(=a=) =Anterior deep pectoral= (portio prescapularis; pectoralis parvus).—This division is prismatic and extends from the anterior part of the lateral surface of the sternum to the cervical angle of the scapula.

_Origin._—The anterior half of the lateral surface of the sternum and the cartilages of the first four ribs.

_Insertion._—The aponeurosis which covers the supraspinatus at its dorsal end, and the scapular fascia.

_Action._—To adduct and retract the limb; when the limb is advanced and fixed, to draw the trunk forward.

_Structure._—The muscle is almost entirely fleshy. It describes a curve (convex anteriorly), passing at first forward, then upward over the front of the shoulder, a little to its inner side, and finally upward and backward along the anterior border of the supraspinatus. It is loosely attached to the latter muscle, and terminates in a pointed end which becomes more firmly attached near the cervical angle of the scapula.

_Relations._—Superficially, the skin and fascia, the panniculus, superficial pectoral, trapezius, and mastoido-humeralis muscles, the cephalic vein, and the inferior cervical artery; deeply, the posterior deep pectoral, biceps, supraspinatus, omo-hyoideus, and serratus magnus muscles, the brachial vessels, and the branches of the brachial plexus of nerves.

(=b=) =Posterior deep pectoral= (portio humeralis s. ascendens; pectoralis magnus).—This is much the largest of the pectoral group in the horse. It is somewhat triangular or fan-shaped.

_Origin._—(1) The abdominal tunic; (2) the xiphoid cartilage and ventral aspect of the sternum; (3) the cartilages of the fourth to the ninth ribs.

_Insertion._—(1) The internal tuberosity of the humerus; (2) the external lip of the bicipital groove; (3) the tendon of origin of the coraco-brachialis.

_Action._—To adduct and retract the limb; if the limb is advanced and fixed, to draw the trunk forward.

_Structure._—This muscle is also almost entirely fleshy. Its posterior part is wide and thin, but as the muscle is traced forward, it becomes narrower and much thicker. It passes forward and slightly upward in a gentle curve to its insertion. The humeral insertion is just below that of the inner division of the supraspinatus. Part of the fibers are inserted by means of a tendinous band which binds down the tendon of the biceps and is attached to the external lip of the bicipital groove, and a small part is attached to the tendon of origin of the coraco-brachialis.

_Relations._—Superficially, the skin, panniculus, and superficial pectoral; deeply, the abdominal tunic, the external oblique, the rectus abdominis et thoracis, the brachial vessels, and branches of the brachial plexus of nerves. The external thoracic vein lies along the outer or upper border.

_Blood-supply._—Internal and external thoracic, inferior cervical, anterior circumflex, and intercostal arteries.

_Nerve-supply._—Pectoral (or thoracic) nerves, from the brachial plexus.

4. =Serratus magnus= (M. serratus ventralis).—This is a large, fan-shaped muscle, situated on the lateral surface of the neck and thorax. It derives its name from the serrated ventral edge of its thoracic portion. It consists of a cervical and a thoracic portion.

(=a=) =Cervical part= (M. serratus cervicis; levator scapulæ hominis).

_Origin._—The transverse processes of the last four or five cervical vertebræ.

_Insertion._—The anterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage.

(=b=) =Thoracic part= (M. serratus thoracis; serratus anterior hominis).

_Origin._—The external surfaces of the first eight or nine ribs.

_Insertion._—The posterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage.

_Action._—The two muscles form a sort of sling in which the trunk is suspended. Contracting together, they raise the thorax; contracting singly, the weight is shifted to the limb on the side of the muscle acting. The two parts can contract separately and are antagonistic in their action on the scapula. The cervical part draws the base of the scapula toward the neck, while the thoracic part has the opposite action; these effects concur in the backward and forward swing of the limb respectively. With the limb fixed, the cervical part extends (raises) the neck or inclines it laterally. The thoracic part may act as a muscle of forced inspiration.

_Structure._—In the domesticated animals there is no such clear division of the muscle as is found in man and the apes. On account of the difference in action, however, it seems desirable to distinguish the two portions. The cervical part is thick and almost entirely fleshy. The thoracic part has on its superficial face a thick, tendinous layer which may sustain the weight of the trunk when the muscle substance relaxes. The ventral edge presents distinct digitations, the last four of which alternate with those of the obliquus externus abdominis, and are covered by the abdominal tunic. The fourth, fifth, and sixth digitations extend nearly to the distal ends of the ribs. The last digitation is small and may be absent. Exceptionally additional digitations may be attached to the tenth or eleventh rib or to the fascia over the intercostal muscles. The fibers converge to the insertion, which is thick and is intersected by elastic lamellæ derived from the dorso-scapular ligament.

_Relations._—Superficially, the mastoido-humeralis, trapezius, deep pectoral, subscapularis, teres major, latissimus dorsi, panniculus carnosus, the abdominal tunic, the brachial vessels, and the long thoracic nerve; deeply, the splenius, complexus, longissimus, transversalis costarum, the ribs and external intercostal muscles, and branches of the superior cervical and dorsal arteries.

_Blood-supply._—Superior cervical, dorsal, vertebral, and intercostal arteries.

_Nerve-supply._—Brachial plexus.

II. THE MUSCLES OF THE SHOULDER

Under this head will be described those muscles which arise on the scapula and end on the arm; they may be divided into two groups—one covering the dorsum, the other the venter of the scapula.

The =superficial fascia= of the shoulder and arm contains the panniculus carnosus of this region, and may be considered to be continued on the inner side of the limb by the subscapular fascia.

The =thoracic= or =scapulo-humeral portion= of the =panniculus= (M. cutaneus scapulæ et humeri) arises by a thin aponeurosis from the ligamentum nuchæ at the withers. The fleshy portion begins over the upper part of the scapula and extends to the elbow. Its fibers have in general a dorso-ventral direction. It is continuous behind with the abdominal portion.

The =deep fascia= of the shoulder and arm (Fascia omobrachialis) is much more developed and important. It is strong and tendinous, and is intimately adherent to the muscles on the outer surface of the scapula, between which it detaches =intermuscular septa=, which are attached to the spine and borders of the scapula. The brachial portion is, for the most part, only loosely attached to the underlying muscles, for which it forms sheaths; it is attached to the humerus, especially to the lips of the bicipital groove and the deltoid tuberosity. It blends distally with the tendon of insertion of the biceps, and is continued by the antibrachial fascia.

A. EXTERNAL GROUP (Figs. 178, 179)

1. =Deltoid= (M. deltoideus; long abductor of the arm; scapular portion of the deltoid of man).—This lies partly on the triceps in the angle between the scapula and humerus, partly on the infraspinatus and teres minor.

_Origin._—(1) The upper part of the posterior border of the scapula; (2) the spine of the scapula, by means of the strong aponeurosis which covers the infraspinatus.

_Insertion._—The deltoid tuberosity of the humerus.

_Action._—To flex the shoulder joint and abduct the arm.

_Structure._—The origin of the muscle is partly aponeurotic, partly fleshy. The aponeurosis fuses with that of the infraspinatus; the posterior part is attached to the scapula immediately in front of the origin of the long head of the triceps. The belly of the muscle lies for the most part in a cavity formed in the triceps. It is widest about its middle.

_Relations._—Superficially, the skin, fascia, panniculus, and mastoido-humeralis; deeply, the infraspinatus, teres minor, triceps, and brachialis muscles, and branches of the posterior circumflex artery and axillary nerve.

_Blood-supply._—Subscapular artery (chiefly through the posterior circumflex).

_Nerve-supply._—Suprascapular and axillary nerves.

2. =Supraspinatus.=—This muscle occupies the supraspinous fossa, which it fills, and beyond which it extends, thus coming in contact with the subscapularis.

_Origin._—The supraspinous fossa, the spine, and the lower part of the cartilage of the scapula.

_Insertion._—The inner and outer lips of the bicipital groove.

_Action._—To extend the shoulder joint. It also assists in preventing dislocation.

_Structure._—The surface of the muscle is covered by a strong aponeurosis, from the deep face of which many fibers arise. The muscle is thin at its origin from the cartilage, but becomes considerably thicker below. At the neck of the scapula it divides into two branches, between which the tendon of origin of the biceps emerges. These branches, fleshy superficially, tendinous deeply, are inserted into the lips of the bicipital groove. They are united by a fibrous membrane already mentioned in connection with the deep pectoral muscle; some fibers are attached to this membrane and the capsule of the shoulder joint.

_Relations._—Superficially, the skin, fascia, panniculus, trapezius, and mastoido-humeralis; deeply, the scapula and its cartilage, the subscapularis muscle, and the suprascapular vessels and nerve; in front, the anterior deep pectoral muscle; behind, the spine of the scapula and infraspinatus muscle.

_Blood-supply._—Suprascapular and posterior circumflex arteries.

_Nerve-supply._—Suprascapular nerve.

3. =Infraspinatus.=—This muscle occupies the greater part of the infraspinous fossa.

_Origin._—The infraspinous fossa and the scapular cartilage.

_Insertion._—(1) The outer tuberosity of the humerus, distal to the outer insertion of the supraspinatus; (2) the posterior eminence of the outer tuberosity.

_Action._—To abduct the arm and rotate it outward.[57] It also plays the part of a lateral ligament.

_Structure._—This muscle is also covered by a strong aponeurosis, from which many fibers arise, and by means of which the deltoid is attached to the spine of the scapula. A thick tendinous layer partially divides the muscle into two strata, and, coming to the surface at the shoulder joint, constitutes the chief means of insertion. This tendon, an inch or more (3 cm.) in width, passes over the posterior eminence of the external tuberosity of the humerus; it is bound down by a fibrous sheet, and a =synovial bursa= is interposed between the tendon and the bone. When the long insertion is cut and reflected, the short insertion, partly tendinous, partly fleshy, is exposed.

_Relations._—Superficially, the skin, fascia, panniculus, trapezius, and deltoid; deeply, the scapula and its cartilage, the shoulder joint and capsule, the long head of the triceps, the teres minor, and the nutrient artery of the scapula.

_Blood-supply._—Subscapular artery.

_Nerve-supply._—Suprascapular and axillary nerves.

4. =Teres minor.=—This is a much smaller muscle than the foregoing. It lies chiefly on the triceps, under cover of the deltoid and infraspinatus.

_Origin._—(1) The rough lines on the distal and posterior part of the infraspinous fossa; (2) a small part of the posterior border of the scapula, about its middle; (3) a tubercle near the rim of the glenoid artery.

_Insertion._—The deltoid tuberosity and a small area just above it.

_Action._—To flex the shoulder joint and to abduct the arm; also to assist in outward rotation.