Chapter XIV: Introduction (12)

_Structure._—Similar to the preceding muscle. The fibers are directed downward and forward and terminate in seven or eight digitations, one or two of which cover the posterior teeth of the anticus. The aponeurosis blends with that of the latissimus dorsi.

_Relations._—Superficially, the latissimus dorsi and external oblique; deeply, the longissimus dorsi, transversalis costarum, external intercostals, serratus anticus, and the ribs.

_Blood-supply._—Intercostal and lumbar arteries.

_Nerve-supply._—Thoracic nerves.

THIRD LAYER

6. =Transversalis costarum= (M. ilio-costalis).—This long, segmental muscle extends, as its name indicates, across the series of ribs, in contact with the outer edge of the longissimus dorsi.

_Origin._—(1) The transverse processes of the second and third lumbar vertebræ and the deep layer of the lumbo-dorsal fascia. (2) The anterior borders and external surface of the last fifteen ribs.[45]

_Insertion._—The posterior borders of the ribs and the transverse processes of the last two or three cervical vertebræ.

_Action._—Chiefly to depress and retract the ribs and so help in expiration. Acting together, they may assist in extending this spine, acting singly in inclining it laterally.

_Structure._—This muscle presents a distinct segmental arrangement. It is composed of a series of bundles, the fibers of which are directed forward and a little downward and outward. From these are detached two sets of tendons. The superficial tendons spring from the outer edge of the muscle. They are flat and are about half an inch in width. Each crosses two or three intercostal spaces, to be inserted on the posterior border of a rib. The deep tendons are detached from the dorsal part of the deep face of the muscle. Each passes backward across one or two intercostal spaces to its origin on the anterior border or outer surface of a rib. Small bursæ may be found between the ribs and tendons.

_Relations._—Superficially, the dorsal serrati and the complexus; deeply, the external intercostals and the ribs. The lumbar origin is covered by the longissimus. The superior (deep) cervical and dorsal vessels cross the surface of the muscle at the first and second intercostal spaces respectively, and branches of the intercostal vessels and nerves emerge between the transversalis and longissimus; here a fascial layer dips in between the two.

_Blood-supply._—Intercostal arteries.

_Nerve-supply._—Dorsal branches of the thoracic nerves.

7. =Longissimus.=[46]—This is the largest and longest muscle in the body. It extends from the sacrum and ilium to the neck, filling up the space between the spinous processes medially and the lumbar transverse processes and the upper ends of the ribs below; consequently it has the form of a three-sided prism.

_Origin._—(1) The internal angle, crest, and adjacent part of the ventral surface of the ilium; (2) the first three sacral spines; (3) the lumbar and thoracic spines and the supraspinous ligament.

_Insertion._—(1) The lumbar transverse and articular processes; (2) the thoracic transverse processes; (3) the spinous and transverse processes of the last four cervical vertebræ; (4) the outer surfaces of the ribs, except the first.

_Action._—Acting with its fellow, it is the most powerful extensor of the back and loins; by its cervical attachment it assists in extending the neck. By its costal attachment it may also assist in expiration. Acting singly, it flexes the spine laterally.

FIG. 181.—RIGHT PORTION OF CROSS-SECTION OF BACK OF HORSE. SECTION IS
CUT THROUGH SEVENTH THORACIC VERTEBRA.

_a_, Ligamentum nuchæ; _b_, trapezius muscle; _c_, cartilage of
scapula; _d_, latissimus dorsi; _e_, panniculus carnosus; _f_,
rhomboideus dorsi; _g_, serratus magnus; _h_, serratus anticus;
_h′_, lumbo-dorsal fascia, which divides below into three layers;
_i_, transversalis costarum; _k_, levator costæ; _k′_, internal
intercostal muscle; _l_, longissimus dorsi; _m_, _m′_, seventh
thoracic vertebra; _n_, head of eighth thoracic vertebra; _o_, head
of eighth rib; _p_, seventh rib; _r_, intercostal artery and nerve;
_s_, skin. The fasciæ are indicated by dotted lines. (After
Ellenberger, in Leisering’s Atlas.)
]

_Structure._—This is quite complex. The posterior part of the muscle is greatly developed and constitutes the common mass of the loins. This is covered by a strong aponeurosis which blends with the supraspinous and sacro-iliac ligaments, and is attached to the crest and inner (sacral) angle of the ilium and the first and second sacral spines; it furnishes origin to the lumbar portion of the middle gluteus. In its course further forward the muscle receives fasciculi from the lumbar and thoracic spines, but diminishes somewhat in volume. At the withers it divides into two parts. The dorsal division (spinalis et semispinalis), reinforced by bundles from the first four thoracic spines, passes forward under the complexus to be inserted into the spines of the last four cervical vertebræ. The ventral division passes forward and downward underneath the serratus magnus to be inserted into the ribs and the transverse processes of the last four cervical vertebræ. Three sets of fasciculi may be distinguished, viz.: (1) spinal, which are superficial and internal; (2) transverse, attached to the transverse and articular processes, which are internal and deep; (3) costal, which are external.

_Relations._—Superficially, the middle gluteus, the lumbo-dorsal fascia, the latissimus dorsi, dorsal serrati, serratus magnus, and complexus; deeply, the multifidus, intertransversales, external intercostals, levatores costarum, the ligamentum nuchæ, and its fellow of the opposite side (in the neck).

_Blood-supply._—Dorsal, superior cervical, intercostal, and lumbar arteries.

_Nerve-supply._—Dorsal branches of the thoracic and lumbar nerves.

8. =Multifidus dorsi= (semispinalis of the back and loins).—This is a long segmental muscle which covers the sides of the spinous processes of the vertebræ from the sacrum to the neck.

_Origin._—(1) The lateral portion of the sacrum; (2) the articular processes of the lumbar vertebræ; (3) the transverse processes of the thoracic vertebræ.

_Insertion._—The spinous processes of the first two sacral, the lumbar, thoracic, and last cervical vertebræ.

_Action._—Acting with its fellow, it is an extensor of the spine; acting singly, it flexes it laterally.

_Structure._—It is composed of a series of bundles which are directed obliquely forward and upward. Each fasciculus passes over several vertebræ to its insertion. In the posterior part of the series the bundles cross two or three vertebræ and are inserted into the summits of the spines. Anteriorly, the bundles have a more horizontal direction, cross three to five vertebræ, and are inserted into the sides of the spines considerably below their summits. A further complication consists in the fusion of several bundles into a common insertion.

_Relations._—Superficially, the longissimus; deeply, the vertebral spines.

_Blood-supply._—Intercostal and lumbar arteries.

_Nerve-supply._—Dorsal branches of the thoracic and lumbar nerves.

9. =Intertransversales lumborum.=—These are very thin muscular and tendinous strata, which occupy the spaces between the transverse processes of the lumbar vertebræ except the fifth and sixth.

_Action._—To assist in flexing the loins laterally.

_Relations._—Superficially, the longissimus; deeply, the quadratus lumborum.

_Blood-supply._—Lumbar arteries.

_Nerve-supply._—Lumbar nerves.

THE FASCIA AND MUSCLES OF THE TAIL

The muscles of the tail are inclosed in the strong =coccygeal fascia=, which is continuous in front with the gluteal fascia and blends with the lateral sacro-iliac ligament. At the root of the tail it is loosely attached to the subjacent muscles, but further back it is intimately adherent to them. From its deep face are detached septa which pass between the muscles to become attached to the vertebræ.

1. =Coccygeus= (Compressor coccygis; ischio-coccygeus).—This is a flat, triangular muscle which lies chiefly between the sacro-sciatic ligament and the rectum.

_Origin._—The pelvic surface of the sacro-sciatic ligament near the ischiatic spine.

_Insertion._—The first four coccygeal vertebræ and the coccygeal fascia.

_Action._—Acting together, to depress (flex) the tail, compressing it over the perineum; acting singly, to depress and incline it to the same side.

_Structure._—The origin of the muscle is aponeurotic. Becoming fleshy, its fibers pass upward and backward and divide into two layers. The outer layer is attached to the vertebræ, the inner to the fascia; included between the two lie the intertransversales. When the tail is raised, the ventral edges of the muscles produce a distinct ridge at either side of the anus.

_Relations._—Outwardly, the sacro-sciatic ligament and the semimembranosus; inwardly, the rectum and the sacro-coccygeus ventralis muscle. The internal pudic artery crosses the outer face of the origin of the muscle.

2. =Sacro-coccygeus superior= (M. sacro-coccygeus dorsalis medialis; erector s. extensor coccygis).—This muscle lies along the dorso-median aspect of the tail, in contact with its fellow.

_Origin._—The last three sacral spines and some of the coccygeal spines.

_Insertion._—The dorsal surface of the coccygeal vertebræ.

_Action._—Acting together, to elevate (extend) the tail; acting singly, to elevate and incline it laterally.

_Structure._—The muscle has a strong rounded belly. It is inserted by means of short tendons which fuse with those of the next muscle.

_Relations._—Superficially, the coccygeal fascia; internally, its fellow; laterally, the sacro-coccygeus lateralis; deeply, the vertebræ.

3. =Sacro-coccygeus lateralis= (M. sacro-coccygeus dorsalis lateralis; curvator coccygis).—This muscle lies immediately lateral to the preceding.

FIG. 182.—CROSS-SECTION OF TAIL OF HORSE CLOSE TO ANUS.

_34_, Coccygeal vertebra; _35_, sacro-coccygeus superior; _36_,
intertransversalis; _37_, sacro-coccygeus inferior; _38_, coccygeus;
_39_, recto-coccygeus; _40_, coccygeal fascia. (After
Ellenberger-Baum, Anat. für Künstler.)
]

FIG. 183.—MUSCLES OF PERINEUM OF HORSE.

_a_, Coccygeus; _b_, retractor ani; _c_, _c′_, sphincter and externus;
_d_, recto-coccygeus; _e_, sacro-coccygeus inferior lateralis; _f_,
retractor penis; _g_, bulbo-cavernosus; _h_, ischio-cavernosus; _i_,
internal pudic artery; _k_, anus; _l_, penis. (After
Ellenberger-Baum, Top. Anat. d. Pferdes.)
]

_Origin._—The sides of the sacral spines, with the multifidus, and the transverse processes of the sacral and coccygeal vertebræ.

_Insertion._—The lateral surface of the coccygeal vertebræ, except the first four.

_Action._—Acting with its fellow, to assist the preceding muscle in elevating the tail; acting singly, to incline it to the same side.

_Structure._—This muscle appears to be a direct continuation of the multifidus dorsi. The belly is fusiform and receives reinforcing fasciculi from the transverse processes of the sacrum. This is succeeded by bundles of tendons, as many as four lying alongside of each other.

_Relations._—Superficially, the lateral sacro-iliac ligament and the coccygeal fascia; dorsally, the sacro-coccygeus superior; ventrally, the intertransversales; deeply, the vertebræ and a branch of the lateral coccygeal artery and accompanying vein and nerve.

4. =Intertransversales caudæ= (Mm. intertransversarii caudæ).—These consist of muscular bundles which lie on the lateral aspect of the tail, between the preceding muscle and the sacro-coccygeus inferior. They begin on the lateral edge of the sacrum and occupy the spaces between the transverse processes, to which they are attached. They are, however, not arranged in a strict segmental manner.

_Action._—Acting together, to fix the coccygeal vertebræ; acting singly, to assist in lateral flexion.

5. =Sacro-coccygeus inferior= (Mm. sacro-coccygei ventrales; depressor coccygis).—This muscle lies on the ventral aspect of the sacrum and coccyx. It is composed of two portions, described by Bourgelat and the German anatomists as separate muscles.

(_a_) The =outer portion= (M. coccygeus ventralis lateralis) is much the larger of the two. It arises from the outer part of the ventral surface of the sacrum, about as far forward as the third foramen, and is inserted into the transverse processes and ventral surface of the coccygeal vertebræ.

(_b_) The =inner portion= (M. sacro-coccygeus ventralis medialis) arises from the ventral surface of the sacrum internal to the preceding muscle and the first eight coccygeal vertebræ, and is inserted into the ventral surfaces of the coccygeal vertebræ.

_Action._—Acting together, to depress (flex) the tail; acting singly, to incline it laterally also.

_Structure._—The outer portion has a somewhat compressed belly, and receives bundles from the transverse processes of the coccygeal vertebræ. The inner portion is much smaller and shorter, reaching only about to the middle of the tail.

_Relations._—Ventrally, the pelvic and coccygeal fasciæ; dorsally, the sacrum, coccygeal vertebræ, and the intertransversales; laterally, the sacro-sciatic ligament, the coccygeus, and the coccygeal fascia; medially, its fellow, the recto-coccygeus, and the middle coccygeal vessels. Branches of the lateral coccygeal vessels and nerves lie between the outer division of the muscle and the intertransversales.

_Blood-supply._—Middle and lateral coccygeal arteries.

_Nerve-supply._—Coccygeal nerves.

THE MUSCLES OF THE THORAX

These consist of seven muscles or sets of muscles, which are attached to the thoracic vertebræ, to the ribs and their cartilages, and to the sternum. Functionally, they are muscles of respiration.

1. =Levatores costarum.=—These constitute a series of small muscles which occupy and overlie the upper ends of the intercostal spaces.

_Origin._—The transverse processes of the thoracic vertebræ.

_Insertion._—The external surfaces of the upper ends of the ribs posterior to the vertebral origin.

_Action._—To draw the ribs forward in inspiration.

_Structure._—Arising by tendinous fibers, each muscle passes backward and outward and expands at its insertion. Some fibers pass over one rib and are inserted on a succeeding one. At the first and last spaces the muscle cannot be distinguished from the external intercostal, of which it is in reality only a specially developed part.

_Relations._—Superficially, the longissimus dorsi; deeply, the ribs, internal intercostal muscles, and the intercostal vessels and nerves.

_Blood-supply._—Intercostal arteries.

_Nerve-supply._—Intercostal nerves.

2. =External intercostals= (Mm. intercostales externi).—Each of these occupies an intercostal space, from the levatores to the sternal extremity of the rib. They do not occupy the intercartilaginous spaces.

_Origin._—The posterior borders of the ribs.

_Insertion._—The anterior borders and external surfaces of the succeeding ribs.

_Action._—To draw the ribs forward in inspiration.

_Structure._—The fibers are directed downward and backward. There is a considerable admixture of tendinous tissue. The thickness of the muscles gradually diminishes toward the lower ends of the spaces.

FIG. 184.—DORSAL AND LUMBAR REGIONS OF HORSE, DORSAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

_Relations._—Superficially, the serratus magnus, latissimus dorsi, serratus anticus and posticus, longissimus dorsi, transversalis costarum, rectus thoracis, deep pectorals, obliquus abdominis externus, and panniculus; deeply, the internal intercostals and (in the upper part of the spaces) the intercostal vessels and nerves.

_Blood-supply._—Intercostal and internal thoracic arteries.

_Nerve-supply._—Intercostal nerves.

3. =Internal intercostals= (Mm. intercostales interni).—These occupy the entire length of the intercostal spaces, including their interchondral portion.

_Origin._—The anterior borders of the ribs and their cartilages.

_Insertion._—The posterior borders of the preceding ribs and cartilages.

_Action._—To draw the ribs backward in expiration. It seems probable, however, that the intercartilaginous portion is inspiratory.[47]

_Structure._—The direction of the fibers is oblique downward and forward. There is a smaller amount of tendinous tissue than in the external set, and the thickness diminishes from below upward. In the upper part of the spaces fibers sometimes cross a rib in a fashion similar to the subcostals of man. A thin aponeurosis separates the internal from the external intercostal muscle in each space.

_Relations._—Superficially, the levatores costarum and the external intercostals; deeply, the endothoracic fascia and pleura, the transversus thoracis, diaphragm, transversus abdominis, and the internal thoracic and asternal vessels. In the upper part of the intercostal spaces the intercostal vessels and nerves lie between the internal and external intercostal muscle, but below they lie chiefly on the deep face of the inner muscle.

_Blood-supply._—Intercostal and internal thoracic arteries.

_Nerve-supply._—Intercostal nerves.

4. =Retractor costæ.=—This is a small triangular muscle which lies behind the last rib, chiefly under cover of the serratus posticus.

_Origin._—The transverse processes of the first three or four lumbar vertebræ by means of the lumbar fascia.

_Insertion._—The posterior border of the last rib.

_Action._—To retract the last rib.

_Structure._—The muscle arises by a thin aponeurosis. Its fibers are parallel to those of the adjacent internal oblique.

_Relations._—Superficially, the serratus posticus and external oblique; deeply, the transversus abdominis.

_Blood-supply._—Lumbar arteries.

_Nerve-supply._—Lumbar nerves.

5. =Rectus thoracis= (M. transversus costarum; lateralis sterni).—This is a thin muscle which lies under cover of the deep pectoral muscles. It is directed obliquely backward and downward, and crosses the lower part of the first three intercostal spaces.

_Origin._—The outer surface of the first rib, below the scalenus.

_Insertion._—The cartilage of the third or fourth rib. The aponeurosis usually joins the rectus abdominis. It may reach the sternum.

_Action._—It may assist in inspiration or concur with the rectus abdominis.

_Relations._—Superficially, the deep pectoral muscles; deeply, the intercostal muscles and the ribs.

6. =Transversus thoracis= (Triangularis sterni).—This is a flat muscle situated on the thoracic surface of the sternum and the cartilages of the sternal ribs.

_Origin._—The sternal ligament.

_Insertion._—The cartilages of the ribs, from the second to the eighth inclusive.

_Action._—It draws the costal cartilages inward and backward, thus assisting in expiration.

_Structure._—Each muscle has the form of a scalene triangle, of which the base is the strongly serrated external border. The muscle contains a good deal of tendinous tissue. The anterior bundles are directed forward and outward; the posterior backward and outward.

_Relations._—Superiorly, the endothoracic fascia and pleura; inferiorly, the costal cartilages, the internal intercostal muscles, and the internal thoracic vessels.

_Blood-supply._—Internal thoracic artery.

_Nerve-supply._—The intercostal nerves.

7. =Diaphragm.=—This is a broad, unpaired muscle which forms a partition between the thoracic and abdominal cavities.[48] In outline it has some resemblance to a palm-leaf fan. In form it is dome-shaped, compressed laterally. On a median section it is seen to have a general direction downward and forward from the lumbar vertebræ to the xiphoid cartilage. The thoracic surface is strongly convex, and is covered by the pleura. The abdominal surface is deeply concave, and is covered for the most part by the peritoneum. The muscle consists of a peripheral fleshy portion, two muscular crura, and a tendinous center.

_Attachments._—(1) =Costal part=: The cartilages of the ninth to the fifteenth ribs, and the last three ribs at an increasing distance from their sternal ends.

(2) =Sternal part=: The upper surface of the xiphoid cartilage.

(3) =Lumbar part=: (_a_) The =right crus= is attached to the inferior common ligament, and by this means to the first four or five lumbar vertebræ. (_b_) The =left crus= is attached in a similar fashion to the first and second lumbar vertebræ.

_Action._—It is the principal muscle of inspiration and increases the longitudinal diameter of the chest. The contraction produces a general lessening of the curvature of the diaphragm. In the expiratory phase the costal part and crura lie directly on the body walls, so that the bases of the lungs are in contact with the tendinous center and sternal portion only. In ordinary inspiration the fleshy rim recedes from the chest-wall, so that the bases of the lungs move backward to a line about parallel with the curve formed by the cartilages of the asternal ribs, and about four or five inches (ca. 10 to 12 cm.) therefrom. It is stated that the inspiratory movement affects the tendinous center much less than the fleshy part, since the posterior vena cava is firmly attached to the former. It should be noted, however, that the direction of the thoracic part of the vena cava in the expiratory phase is oblique upward and backward. Thus it would seem that there is no anatomical reason why the diaphragm should not move as a whole in ordinary inspiration at least.

_Structure._—The =costal part= (Pars costalis) consists of a series of digitations which meet, or are separated by a very narrow interval from, the transversus abdominis; between the two are the asternal vessels. From the tenth rib backward the attachments are to the ribs at an increasing distance above the costo-chondral junctions. Thus at the last rib the attachment is four to five inches (10 to 12 cm.) from the lower end. Anteriorly, the origin extends along the ninth costal cartilage to the xiphoid cartilage. From these points of origin the fibers curve inward and forward to join the tendinous center. The =right crus= (Crus dextrum) is about twice as thick as the left one and is also longer. It arises by a strong tendon from the lumbar vertebræ (by means of the inferior common ligament). The tendon is succeeded by a rounded belly which leaves the vertebral column, at the last thoracic vertebra. Passing downward and forward, its fibers spread out and join the tendinous center. The =left crus= (Crus sinistrum) arises by a thin tendon from the inferior common ligament at the first and second lumbar vertebræ. This is succeeded by a triangular belly which joins the central tendon. Between the crura and the attachment to the last rib the edge of the muscle crosses the ventral surface of the psoas muscles without attachment, forming the so-called lumbo-costal arch; here the thoracic and abdominal cavities are separated only by the serous membranes and some areolar tissue. The =tendinous center= (Centrum tendineum) resembles the periphery in outline, but is more elongated. It is partially divided into right and left halves by the descent of the crura into it. It is composed largely of radiating fibers, but many interlace in various directions; this is specially evident around the foramen venæ cavæ, which is encircled by fibers. A strong tendinous layer extends across below the hiatus œsophageus.

FIG. 185.—DIAPHRAGM OF HORSE, ABDOMINAL SURFACE.

_1_, Inferior common ligament; _2_, _2′_, tendons of crura; _3_,
lumbar sympathetic trunks; _4_, external spermatic nerve; _5_, _5′_,
great splanchnic nerves; _6_, cisterna chyli (opened); _7_, _7′_,
œsophageal continuations of vagus nerves; _8_, lymph gland; _9_,
coronary ligament of liver (cut); _10_, right lateral ligament of
liver (cut); _11_, left lateral ligament of liver (cut); _12_,
falciform ligament of liver (cut); _A.l._, lumbo-costal arch;
_N.i._, intercostal nerve; _C.d._, right crus; _C.s._, left crus;
_A_, aorta; _Ca_, cœliac artery; _Oe._, œsophagus; _V.c._, posterior
vena cava; _V.p._, phrenic veins. (After Schmaltz, Atlas d. Anat. d.
Pferdes.)
]

The diaphragm is pierced by three foramina. (1) The =hiatus aorticus= is an interval between the two crura and below the last thoracic vertebra. It contains the posterior aorta, vena azygos, and cisterna chyli. (2) The =hiatus œsophageus= (or foramen sinistrum) perforates the right crus near its junction with the tendinous center. It is situated a little to the left of the median plane and two or three inches below the thirteenth thoracic vertebra (in expiration). It transmits the œsophagus, the vagus nerves, and the œsophageal branch of the gastric artery. (3) The =foramen venæ cavæ= (s. dextrum) pierces the tendinous center about an inch to the right of the median plane, and about six inches below the twelfth thoracic vertebra (in expiration). The vena cava is firmly attached to the margin of the opening.[49]

_Relations._—The thoracic surface is related to the endothoracic fascia, pleuræ, pericardium, the bases of the lungs, and the ribs in part. The abdominal surface is in great part covered by the peritoneum, and is related chiefly to the liver, stomach, spleen, pancreas, kidneys and adrenals, and the anterior flexures of the colon. The sympathetic and splanchnic nerves pass between the crus and the psoas muscles on each side. The asternal vessels perforate the edge of the muscle at the ninth costo-chondral joint.

_Blood-supply._—Phrenic and asternal arteries.

_Nerve-supply._—Phrenic nerves (from the fifth, sixth, and seventh cervical nerves).

THE ABDOMINAL MUSCLES

The =superficial fascia= covering the lateral and ventral walls of the abdomen is continuous dorsally with the lumbo-dorsal fascia, in front with the thoracic fascia, and behind with the gluteal fascia. In the inguinal region it forms part of the fascia of the penis or of the mammary glands. At the lower part of the flank it forms a fold which is continuous with the fascia of the thigh near the stifle joint. In this fold are the precrural lymph glands. Medially it blends with the linea alba. It contains the abdominal portion of the panniculus carnosus.

The =abdominal panniculus= (M. cutaneus maximus) covers a large part of the lateral surface of the abdomen and thorax. The general direction of its fibers is longitudinal. Its posterior extremity forms the basis of the fold of the flank. Its anterior extremity is inserted by a thin tendon into the internal tuberosity of the humerus, with the posterior deep pectoral muscle. Its dorsal edge may be indicated by a line drawn from the upper end of the thirteenth rib to the fold of the flank. Its ventral limit corresponds to a line drawn from the fold to a point about a handbreadth external to the umbilicus, and from here to a point a little above the level of the elbow. Behind the shoulder the fibers become oblique and blend with the scapular portion. The aponeurosis extends ventrally to the linea alba, dorsally to the supraspinous ligament. The muscle is intimately adherent to the skin, so that special care is necessary in removing the latter. Its deep face, on the other hand, is loosely attached to the underlying structures by a quantity of areolar tissue which is more or less loaded with fat (panniculus adiposus) in animals in good condition. The large external thoracic (“spur”) vein is partially embedded in the lower part of the muscle. Its action is to twitch the skin.

The =deep fascia= is represented chiefly by the =abdominal= tunic (Tunica flava abdominis). This is a sheet of elastic tissue which assists the muscles in supporting the great weight of the abdominal viscera. It is practically coextensive with the obliquus externus, which it covers. Ventrally it is thick, and is intimately adherent to the aponeurosis of the muscle. Laterally it becomes thinner and is more easily separated, although fibers from it dip in between the muscle-bundles. It is continued for some distance upon the intercostals and serratus magnus. Traced forward, it passes as a thin layer beneath the posterior deep pectoral muscle. Posteriorly it is attached to the external angle of the ilium. In the inguinal region it forms the deep fascia of the prepuce or of the mammary glands.

The =linea alba= is a median fibrous raphé which extends from the xiphoid cartilage to the symphysis pubis. It is formed chiefly by the junction of the aponeuroses of the oblique and transverse muscles, but partly by longitudinal fibers. A little behind its middle is a cicatrix which indicates the position of the umbilical opening of the fœtus.

1. =Obliquus abdominis externus= (great oblique; external oblique of the abdomen).—This is the most extensive of the abdominal muscles. It is a broad sheet, irregularly triangular in shape, widest behind. Its fibers are directed chiefly downward and backward.

_Origin._—(1) The outer surfaces of the last fourteen ribs, and the fascia over the external intercostal muscles; (2) the lumbo-dorsal fascia.

_Insertion._—(1) The linea alba and the prepubic tendon; (2) the external angle and shaft of the ilium; (3) the internal femoral fascia.

_Action._—(1) To compress the abdominal viscera, as in defecation, micturition, parturition, and expiration; (2) to flex the trunk (arch the back); (3) acting singly, to flex the trunk laterally.

_Structure._—The muscle is composed of a fleshy portion and an aponeurosis. The =muscular portion= lies on the lateral wall of the thorax and abdomen. It arises by a series of digitations, the anterior four of which alternate with those of the serratus magnus. The origin may be indicated by a slightly curved line (concave above) drawn from the lower part of the fifth rib to the external angle of the ilium. The fibers are directed downward and backward and terminate on the aponeurosis, except in the flank, where they are almost horizontal in direction. The line of junction is a curve (concave above) extending from the upper edge of the posterior deep pectoral muscle toward the external angle of the ilium. The =aponeurosis= is intimately attached to the abdominal tunic, and its fibers are largely interwoven ventrally with those of the aponeurosis of the internal oblique. By this fusion is formed the outer sheath of the rectus abdominis, which blends at the linea alba with that of the opposite side. In the inguinal region the aponeurosis divides into two chief layers; one of these curves upward and backward and is inserted into the external angle of the ilium and the prepubic tendon. Between these points the aponeurosis is much strengthened and is called the =inguinal= (Poupart’s) =ligament= (Ligamentum inguinale). This curves upward and somewhat forward, becomes thin, and blends with the iliac fascia. It forms the posterior wall of the inguinal canal. About an inch (ca. 2 to 3 cm.) in front of the pubis and about two inches (ca. 4 to 5 cm.) from the median plane the aponeurosis is pierced by a slit-like opening,[50] the =external inguinal ring= (Annulus inguinalis subcutaneus). This is the external orifice of the inguinal canal. Its long axis is directed outward and forward, and is about four inches (ca. 10 cm.) in length. The inner angle is rounded and is well defined by the junction of the inguinal ligament with the prepubic tendon, but the outer angle is not so sharply defined. The borders or pillars are constituted by arciform fibers of the aponeurosis of the external oblique (Crus mediale, laterale). The =femoral layer= of the aponeurosis (Lamina femoralis) passes on to the inner surface of the thigh, where it blends with the femoral fascia. A thin =iliac layer= (Lamina iliaca) passes over the outer margin of the iliacus to the external border of the ilium.

_Relations._—Superficially, the skin, the panniculus carnosus, the abdominal tunic, and the posterior deep pectoral muscle; deeply, the ribs and their cartilages, the intercostal muscles, the internal oblique, the contents of the inguinal canal, and the sartorius and gracilis.

_Blood-supply._—Intercostal and lumbar arteries.

_Nerve-supply._—Intercostal and lumbar nerves.

2. =Obliquus abdominis internus= (small oblique; internal oblique of the abdomen).—This muscle is situated under the preceding one. Its fibers are directed downward, forward, and inward. It forms a triangular curved sheet with the base behind.

FIG. 186.—VENTRAL MUSCLES OF HORSE, AFTER REMOVAL OF GREATER PART OF
PANNICULUS CARNOSUS AND ABDOMINAL TUNIC. SARTORIUS AND GRACILIS
REMOVED FROM RIGHT THIGH.

_c_, Mastoido-humeralis; _g_, anterior superficial pectoral; _g′_,
posterior superficial pectoral; _h_, posterior deep pectoral; _l′_,
aponeurosis of obliquus abdominis externus; _t_, pectineus; _u_,
ilio-psoas; _v_, semimembranosus; _w_, gracilis; _x_, sartorius;
_y_, vastus internus; _z_, _z′_, adductor; _14_, sternum; _14′_,
cariniform cartilage. (After Ellenberger-Baum, Anat. f. Künstler.)
]

_Origin._—The external angle of the ilium and the adjacent part of the inguinal (Poupart’s) ligament.

_Insertion._—(1) The cartilages of the last four or five ribs; (2) the linea alba and the prepubic tendon.

_Action._—Similar to that of the preceding muscle.

_Structure._—Like the external oblique, it is composed of a fleshy portion and an aponeurosis. The fleshy portion is fan-shaped, and is situated chiefly in the flank. At its iliac origin it is covered by a glistening aponeurosis. Traced inward and downward along the abdominal surface of the inguinal ligament, the muscular origin is found to become much thinner, and also becomes loosely attached to the ligament. About four or five inches (ca. 10 to 12 cm.) from the linea alba the muscle separates from the ligament and forms the anterior wall of the inguinal canal. The abdominal orifice of the canal, the =internal inguinal ring=[51] (Annulus inguinalis abdominalis), is found here. It is normally a narrow slit, bounded in front by the edge of the internal oblique, and behind by the inguinal ligament. The =aponeurosis= is to a great extent blended with that of the external oblique, being, indeed, considerably interwoven with it ventrally. Where it covers the rectus abdominis it is attached to the tendinous inscriptions of that muscle.

_Relations._—Superficially, the external oblique; deeply, the rectus abdominis, transversus abdominis, and the peritoneum.

_Blood-supply._—Circumflex iliac, lumbar, and intercostal arteries.

_Nerve-supply._—Ventral branches of the lumbar nerves.

3. =Rectus Abdominis.=—This muscle is confined to the ventral part of the abdominal wall; it extends from the lower part of the chest-wall to the pubis.

_Origin._—The cartilages of the fifth to the ninth ribs inclusive, and the adjacent surface of the sternum.

_Insertion._—The pubis, by means of the prepubic tendon.

_Action._—Similar to that of the oblique muscles. It is specially adapted to flex the lumbo-sacral joints and the lumbar and thoracic parts of the spine.

_Structure._—The fibers of the muscle are directed longitudinally. Nine to eleven transverse bands of fibrous tissue extend in an irregular manner across the muscle. These are termed =inscriptiones tendineæ=. They strengthen the muscle and serve to prevent separation of its fibers. The width of the muscle is greatest about its middle.

_Relations._—Superficially, the aponeuroses of the oblique muscles (which constitute the external rectus sheath), and the posterior deep pectoral; deeply, the transversus, intercostals, the cartilages of the ribs, and the sternum. The posterior abdominal artery runs along the outer edge of the muscle posteriorly, and the anterior abdominal artery on or in its anterior part.

_Blood-supply._—Anterior and posterior abdominal arteries.

_Nerve-supply._—Intercostal and lumbar nerves.

4. =Transversus abdominis.=—This muscle, named from the general direction of its fibers, is a triangular curved sheet. Its lateral part is muscular, its ventral aponeurotic.

_Origin._—(1) The inner surfaces of the distal ends or the cartilages of the asternal ribs, meeting the costal attachment of the diaphragm; (2) the transverse processes of the lumbar vertebræ, by means of the deep layer of the lumbo-dorsal fascia.

_Insertion._—The xiphoid cartilage and the linea alba.

_Action._—Similar to that of the oblique muscles.

_Structure._—The =muscular part= is a sheet of parallel bundles of fibers, directed downward and inward. It is thickest over the cartilages of the ribs, and from here it thins out greatly toward the aponeurosis and the lumbar region. The fibers of the =aponeurosis= directly continue those of the fleshy part. Posteriorly it becomes extremely thin and blends with the aponeuroses of the oblique muscles. It covers the deep face of the rectus, so forming the internal rectus sheath.

_Relations._—Superficially, the oblique and straight muscles, the retractor costæ, the cartilages of the asternal ribs, and the internal intercostal muscles; deeply, the transversalis fascia and the peritoneum. The =transversalis fascia= is little developed in the horse, and is very thin in emaciated subjects, but in animals in good condition it contains a good deal of fat. It blends with the iliac fascia and descends into the inguinal canal. The asternal artery runs along the interval between the origin of the transversus and the costal part of the diaphragm. The intercostal nerves pass down over the external surface of the muscle, to which they give branches. Branches of the first three lumbar nerves are similarly disposed further back.

_Blood-supply._—Intercostal, lumbar, and asternal arteries.

_Nerve-supply._—Intercostal and lumbar nerves.

5. =Cremaster externus.=—This small muscle may be regarded as a detached portion of the internal oblique, with which it blends at its origin (Figs. 272, 450).

_Origin._—The iliac fascia, near the origin of the sartorius.

_Insertion._—The tunica vaginalis communis.

_Action._—To raise the tunica vaginalis, and with it the testicle.

_Structure._—The muscle arises by a thin aponeurosis which is succeeded by a flat muscular belly about one and a half to two inches (ca. 4 to 5 cm.) in width.[52] It passes down the inguinal canal on the postero-external surface of the tunica vaginalis, to which it is rather loosely attached. On reaching the point where the tunic is reflected on to the tail of the epididymis, the muscle is firmly attached to the outer surface of the tunic by short tendinous fibers.

_Relations._—The muscle lies between the peritoneum and the fascia transversalis in front and the iliac fascia and inguinal ligament behind. On reaching the internal ring it descends the inguinal canal on the postero-external surface of the tunica vaginalis communis.

_Blood-supply._—External spermatic or cremasteric artery.

_Nerve-supply._—External spermatic nerve.

=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