Part 27
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.