III. THE MUSCLES OF THE THORAX.
| Intercostales externi | Levatores costarum |
| Intercostales interni | Serratus posterior superior |
| Transversus thoracis | Serratus posterior inferior |
| Sternocostalis | Diaphragm |
| Intercostales intimi | Subcostales |
The Intercostales, externi et interni (fig. 620) are two thin layers of muscular and tendinous fibers occupying each of the intercostal spaces. They are named external and internal from their surface relations–the external being superficial to the internal.
The Intercostales externi are eleven in number on each side. Their attachments extend from the tubercles of the ribs behind, to near the cartilages of the ribs in front, where each is replaced by an aponeurotic layer named the anterior intercostal membrane, which is continued forwards to the sternum. Each muscle arises from the lower border of one rib, and is inserted into the upper border of the rib below. In the lower two spaces they extend to the ends of the .rib-cartilages, and in the upper two or three spaces they do not quite reach the ends of the ribs. They are thicker than the Internal intercostals, and their fibers are directed obliquely downwards and laterally on the back of the thorax, and downwards, forwards, and medially on the front.
Nerve-supply:-Both groups are supplied by the intercostal nerves.
Actions.-The actions of the External and Internal intercostal muscles are very difficult to determine. It is generally believed that both groups act as elevators of the ribs, although some investigators have attributed the opposite action to the Internal intercostals. In addition, they form strong elastic supports, which prevent the intercostal spaces being drawn in or bulged out during respiration. The anterior portions of the Internal intercostals probably have an additional function in keeping the sternocostal and interchondral joint surfaces in apposition, the posterior portions of the External intercostals performing a similar function for the costovertebral joints.
The Transversus thoracis is a thin plane of muscular and tendinous fibers situated upon the inner surface of the chest wall. It forms an incomplete investment and is divisible into three parts, viz.: the Subcostales, the Intercostales intimi and the Sternocostalis.
The Subcostales consist of muscular and aponeurotic fasciculi, and are usually well developed only in the lower part of the thorax; each arises from the inner surface of one rib near its angle, and is inserted into the inner surface of the second or third rib below. Their fibers run in the same direction as those of the Internal intercostals. They intervene between the intercostal vessels and nerves and the pleura.
Nerve -supply.-The Subcostals are supplied by intercostal nerves.
Actions.-The Subcostals depress the ribs.
The Intercostales intimi are placed on the deep surfaces of the Internal intercostals and their fibers pass in the same direction. Each is attached to the inner surfaces of two adjoining ribs. Poorly developed, sometimes absent, in the upper intercostal spaces, the Intercostales intimi become gradually more extensive from above downwards. Below each covers middle two-fourths of the intercostal space, and its posterior border is edge to edge with the lateral border of the corresponding Subcostalis muscle. Like the Subcostals, the Intercostales intimi separate the intercostal vessels and nerves from the pleura (fig. 598).
Actions. – The Intercostales intimi act in the same way as the Internal intercostals.
The Sternocostalis (Transversus thoracis) is situated upon the inner surface of the front wall of the in both the spaces shown a part of the Intercostalis chest (fig. 597). It arises from the lower third of the posterior surface of the body of the sternum, from the posterior surface of the xiphoid process, and from the posterior surfaces of the costal cartilages of the lower three or four true ribs near their sternal ends. Its fibers diverge as they pass upwards and laterally, to be inserted by slips into the lower borders and inner surfaces of the costal cartilages of the second, third, fourth, fifth, and sixth ribs. The lowest fibers of this muscle are horizontal, and are continuous with those of the Transversus abdominis; the intermediate fibers are oblique, while the highest are almost vertical. This muscle varies in its attachments not only in different subjects but on opposite sides of the same subject. Like the other parts of the Transversus thoracis the Sternocostalis separates the intercostal nerves from the pleura.
Nerve-supply – The Sternocostalis is supplied by intercostal nerves.
Actions-The Sternocostalis draws down the costal cartilages to which it is attached.
The Levatores costarum (fig. 595), twelve in number on each side, are strong bundles which arise from the ends of the transverse processes of the seventh cervical and upper eleven thoracic vertebrae; they pass obliquely downwards and laterally, parallel with the posterior borders of the External intercostals, and each is inserted into the upper edge and outer surface of the rib immediately below the vertebra from which it takes origin, between the tubercle and the angle (Levatores costarum brevis). Each of the four lower muscles divides into two fasciculi, one of which is inserted as above described: the other passes down to the second rib below its origin (Levatores costarum longus).
Nerve supply – The Levatores costarum are supplied by the intercostal nerves.
Actions.–The Levatores costarum being inserted near the fulcra of the ribs can have little or no elevating action on the ribs, but they may rotate the necks of the ribs in a forward direction; they are said to act as rotators and lateral flexors of the vertebral column.
The Serratus posterior superior is a thin, quadrilateral muscle, situated at the upper and posterior part, of the thorax. It arises by a thin aponeurosis from the lower part of the ligamentum nuchae, from the spines of the seventh cervical and upper two or three thoracic vertebrae and from the supraspinous ligament. Inclining downwards and laterally it is inserted, by four fleshy digitations, into the upper borders and outer surfaces of the second, third, fourth, and fifth ribs, a little beyond their angles. It lies superficial to the upward continuation of the lumbar fascia and is hidden by the Rhomboid muscles.
Nerve-supply.-The Serratus posterior superior is supplied by the second, third, fourth and fifth intercostal nerves.
Actions.-The Serratus posterior superior elevates the ribs to which it is attached.
The Serratus posterior inferior (fig. 618) is situated at the junction of the thoracic and lumbar regions: it is of an irregularly quadrilateral form, broader than the preceding muscle, and separated from it by a wide interval. It arises by a thin aponeurosis from the spines of the lower two thoracic and upper two or three lumbar vertebrae and from the supraspinous ligament; this aponeurosis is intimately blended with the lumbar fascia. Passing obliquely upwards and laterally, it becomes fleshy, and is inserted by four digitations into the inferior borders and outer surfaces of the lower four ribs, a little beyond their angles.
Nerve-supply.-The Serratus posterior inferior is supplied by the anterior primary rami of the ninth, tenth, eleventh and twelfth thoracic nerves.
Actions.-The Serratus posterior inferior draws the lower ribs downwards and backwards and thus elongates the thorax; it also fixes the lower ribs, thus assisting the inspiratory action of the Diaphragm and resisting the tendency of the latter to draw the lower ribs upwards and forwards.
The Diaphragm (fig. 599) is a dome-shaped, musculofibrous septum which separates the thoracic from the abdominal cavity, its convex upper surface forming the floor of the former, and its concave under surface the roof of the latter. Its peripheral part consists of muscular fibers which take origin from the circumference of the thoracic outlet and converge to be inserted into a central tendon. The muscular fibers may be grouped according to their origins into three parts–sternal, costal, and vertebral. The sternal part arises by two fleshy slips from the back of the xiphoid process; the costal part from the inner surfaces of the cartilages and adjacent portions of the lower six ribs on each side, interdigitating with the Transversus abdominis (fig. 597); and the vertebral part from aponeurosic arches, named the arcuate ligaments (lumbocostal arches), and from the lumbar vertebrae by two pillars or crura. There are two arcuate ligaments, a lateral and a medial, on each side, and, in addition, a median arcuate ligament connects the two crura across the median plane.
The lateral arcuate ligament (lateral lumbocostal arch), which is a thickened band in the fascia covering the Quadratus lumborum, arches across the upper part of that muscle, and is attached, medially, to the front of the transverse process of the first lumbar vertebra, and, laterally, to the lower margin of the twelfth rib.
The medial arcuate ligament (medial lumbocostal arch) is a tendinous arch in the fascia covering the upper part of the Psoas major; medially, it is continuous with the lateral tendinous margin of the corresponding crus, and is attached to the side of the body of the first or second lumbar vertebra; laterally, it is fixed to the front of the transverse process of the first lumbar vertebra.
The crura.–At their origins the crura are tendinous in structure, and blend with the anterior longitudinal ligament of the vertebral column. The right crus, larger and longer than the left, arises from. the anterior surfaces of the bodies and intervertebral discs of the upper three lumbar vertebrae, while the left crus arises from the corresponding parts of the upper two only. The medial tendinous margins of the crura meet in the median plane to form an arch across the front of the aorta, which is termed the median arcuate ligament; it is often poorly defined.
From this series of origins the fibers of the Diaphragm converge to be inserted. into the central tendon. The fibers arising from the xiphoid process are very short; and occasionally aponeurotic; those from the medial and lateral arcuate ligaments, and more especially those from the ribs and their cartilages, are longer; and curve as they ascend and converge to their insertion. The fibers arising from the crus diverge as they ascend, the most lateral being directed upwards and laterally to the central tendon. The medial fibers of the right crus ascend on the left side of the esophageal opening, and occasionally a fleshy fasciculus from the medial side of the left crus crosses the aorta and runs obliquely through the fibers of the right crus towards the venacaval opening, but this fasciculus “is never continued upwards to help to bound the esophageal passage on the right side”.
Openings in the Diaphragm (fig. 599).–The Diaphragm is pierced by apertures for the passage of structures between the thorax and abdomen. Three large openings-the aortic, the esophageal, and the venacaval and a number of small ones are present.
The aortic opening is the lowest and most posterior of the large apertures; it lies at the level of the lower border of the twelfth thoracic vertebra, slightly to the left of the median plane. Strictly speaking, it is an osseo-aponeurotic opening between the vertebral column and the Diaphragm, and therefore behind the latter; occasionally some tendinous fibers from the medial parts of the crura pass behind the aorta, and convert the opening into a fibrous ring. The aortic opening transmits the aorta and the thoracic duct.
The esophageal opening is situated in the muscular part of the Diaphragm at the level of the tenth thoracic vertebra; it is elliptical in shape, and is ” formed by the splitting of the medial fibers of the right crux ” (Low). It is placed above, in front, and a little to the left of the aortic opening, and transmits the esophagus, the vagus nerves, and the esophageal branches of the left gastric artery.
The venacaval opening, the highest of the three large openings, is situated about the level of the disc between the eighth and ninth thoracic vertebrae. It is quadrilateral in form, and is placed at the junction of the right leaflet with the central. area, so that its margins are tendinous, It transmits the inferior vena caves, the wall of which is adherent to the margin of the opening, and some branches of the right phrenic nerve.
There are two lesser apertures in each crux; of these one transmits the greater and the other the lesser splanchnic nerve. The ganglionated trunks of the sympathetic usually enter the abdominal cavity behind the Diaphragm, deep to the medial arcuate ligaments. Openings for minute veins are frequently seen in the central tendon.
Blair (loc. cit.) describes one of varying size, but of constant position, and present in ten out of twelve specimens he examined. It occurs in the angle between the left pair of bands as they diverge from the central point of decussation, and he suggests that the vein it transmits may represent the suprahepatic part of the left vitelline vein.
On each side are two small areas where the muscular fibers of the Diaphragm are deficient and are replaced by areolar tissue. One between the sternal and costal parts transmits the superior epigastric branch of the internal mammary artery, and some lymph vessels from the abdominal wall and convex surface of the liver. The other, between the fibers springing from the medial and lateral accuate ligaments, is less constant; when this interval exists, the upper and back part of the kidney is separated from the pleura by areolar tissue only.
Relations.-The upper surface of the Diaphragm is in relation with three serous membranes, viz. on each side with the pleura, which separates it from the base of the corresponding lung, and on the middle leaflet of the central tendon with the pericardium, which intervenes between it and the heart. The central portion lies on a slightly lower level than the summits of the lateral portions. The greater part of the under surface is covered by the peritoneum. The right side is accurately moulded over the convex surface of the right lobe of the liver, the right kidney, and right suprarenal gland; the left over the left lobe of the liver, the fundus of the stomach, the spleen, the left kidney, and the left suprarenal gland.
Nerve-supply.—The Diaphragm is supplied by the phrenic nerve and the lower six or seven intercostal nerves, the latter being distributed to the peripheral part of the muscle.
Actions.-The Diaphragm is the principal muscle of inspiration, and presents the form of a dome concave towards the abdomen. The central part of the dome is tendinous, and the pericardium is attached to its upper surface, the circumferential part is muscular. During inspiration the lowest ribs are fixed; and from these and the crura the muscular fibers contract and draw downwards and forwards the central tendon with the attached pericardium. In this movement the, curvature of the Diaphragm is scarcely altered, the dome moving downwards nearly parallel to its original position and. pushing before it the abdominal viscera. The descent of the abdominal viscera is permitted by the extensibility of the abdominal wall, but the limit of this is soon reached. The central tendon, applied to the abdominal viscera, then becomes a fixed point for the action of the Diaphragm, the effect of which is to elevate the lower ribs and through them to push forwards the body of the sternum and the upper ribs. The right cupola of the Diaphragm, lying on the liver, has a greater resistance to overcome than the left, which lies over the stomach, but to compensate for this the right crus and the fibers of the right side generally are stronger than those of the left.
In all expulsive acts the Diaphragm is called into action to give additional power to each effort. Thus, before sneezing, coughing, laughing, crying, or vomiting, and previous to the expulsion of urine, or feces, or of the fetus from the uterus, a deep inspiration takes place.
According to Whillis the fibers of the right crux exert a sphincteric action on the lower end of the esophagus in man. The act of expiration, which immediately succeeds the act of swallowing, relaxes these fibers and allows the contents of the esophagus to pass into the stomach.
The height of the Diaphragm is constantly varying during respiration; it also varies with the degree of distension of the stomach and intestines and with the size of the liver. After a forced expiration the right cupola is on a level in front with the fourth costal cartilage, at the side with the fifth, sixth, and seventh ribs, and behind with the eighth rib : the left cupola is a little lower than the right. Halls Dally** states that the absolute range of movement between deep inspiration and deep expiration averages in the male and female 30 mm. on the right side and 28 mm. on the left; in quiet respiration the average movement is 12-5 mm. on the right side and 12 mm. on the left.
Skiagraphy shows that the height of the Diaphragm in the thorax varies considerably with the position of the body. It stands highest when the body is horizontal and the patient on his back, and in this position it performs the largest respiratory excursions with normal breathing. When the body is erect the dome of the Diaphragm falls, and its respiratory movements become smaller. The dome falls still lower when the sitting posture is assumed, and in this position its respiratory excursions are smallest. When the body is horizontal and the patient on his side, the two halves of the Diaphragm do not behave alike. The uppermost half sinks to a level lower even than when the patient sits, and moves little with respiration; the lower half rises higher in the thorax than it does when the patient is supine, and its respiratory excursions are much increased.
It appears that the position of the Diaphragm in the thorax depends upon three main factors, vii.: (1) the elastic retraction of the lung-tissue, tending to pull it upwards (2) the pressure exerted on its under surface by the viscera: this naturally tends to be a. negative pressure, or downward suction, when the patient sits or stands, and a positive, or upward pressure, when he lies; (3) the intra-abdominal tension due to the abdominal muscles. These muscles are in a state of contraction in the standing position and not in the sitting; hence the Diaphragm is pushed up higher in the former position.
Applied Anatomy.–The oblique rise of the Diaphragm from the costal margin to the level of the fifth costal cartilage on the right side, and the sixth on the left, has to be borne in mind in opening an empyema. If the drainage tube be put in too low down, when the abscess cavity contracts the Diaphragm is drawn up against and blocks the tube before the abscess is cured.
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