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The superior oblique portion arises from the anterior tubercles of the transverse processes of the third, fourth, and fifth cervical vertebræ.
It is directed upwards, to be inserted into the anterior tubercle of the atlas.
Nerve-Supply. It is supplied by nerves from the anterior rami of the second, third, and fourth cervical nerves.
Action. A flexor of the vertebral column.
M. Rectus Capitis Lateralis.-The rectus capitis lateralis, in series with the posterior inter-transverse muscles in the neck, arises from the transverse process of the atlas.
It is inserted into the inferior surface of the jugular process of the occipital bone. It is placed alongside the rectus capitis anterior, separated from it by the anterior ramus of the first cervical nerve.
Nerve-Supply. The loop between the anterior rami of the first two cervical nerves. Actions. A lateral flexor of the head and vertebral column. The movements produced by these muscles are considered along with those of other muscles acting on the head, vertebral column, and thorax (pp. 445, 446).
THE MUSCLES OF THE THORAX.
Muscles of Respiration.
The muscles which complete the boundaries of the thorax are the diaphragm and intercostal muscles (external and internal), along with three series of smaller muscles-the transversus thoracis, the levatores costarum, and the subcostal muscles.
Mm. Intercostales. The intercostal muscles are arranged in eleven pairs, which occupy the intercostal spaces.
Each external muscle arises from the sharp lower border of a rib, and is directed inferiorly and anteriorly, to be inserted into the external edge of the superior border of the rib below. It extends from the tubercle of the rib posteriorly nearly to the costal cartilage anteriorly. The anterior intercostal aponeurosis is continuous with it anteriorly, and extends forwards to the side of the sternum.
Each internal muscle arises from the costal cartilage and the internal or superior edge of the costal groove, and is directed inferiorly and posteriorly, to be inserted into the internal edge of the superior border of the rib and costal cartilage below. It extends from the side of the sternum anteriorly to the angle of the rib posteriorly, where it is replaced by the posterior intercostal aponeurosis extending to the tubercle of the rib.
The superficial surface of the external muscle is covered by the muscles of the chest, axilla, abdomen, and back. The deep surface of the internal muscle is in contact with the pleura.
Mm. Levatores Costarum.-The levatores costarum are in series with the external intercostal muscles. They are twelve small slips arising from the transverse processes of the seventh cervical and upper eleven thoracic vertebræ. Each spreads out in a fan-like manner as it descends to the lateral surface of the rib immediately below where it is inserted posterior to the angle.
Mm. Subcostales.-The subcostal muscles are slips of muscles found on the internal surface of the lower ribs near their angles. They are in series with the internal intercostal muscles, but pass over the deep surface of several ribs.
M. Transversus Thoracis. The transversus thoracis (O.T. triangularis sterni) occupies the posterior aspect of the anterior thoracic wall, and is separated from the costal cartilages by the internal mammary vessels. It arises from the posterior surface of the xiphoid process and body of the sternum as high as the level of the third costal cartilage.
From that origin its fibres radiate laterally, the lower horizontally, the upper fibres obliquely upwards, to be inserted into the second, third, fourth, fifth, and sixth costal cartilages. The muscle is continuous below with the transversus abdominis.
Diaphragma. The diaphragm is the great membranous and muscular partition separating the cavities of the thorax and abdomen. It forms a thin lamella arching over the abdominal cavity, and clothed on that surface, for the most part, by peritoneum. It is related, on its inferior concave surface, to the liver, stomach, and spleen, the kidneys and suprarenal glands, and the duodenum and pancreas. Its superior convex surface projects into the thoracic cavity, rising higher on the right than on the left side, and is related to the pericardium and pleuræ, and along its margin to the chest wall. The oesophagus and thoracic aorta are in contact with it posteriorly.
It possesses a peripheral origin from the sternum, ribs, and vertebral column,
FIG. 418.-THE MUSCLES OF THE RIGHT SIDE OF THE THORACIC WALL.
and an insertion into a central tendon. It arises (1) anteriorly (pars sternalis) from the posterior surface of the xiphoid process by two slender fleshy slips, directed backwards; (2) laterally (pars costalis), from the deep surface of the lower six costal cartilages on each side by fleshy bands which interdigitate with those of the transversus abdominis; (3) posteriorly (pars lumbalis), from the lumbar vertebræ, by the crura, and the medial and lateral lumbo-costal arches. The crura are two elongated fibro-muscular bundles which arise, on each side of the aorta, from the anterior surface of the bodies of the lumbar vertebræ, on the right side from the first three, on the left side from the first two lumbar vertebræ. They are directed upwards and decussate across the median plane in front of the aorta, the fibres of the right crus passing anterior to those of the left crus. The fibres then encircle the oesophagus, forming an elliptical opening for its passage, and finally join the central tendon, after a second decussation anterior to the gullet.
The medial part of each crus is wholly tendinous and is sometimes called the crus mediale; it is connected with its fellow of the opposite side by a tendinous band called the middle arcuate ligament, which arches between them, in front of the aorta, and gives origin to fibres which join the crura as they decussate to encircle the gullet. The most outlying part of the crus is sometimes called the crus laterale; its infero-lateral margin is continuous with the medial lumbo-costal arch. The intermediate part of the crus is the crus intermedium; the splanchnic nerves pierce the diaphragm between it and the medial crus. The sympathetic trunk sometimes pierces the diaphragm between the intermediate and lateral crura.
The arcus lumbocostalis medialis (O.T. internal arcuate ligament) is a thickening formed by the attachment of the psoas fascia to the body of the first lumbar vertebra medially and its transverse process laterally. Stretching across the superior end of the psoas muscle, the ligament gives origin to muscular fibres which join the fibres of the crus.
The arcus lumbocostalis lateralis (O.T. external arcuate ligament) is the thickened superior border of the fascia over the quadratus lumborum muscle
and is attached medially to the transverse process of the first lumbar vertebra, and laterally to the last rib. It gives origin to a broad band of muscular fibres, separated by an interval from the fibres arising from the medial lumbocostal arch which sweep upwards to the central tendon.
From this extensive origin the muscular fibres of the diaphragm converge to an insertion into a large trilobed central tendon called the centrum tendineum. Of its lobes the right one is the largest, the middle or anterior is intermediate in size, and the left is the smallest. It does not occupy the centre of the muscle, being placed nearer the front than the back. The fibres of the crura are consequently the longest; those from the xiphoid process are the shortest.
The diaphragm is pierced by numerous structures. The superior epigastric artery enters the sheath of the rectus abdominis between its sternal and costal origins; the musculo-phrenic artery passes between its attachments to the seventh and eighth ribs. The sympathetic trunk and the splanchnic nerves pierce, or pass posterior to the diaphragm; the last thoracic nerve passes behind the lateral lumbo-costal arch; and the aorta, the azygos vein, and thoracic duct pass between the crura, underneath the middle arcuate ligament (hiatus aorticus or aortic opening). The special foramina are two in number. The foramen vena cava (O.T. foramen
quadratum) in the right lobe of the central tendon transmits the inferior vena cava, and small branches of the right phrenic nerve. The hiatus œsophageus (asophageal opening) is in the muscular substance of the diaphragm, posterior to the central tendon, and is surrounded by a sphincter-like arrangement of the crural fibres. Besides the oesophagus, this opening transmits the two vagi nerves.
The diaphragm is found as a complete septum between the thorax and abdomen only in mammals. It is occasionally deficient in the human subject, producing hernia of the diaphragm, either into the pericardial cavity through the central tendon, or into the pleural cavity through the lateral portions of the muscle. A rare condition is congenital deficiency of a part of the lateral half of the muscle, generally placed posteriorly, and on the left side. This produces, by continuity of the pleural and peritoneal cavities behind the diaphragm, a congenital diaphragmatic
Nerve-Supply.-The intercostal muscles, levatores costarum, subcostal muscles, and transversus thoracis, are all supplied by the anterior rami of the thoracic nerves. The diaphragm receives its chief, if not its entire, motor supply from the phrenic nerves (C. 3. 4. 5.). It is innervated also by the diaphragmatic plexus of the sympathetic, and is sometimes said to receive fibres from the lower thoracic nerves.
Actions. The act of respiration consists of two opposite movements-inspiration and expiration.
1. The movement of expiration is performed by (1) the elasticity of the lungs, (2) the weight of the chest walls, (3) the elevation of the diaphragm, (4) the action of muscles-transversus thoracis and muscles of the abdominal wall. It is sometimes stated that the interosseous fibres of the internal intercostal muscles are depressors of the ribs.
2. The movement of inspiration results in the enlargement of the thoracic cavity in all its diameters. Its antero-posterior and transverse diameters are increased by the elevation and forward movement of the sternum, and by the elevation and eversion of the ribs, while its vertical diameter is increased by the descent of the diaphragm.
The muscles of inspiration are divided into two series-ordinary and accessory.
Of the ordinary muscles the diaphragm is the most important. Its action is twofoldcentrifugal, elevating the ribs and increasing the transverse and antero-posterior diameters of the thorax, and centripetal, drawing downwards the central tendon and increasing the vertical diameter of the thorax. Of the two movements the former is the more important. There has been considerable diversity of opinion regarding the action of the intercostal muscles. It is generally agreed that the external muscles elevate the ribs; it is probable that the whole of each internal muscle acts in the same way, although it has been stated by different observers that the whole internal muscle is a depressor; or that the interosseous part is a depressor, the interchondral portion of the muscle an elevator of the ribs.
FASCIÆ AND MUSCLES OF THE ABDOMINAL WALL.
The space between the base of the bony thorax and the pelvis is filled up by a series of muscular sheets, covered externally and internally by fasciæ.
The fascia of the abdominal wall are-externally, the superficial and deep fasciæ; internally, the fascia transversalis, which clothes the interior of the abdominal cavity, and is continuous with the diaphragmatic, lumbo-dorsal, psoas, iliac, and pelvic fasciæ, and is lined within by the subserous coat of extra-peritoneal tissue.
The superficial fascia of the abdomen is liable to contain a large quantity of fat. In the groin it is separated into two layers: a superficial fatty layer continuous over the inguinal ligament with the fascia of the anterior surface of the thigh (p. 402), and a deeper membranous layer attached to the medial.half of the inguinal ligament, and more laterally to the fascia lata of the thigh distal to the inguinal ligament. The two layers are separated by the lymph glands and the superficial vessels of the groin. Higher up in the abdominal wall the two layers blend together. As they pass downwards over the spermatic funiculus, they unite to form the fascia and dartos muscle of the scrotum. The attachment of the fascia to the groin prevents the passage into the thigh of fluid extravasated in the abdominal wall.
The deep fascia of the abdominal wall resembles similar fasciæ in other situations. It forms an investment for the obliquus externus muscle, and becomes thin and almost imperceptible in relation to the aponeurosis of that muscle.