MUSCLES OF THE THORAX.

The principal muscles situated on the front and sides of the thorax belong in their actions to the upper extremity, with which they will be described. They are the pectoralis major and minor, subclavius and serratus magnus. The true thoracic muscles, which appertain exclusively to the actions of the ribs, are, the

Intercostales externi,

Intercostales interni,

Subcostales,
Triangularis sterni.

The intercostal muscles are two planes of muscular and tendinous fibres, directed obliquely between adjacent ribs and closing the intercostal spaces. They are seen partially on the reflection of the pectoral muscles, or on the inuer surface of the chest. The triangularis sterni is within the chest, and requires the removal of the anterior part of the thorax to bring it into view.

The INTERCOSTALES EXTERNI, eleven on each side, commence posteriorly at the tubercles of the ribs, and advance forwards to the costal cartilages, where they terminate in a thin aponeurosis which is continued onwards to the sternum. Their fibres are directed obliquely downwards and inwards, pursuing the same line with those of the external oblique muscle of the abdomen. They are thicker than the internal intercostals, and more tendinous in structure.

The INTERCOSTALES INTERNI, also eleven on each side, commence anteriorly at the sternum, and extend backwards as far as the angles of the ribs, whence they are prolonged to the vertebral column by a thin aponeurosis. Their fibres are directed obliquely downwards and backwards, corresponding with those of the internal oblique muscle of the abdomen, and crossing those of the external intercostals.

In structure the intercostal muscles consist of an admixture of muscular and tendinous fibres. They arise from the two lips of the lower border of the rib, the external from the outer lip, the internal from the inner; and are inserted into the upper border of the rib below, encroaching somewhat on its surfaces.

The SUBCOSTALES (infracostales; intracostales) are nine or ten small muscles situated within the thorax at its posterior part, and lying upon the ribs. They increase in size from above downwards, and the direction of their fibres corresponds with that of the intercostales interni. Each muscle arises from the front of a rib, and is inserted into the front of the rib but one below; the first subcostalis is often absent.

Relations. The external intercostals, by their external surface, with the muscles which immediately invest the chest, viz., pectoralis major and minor, serratus magnus, serratus posticus superior and inferior, scalenus posticus, sacrolumbalis and longissimus dorsi with their continuations, cervicalis ascendens and transversalis cervicis, levatores costarum, and obliquus externus abdominis. By their internal surface with the internal intercostals, intercostal vessels and nerves, and posteriorly with the pleura, a thin aponeurosis being interposed. The internal intercostals, by their external surface with the external intercostals, and intercostal vessels and nerves; by their internal surface with the pleura costalis, triangularis sterni, subcostales, and diaphragm.

The TRIANGULARIS STERNI (sterno-costalis), situated upon the inner wall of the front of the chest, arises by a thin aponeurosis from the side of the sternum, ensiform cartilage, and sternal extremities of the costal cartilages from the third to the sixth or seventh; it is inserted by fleshy digitations into the second, third, fourth, and fifth costal cartilages and corresponding ribs.

Relations. By its external surface with the sternum, ensiform cartilage, costal cartilages, internal intercostal muscles, and internal mammary vessels. By its internal surface with the pleura costalis, cellular tissue of the anterior mediastinum, and diaphragm. The lower fibres of the triangularis sterni are continuous with those of the transversalis abdominis.

Actions. The intercostal muscles raise the ribs when they act from above, and depress them when they take their fixed point from below. They are, therefore, both inspiratory and expiratory muscles. The triangularis sterni draws down the costal cartilages, and is therefore an expiratory muscle.

The muscles of respiration, according to Dr. Barclay, are as follows:

To which may be added, as expiratory powers, the costal cartilages; acting by virtue of their elasticity.

MUSCLES OF THE ABDOMEN.

The muscles of the abdominal region are, the

Obliquus externus (descendens),
Obliquus internus (ascendens),

Cremaster,

Transversalis,

Rectus,
Pyramidalis,

Quadratus lumborum,
Psoas parvus,

Diaphragma.

Dissection. The dissection of the abdominal muscles is to be commenced by making three incisions: The first, vertical, in the middle line, from over the lower part of the sternum to the pubes; the second, oblique, from the umbilicus, upwards and outwards, to the outer side of the chest, as high as the fifth or sixth rib; the third, oblique, from the umbilicus, downwards and outwards, to the crest of the ilium and along the crest to its middle. The three flaps included by these incisions should then be dissected back in the direction of the fibres of the external oblique muscle, beginning at the angle of each. The integument and superficial fascia should be dissected off together, so as to expose the fibres of the muscle at once.

When the external oblique muscle is dissected on both sides, a white tendinous line will be seen along the middle of the abdomen, extending from the ensiform cartilage to the pubes; this is the linea alba. A little external to it, on each side, two curved lines will be observed extending from the eighth rib to the spine of the pubes, and bounding the recti muscles; these are the lineæ semilunares. Some transverse lines, lineæ transversa, three in number, connect the lineæ

semilunares with the linea alba at and above the umbilicus; [sometimes a fourth and even a fifth are found below the umbilicus].

The EXTERNAL OBLIQUE MUSCLE (obliquus externus abdominis, descendens: costo-abdominalis) is the external flat muscle of the abdomen. Its name is derived from the obliquity of its direction, and the descending course of its fibres. It arises by fleshy digitations from the external surface of the eight inferior ribs; the five upper digitations being received between corresponding processes of the serratus magnus, the three lower, of the latissimus dorsi. The fleshy fasciculi proceeding from this extensive origin terminate on the front of the abdomen in a broad aponeurosis, and posteriorly are inserted into the outer lip of the crest of the ilium, for two-thirds its length, and into the anterior superior spinous process. The aponeurosis is united, in front, by its under surface, with that of the obliquus internus, forming the anterior wall of the sheath of the rectus, and is inserted into the linea alba, front of the pubes, spine of the pubes and pectineal line.

The lower border of the aponeurosis, which is stretched between the anterior superior spinous process of the ilium, and the spine of the pubes, is round from being folded inwards, and forms Poupart's ligament (ligamentum Fallopii; arcus cruralis); the insertion into the pectineal line is Gimbernat's ligament [see fig. 129, 6, 7].

Just above the crest of the pubes is the external abdominal ring (annulus abdominalis externus), a triangular opening formed by the separation of the fibres of the aponeurosis of the external oblique. It is oblique in direction, and corresponds with the course of the fibres of the aponeurosis. It is bounded below by the crest of the pubes; on either side by the borders of the aponeurosis, which are termed pillars; and above by some curved fibres (intercolumnar) which originate from Poupart's ligament, and cross the upper angle of the ring, to give it strength. The external pillar, which is at the same time inferior from the obliquity of the opening, is inserted into the spine of the pubes; the internal or superior pillar forms an interlacement with its fellow of the opposite side over the front of the symphysis pubis. The external abdominal ring gives passage to the spermatic cord in the male, and the round ligament in the female; they are both invested in their passage through it by a prolongation of the fascia of the external oblique muscle, the intercolumnar fascia, or fascia spermatica.

The pouch of inguinal hernia, in passing through this opening, receives the intercolumnar fascia as one of its coverings.

Relations. By its external surface with the superficial fascia, integument, cutaneous vessels and nerves, particularly the superficial epigastric and superficial circumflexa ilii vessels, and latissimus dorsi, by which it is overlapped posteriorly. By its internal surface with the internal oblique, lower part of the eight inferior ribs and intercostal muscles, cremaster, spermatic cord in the male, and round ligament in the female. The upper border of the external oblique is continuous with the pectoralis major.

The external oblique is now to be removed by making an incision across the ribs, just below its origin, to its posterior border; and another along Poupart's ligament and the crest of the ilium. Poupart's ligament should be left entire, as it gives attachment to the next muscles. The muscle may then be turned forwards towards the linea alba, or removed altogether.

The INTERNAL OBLIQUE MUSCLE (obliquus internus abdominis, ascendens; ilio-abdominalis) is the middle flat muscle of the abdomen. It arises from the outer half of Poupart's ligament, from the middle of the crest of the ilium for two-thirds its length, and from the fascia lumborum [see p. 217]. Its fibres diverge from their origin, those from Poupart's ligament curving downwards, those from the anterior part of the crest of the ilium passing transversely, and the rest ascending obliquely. The muscle is inserted into the pectineal line and crest of the pubes, linea alba, and lower border of the four inferior ribs. At its origin

from the fascia lumborum it becomes the bond of union between the aponeurosis of the transversalis and that of the latissimus dorsi.

MUSCLES OF THE ANTERIOR ASPECT OF THE TRUNK; on the left side the superficial layer is seen, on the right the deeper layer. 1. Pectoralis major. 2. Deltoid; the interval between these muscles lodges the cephalic vein. 3. Anterior border of the latissimus dorsi. 4. Serratus magnus. 5. Subclavius, right side. 6. Pectoralis minor. 7. Coraco-brachialis. 8. Upper part of the biceps, showing its two heads. 9. Coracoid process of the scapula. 10. Serratus magnus, right side. 11. External intercostal muscle of the fifth intercostal space. 12. External oblique. 13. Its aponeurosis; the median line to the left of this number is the linea alba; the curved line to the right, the linea semilunaris; the transverse lines above and below the cipher, the lineæ transversæ. 14. Poupart's ligament. 15. External abdominal ring; the margin above the ring is the superior or internal pillar; the margin below the ring, the inferior or external pillar; the curved intercolumnar fibres are seen proceeding upwards from Poupart's ligament to strengthen the ring. The numbers 14 and 15 are placed on the fascia lata of the thigh; the opening immediately to the left of 15 is the saphenous opening. 16. Rectus muscle of the right side brought into view by the removal of the anterior segment of its sheath; posterior segment of its sheath with the divided edge of the anterior segment. 17. Pyramidalis muscle. 18. Internal oblique. 19. Conjoined tendon of the internal oblique and transversalis, descending behind Poupart's ligament to the pectineal line. 20. The arch formed between the lower curved border of the internal oblique muscle and Poupart's ligament; it is beneath this arch that the spermatic cord and oblique inguinal hernia pass.

Along the upper three-fourths of the linea semilunaris, the aponeurosis of the internal oblique separates into two lamellæ, which pass, one in front, the other behind the rectus muscle to the linea alba, where they are inserted; along the lower fourth, the aponeurosis passes in front of the rectus without separation. The two layers, which thus inclose the rectus, consequently form for it a sheath, which is incomplete at its posterior and inferior part.