Muscles of Thorax | Respiratory system | MBBS
MUSCLES
Radiate ligaments
Radiate ligaments connect the anterior parts of each costal head to the bodies of two vertebrae and their intervening intervertebral disc. Each is attached to the head just
beyond its articular surface. Intermediate fibers, shortest and least distinct, are horizontal and attached to the disc.
Costotransverse ligament
The costotransverse ligament fills the costotransverse foramen between the neck of the rib and its adjacent corresponding transverse process. Its numerous short
fibres extend back from the posterior rough surface on the neck to the anterior surface of the transverse process.
A costotransverse ligament is rudimentary or absent in the eleventh and twelfth ribs.
MUSCLES
Intrinsic and extrinsic muscles
The intrinsic muscles of the chest wall are the intercostal muscles, subcostalis, transversus thoracis.
The intercostal muscles occupy each of the intercostal spaces and are named according to their surface relations, i.e. external, internal and innermost. All are innervated by the adjacent intercostal nerves derived from the ventral rami of the thoracic spinal nerves. The intrinsic muscles can elevate or depress the ribs, and are active during respiration,
particularly forced respiration: their primary
action is believed to be to stiffen the chest wall, preventing paradoxical movement during inspiration.
The skeletal framework of the thoracic wall provides extensive attachment sites for muscles associated functionally with the neck, abdomen, back, and upper limbs. Some of them (scalenes, infrahyoid strap muscles, sternocleidomastoid, serratus anterior, pectoralis major and minor, external and internal obliques, and rectus abdominis) function as accessory muscles of respiration
and are usually active only during forced respiration.
INTRINSIC CHEST WALL MUSCLES
Intercostal muscles The intercostal muscles are thin multiple layers of muscular and tendinous fibres that occupy the intercostal spaces; their names are derived from their spatial relationship, i.e. the external, internal
and innermost intercostals.
External intercostals
Eleven pairs of external intercostals extend from the tubercles of the ribs, almost to the costal cartilages, where each continues forwards to the sternum as an aponeurotic layer, the external intercostal membrane. Each muscle passes from the lower border of one rib to the upper border of
the rib below:
their fibres are directed obliquely and downwards, forwards and medially at the front. In the upper two or three spaces, they do not quite reach the ends of the rib. The external intercostals are thicker
than the internal intercostals.
Internal intercostals
Eleven pairs of internal intercostals begin anteriorly at the sternum, in the interspaces between the cartilages of the true ribs, and at the anterior extremities of the cartilages of the ‘false' ribs. Their greatest thickness lies in this intercartilaginous or parasternal part. They continue back as far as the posterior costal angles, where each is replaced by an aponeurotic layer, the internal intercostal
membrane. Each muscle descends from the floor of a costal groove and adjacent costal cartilage, and inserts into the upper border of the rib below: their fibres are directed obliquely, nearly at right angles to those of the external intercostal muscles.
Innermost intercostals
The innermost intercostals were once regarded as internal laminae of the internal intercostal muscles, and fibres in the two layers do coincide in direction. Each muscle is attached to the internal aspects of two adjoining ribs. They are insignificant, and sometimes absent, at highest thoracic levels, but become progressively more substantial below this, typically extending through the middle two quarters of the lower intercostal spaces. Posteriorly, the innermost
intercostals, in those spaces where they are well developed. The innermost intercostals are related internally to the endothoracic fascia and parietal pleura, and externally to the intercostal nerves and vessels.
the diaphragm slopes downward to its costal and vertebral attachments: this slope is most marked posteriorly.
The diaphragm is a curved musculofibrous sheet that separates the thoracic from the abdominal cavity. It’s mainly convex upper surface faces the thorax, and its concave inferior surface is directed towards the abdomen. The positions of the domes or cupolae of the diaphragm are extremely variable because they depend on body build and the phase of ventilation. Thus the
diaphragm will be higher in short, fat people than in tall, thin people. Usually, after forced
expiration the right cupola is level anteriorly with the fourth costal cartilage and therefore the right nipple, whereas the left cupola lies approximately one rib lower. With maximal inspiration, the cupola will descend as much as 10 cm, and the right dome coincides with the tip of the sixth rib.
