The Thoracic Cage-The Diaphragm

The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side and the sternum and costal cartilages in front. Above, it communicates through the ‘thoracic inlet’ with the root (BASE) of the neck; below, it is separated from the abdominal cavity by the diaphragm (Fig. 1).

The diaphragm is the dome-shaped septum (DIVING WALL OR PARTITION) dividing the thoracic from the abdominal cavity. It comprises two portions: a peripheral (OF OR RELATING TO THE SURFACE OR OUTER PART OF A BODY OR ORGAN) muscular part

which arises from the margins of the thoracic outlet and a centrally placed aponeurosis (A SHEETLIKE FIBROUS MEMBRANE RESEMBLING A FLATTENED TENDON THAT SERVES AS A FASCIA TO BIND MUSCULES TOGETHER OR TO CONNECT MUSCLE TO BONE)(Fig. 10).

The muscular fibres are arranged in three parts.

1◊◊Avertebral part from the crura (AN ELEONGATED PART OF AN ANATOMICAL STRUCTURE) and from the arcuate ligaments (REFERS TO AN ARC-SHAPED LIGAMENT).

The right crus (SEE CURA) arises from the front of the bodies of the upper three lumbar vertebrae (ONE OF THE FIVE HUMAN VERTEBRAE GROUPS) and intervertebral discs (A CARTLIGOUS DISC SERVING AS A CUSHION BETWEEN ALL OF THE VERTEBRAE IN THE SPINAL COLUMN EXCLUDING THE FIST TWO) the left crus is only attached to the first two vertebrae.

The arcuate ligaments are a series of fibrous arches, the medial (IN THE MEDIAN PLANE OF THE BODY OR THE MIDLINE F AN ORGAN) being a thickening of the fascia covering psoas major (A LARGE MUSCLE THAT RUNS FROM THE LUMBAR SPINE THROUGH THE GROIN ON EITHER SIDE) and the lateral(SIDE) of fascia overlying quadratus lumborum (A MUSCLE IN THE BACK WHICH ATTACHES TO THE TOP OF THE PELVIS AND THE SPINE IN THE UPPER LUMBAR AREA). The fibrous medial borders of the two crura form a median arcuate ligament over the front of the aorta.

2◊◊Acostal part is attached to the inner aspect of the lower six ribs and costal

cartilages.

3◊◊Asternal portion consists of two small slips from the deep surface of the

Xiphisternum. The central tendon, into which the muscular fibres are inserted, is trefoil

in shape and is partially fused with the under-surface of the pericardium.

The diaphragm receives its entire motor supply from the phrenic nerve (C3, 4, 5) whose long course from the neck follows the embryological migration of the muscle of the diaphragm from the cervical region (see below). Injury or operative division of this nerve results in paralysis and elevation of the corresponding half of the diaphragm.

Radiographically (IN X-RAYS), paralysis of the diaphragm is recognized by its elevation and paradoxical (SELF-CONTRADICTORY) movement; instead of descending on inspiration it is forced upwards by pressure from the abdominal viscera (INTERNAL ORGANS).

The sensory nerve fibres from the central part of the diaphragm also run in the phrenic nerve, hence irritation of the diaphragmatic pleura (PART OF THE PARIETAL PLEURA)(in pleurisy) or of the peritoneum on the undersurface of the diaphragm. Subphrenic collections of pus or blood produces referred pain in the corresponding cutaneous (AFFECTING OR RELATED TO THE SKIN) area, the shoulder-tip. The peripheral part of the diaphragm, including the crura, receives sensory fibres from the lower intercostal nerves.

Openings in the diaphragm

The three main openings in the diaphragm (Figs 10, 11) are:

1◊◊the aortic (at the level of T12) which transmits the abdominal aorta, the thoracic duct and often the azygos vein;

2◊◊the oesophageal (T10) which is situated between the muscular fibres of the right crus of the diaphragm and transmits, in addition to the oesophagus, branches of the left gastric artery and vein and the two vagi (EACH OF THE TENTH PAIR OF CRANIAL NERVES, SUPPLYING THE HEART, LUNGS, UPPER DIGESTIVE TRACT, AND OTHER ORGANS OF THE CHEST AND ABDOMEN);

3◊the opening for the inferior vena cava (T8) which is placed in the central tendon and also transmits the right phrenic nerve.

In addition to these structures, the greater and lesser splanchnic nerves pierce the crura and the sympathetic chain passes behind the diaphragm deep to the medial arcuate ligament.

The development of the diaphragm and the anatomy of diaphragmatic herniae (A CONDITION IN WHICH PART OF AN ORGAN IS DISPLACED AND PROTRUDES THROUGH THE WALL OF THE CAVITY CONTAINING IT).

The development of the diaphragm and the anatomy of diaphragmatic herniae

The diaphragm is formed (Fig. 12) by fusion in the embryo of:

1◊◊the septum transversum (forming the central tendon);

2◊◊the dorsal oesophageal mesentery;

3◊◊a peripheral rim derived from the body wall;

4◊◊the pleuroperitoneal membranes, which close the fetal communication between the pleural and peritoneal cavities.

The septum transversum is the mesoderm (THE MIDDLE LAYER OF AN EMBRYO IN EARLY DEVELOPMENT, BETWEEN THE ENDODERM AND ECTODERM) which, in early development, lies in front of the head end of the embryo. With the folding off of the head, this mesodermal mass is carried ventrally (NEAR THE ABDOMEN) and caudally (TOWARD THE POSTERIOR END OF THE BODY),  to lie in its definitive position at the anterior (TOWARD THE FRONT OF THE BODY) part of the diaphragm.

During this migration, the cervical myotomes (PART OF EMBRYONIC VERTEBRAE) and nerves contribute muscle and nerve supply respectively, thus accounting for the long course of the phrenic nerve (C3, 4 and 5) from the neck to the diaphragm. With such a complex embryological story, one may be surprised to know that congenital abnormalities of the diaphragm are unusual.

However, a number of defects may occur, giving rise to a variety of congenital

herniae through the diaphragm. These may be:

1◊◊through the foramen of Morgagni (UNCOMMON DIAPHRAGMATIC HERNIAE); anteriorly between the xiphoid and costal origins;

2◊◊through the foramen of Bochdalek—the pleuroperitoneal canal—lying posteriorly;

3◊◊through a deficiency of the whole central tendon (occasionally such a hernia may be traumatic in origin);

4◊◊through a congenitally large oesophageal hiatus (PAUSE, BREAK). Far more common are the acquired hiatus herniae (subdivided into sliding and rolling herniae). These are found in patients usually of middle age where weakening and widening of the oesophageal hiatus has occurred (Fig. 13).

In the sliding hernia the upper stomach and lower oesophagus slide upwards into the chest through the lax hiatus when the patient lies down or bends over; the competence of the cardia (THE UPPER OPENING OF THE STOMACH, WHERE THE ESOPHAGUS ENTERS) is often disturbed and peptic juice can therefore regurgitate into the gullet. This may be followed by oesophagitis with consequent heartburn, bleeding and, eventually, stricture formation.

In the rolling hernia (which is far less common) the cardia remains in its normal position and the cardio-oesophageal junction is intact, but the fundus (THE UPPER PART OF THE STOMACH, WHICH FORMS A BULGE HIGHER THAN THE OPENING OF THE ESOPHAGUS) of the stomach rolls up through the hiatus in front of the oesophagus, hence the alternative term of para-oesophageal hernia. In such a case there may be epigastric  discomfort, flatulence and even dysphagia, but no regurgitation because the cardiac mechanism is undisturbed.

The movements of respiration

During inspiration the movements of the chest wall and diaphragm result in an increase in all diameters of the thorax. This, in turn, brings about an increase in the negative intrapleural pressure and an expansion of the lung tissue. Conversely, in expiration the relaxation of the respiratory muscles and the elastic recoil of the lung reduce the thoracic capacity and force air out of the lungs.

In quiet inspiration the first rib remains relatively fixed, but contraction of the external and internal intercostals elevates and, at the same time, everts the succeeding ribs. In the case of the 2nd–7th ribs this principally increases the anteroposterior diameter of the thorax (by the forward thrust of the sternum), like a pump handle.

The corresponding movement of the lower ribs raises the costal margin and leads mainly to an increase in the transverse diameter of the thorax, like a bucket handle. The depth of the thorax is increased by the contraction of the diaphragm which draws down its central tendon. Normal quiet expiration, brought about by elastic recoil of the elevated ribs, is aided by the tone of the abdominal musculature which, acting through the contained viscera, forces the diaphragm upwards.

In deep and in forced inspiration additional muscles attached to the

chest wall are called into play (e.g. scalenus anterior, sternocleidomastoid,

serratus anterior and pectoralis major) to increase further the capacity of

the thorax. Similarly, in deep expiration, forced contraction of the abdominal

muscles aids the normal expulsive factors described above.