Anatomy Of The Thoracic Cavity Notes

Anatomy of the Thoracic Cavity

Thoracic Cavity

The thoracic cavity is cone shaped, with the thoracic inlet situated at the apex. Its walls are formed by the thoracic vertebrae dorsally, the ribs and costal cartilages laterally, and the sternum (consisting of the manubrium, xiphoid process and sternebrae) ventrally. It is separated from the abdomen by the diaphragm.

The thoracic skeleton consists of 13-18 thoracic vertebrae (species dependent), a corresponding number of ribs articulating with 2 successive vertebrae and the sternum.

Ribs

Costae are arranged in pairs and articulate with 2 successive vertebrae (costovertebral joint – ball and socket). More caudally, there may be a tubercle for articulation with the transverse processes of more caudal vertebra (costotransverse joint). Costae join ventrally on the midline at the sternum. The sternebrae is the main body joining ribs on the midline. True ribs articulate directly with the sternum, whilst false ribs articulate indirectly through a connection with the cartilage of the costae directly cranial. The last rib may not have contact with its neighbour, and is termed a floating rib.

Individual ribs have a dorsal bony part, a body of rib and a ventral cartilaginous part (costal cartilage).

Intercostal Muscles

External and internal intercostal muscles occupy the intercostal spaces.

External intercostal mm. lie superficially and their fibres run caudoventrally between the ribs (but not between costal cartilages). Their main action is inspiratory – when contracted, they move the ribcage up and outwards expanding the thoracic cavity.

Internal intercostal mm. lie deep to the external intercostal muscles, and their fibres run cranioventrally. They have an expiratory action (interosseus mm.), but are usually only active during exercise. Their second component, the interchondral mm., have an inspiratory action.

Pleura

The pleura is a serous membrane that lines the thoracic cavity. It consists of a single flat layer of epithelium with and underlying propria (connective tissue).

Each lung is invested by a pleural membrane called the visceral pleura, so there are 2 pleural membranes arranged as closed sacs. The space in between the left and right pleural sac is the mediastinum. The visceral pleura derives from the splanchnic mesoderm.

The visceral pleura is continuous with the mediastinal pleura, which in turn in continuous with diaphragmatic and costal pleura. The mediastinal, costal and diaphragmatic pleura are called the parietal pleura. The parietal pleura effectively lines the thoracic cavity, and is derived from the somatic mesoderm.

The narrow space between the parietal and visceral pleura is called the pleural cavity. It contains a small amount of serous fluid spread over the surface of the pleura, which creates adhesion. The pressure within the pleural cavity is sub-atmospheric. The pleura translates changes in thoracic volume into changes in lung volume.

The costodiaphragmatic recess in the space between costal and diaphragmatic pleura.

Mediastinum

The mediastinum is the partition between the left and right pleural sacs. It contains …

• Heart (pericardial sac)

• Thoracic trachea

• Thoracic oesophagus

• Lymph nodes

• Caudal vena cava

• Right phrenic nerve

And other structures depending on species.

Diaphragm

The diaphragm is a dome-shaped musculotendinous sheet that separates the thoracic and abdominal cavities. The muscular part is peripheral, and the tendinous part is central. The muscle attaches to the xiphoid process and the inside of the lower costal cartilages and ribs, and the vertebral column. The diaphragmatic crura are attached to the lumbar vertebrae.

It receives motor and sensory innervation from the phrenic nerve.

The diaphragm is the major muscle of inspiration. When it contracts, it flattens and moves downwards, increasing the volume of the thoracic cavity.

The diaphragm has 3 openings …

• Aortic haitus – the most dorsal opening. Contains the aorta, azygous vein and thoracic duct.

• Oesophageal hiatus – contains the oesophagus, dorsal and ventral vagal trunks.

• Caval foramen – lies within the central tendinous region of the diaphragm and contains the caudal vena cava. This opening does not allow movement, as the diaphragm is fused with the vessel wall.

Left and Right Thoracic Cavities

Both the left and right thoracic cavities contain …

• Sympathetic trunk

• Aorta

• Brachiocephalic trunk

• Oesophagus

• Vagal nerve

• Laryngeal recurrent nerve

• Trachea

• Vena cava

• Lymph nodes and thoracic duct

• Pericardium with heart

The left thoracic cavity only contains the thymus. The right thoracic cavity only contains the azygous vein.

Lungs

Each lung is contained within a pleural sac. They are attached by the roots only to the mediastinum. The right lung is always larger than the left due to the positioning of the heart. The cranial point of the lungs is termed the apex. Each lung has a concave base relating to the face of the diaphragm.

In most species, the lungs are divided into lobes by the bronchial tree. The left lung has a cranial and caudal lobe, whilst the right lung has a cranial, caudal, middle and accessory lobe (although this pattern varies a little between species).

The majority of lung tissue is provided by bronchi and bronchioles, pulmonary vessels, peribronchial and perivascular connective tissue. The terminal bronchioles have alveoli scattered...

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