The thoracic cavity (cavum thoracis) (), in a narrow sense, is bounded by the subserous endothoracic fascia. In a wider sense, its walls are formed by the ribs, thoracic vertebrae, sternum, and associated muscles, including the diaphragm. It is cone-shaped with the apex between the first pair of ribs and the base at the diaphragm.
Rivero et al. (2005) provided a new reference for interpretation of the normal anatomy of the canine thorax imaged by computed tomography. A similar study by Cardoso et al. (2007) used a helical scanner and intravenous contrast media to visualize the lung.
The endothoracic fascia (fascia endothoracica) is the areolar tissue that attaches the costal and diaphragmatic pleurae to the underlying muscles, ligaments, and bones. The endothoracic fascia is scanty where it closely attaches the costal pleura to the ribs. Dorsally and ventrally it extends into the mediastinal space and becomes the connective tissue that invests the organs and other structures that lie in the mediastinum. Cranially it passes through the thoracic inlet and is continued into the neck where it blends with the deep cervical fascia, particularly with the prevertebral portion of this fascia. Caudally it blends with the transversalis fascia at the hiatuses of the diaphragm and at the lumbosacral arches.
The thoracic wall is formed bilaterally by the ribs and the intercostal muscles, and dorsally by the bodies of the thoracic vertebrae and the intervening fibrocartilages. Cranial to the sixth thoracic vertebra the right and left longus colli muscles cover the thoracic vertebral bodies and lie directly deep to the pleura and endothoracic fascia. Ventrally, the narrow sternum and the paired fat transversus thoracis muscles contribute to the thoracic wall. Caudally, the base of the thoracic cavity is formed by the dome-shaped, obliquely placed, musculotendinous diaphragm.
The shape of the thoracic cavity of the dog () varies between breeds of dogs. Its walls are laterally compressed, with the result that in dogs of usual proportions, its average dorsoventral dimension is greater than either the average lateral or craniocaudal measurement. Cranially, the thoracic cavity opens to the exterior at the thoracic inlet. The external contour of the thorax differs from the internal limits of the thoracic cavity. In cross-section the thorax is roughly oval in shape, wider dorsally than ventrally, and long dorsoventrally. A transverse section of the thoracic cavity cranial to the diaphragm is heart-shaped, with the apex located ventrally (). The base of the transverse section, located dorsally, is widened to accommodate the epaxial muscles, thoracic vertebral bodies, aorta, and smaller associated structures. The thorax is a laterally compressed cone with a base (diaphragm) that is convex cranially. The greatest cranial encroachment of the diaphragm is to a transverse plane through the sixth intercostal spaces and approximately 5 cm dorsal to the sternum. In addition to being convex, the diaphragm is oblique in position. The most cranial dorsal attachment is approximately eight vertebral segments caudal to its most cranial ventral attachment. From an origin on the medial surfaces of the ribs, including the costal cartilages and the lumbar vertebrae, the diaphragm bilaterally extends almost directly cranially, forming a slitlike space or recess at its attachments. The recess is formed by the diaphragm centrally, and the ribs and intercostal structures and lumbar vertebrae peripherally. In normal respiration the diaphragm undergoes a craniocaudal excursion of approximately one and a half vertebral segments, yet even in forced inspiration the margin of the lungs never completely invades the recesses so created. In a Beagle-type dog the diaphragm protrudes cranially from its costal attachment for a maximum distance of 6 cm. For the details of the intrinsic structure of the diaphragm. The thoracic cavity contains the trachea, lungs, heart, thymus gland, esophagus, lymph nodes, vessels, and nerves. The structures that partly or completely traverse the thoracic cavity are the aorta, cranial vena cava and caudal vena cava, azygos and hemiazygos veins, thoracic duct and smaller lymph vessels, esophagus, and vagal, phrenic, and sympathetic nerves.
The thoracic inlet (apertura thoracis cranialis) is the roughly oval opening into the cranial part of the thoracic cavity. It is bounded bilaterally by the first pair of ribs with their cranially extending costal cartilages. In a Beagle-type dog its dorsoventral dimension is approximately 4 cm. Its greatest width is approximately one-fourth less than its dorsoventral dimension. The aperture is wider dorsally than ventrally. The first thoracic vertebra and the paired longus colli muscles bound the thoracic inlet dorsally; the manubrium of the sternum bounds it ventrally. Traversing the aperture are the trachea, esophagus, vagosympathetic nerve trunks, recurrent laryngeal nerves, phrenic nerves, first two thoracic nerves, and several vessels. The apices of the pleural cavities lie in the thoracic inlet.