THE THORACIC CAVITY 1235 



blood coming from the capillary network of the pulmonary pleura and from the capillary net- 

 work of the bronchi (fig. 992). Thus it will be seen that while the pulmonary vein carries 

 maiuh" arterial blood, it carries also some venous blood. The pulmonary veins (see p. 529) 

 follow the bronchial tree on the side opposite the arteries to the hilus, where, having converged 

 to two large trunks located in the root of the lung below the plane of the artery, they pass to the 

 left atrium. The pulmonary veins have no valves. 



Lymphatics. — Miller has found the lymphatic vessels forming a closed tube system in the 

 walls of the bronchi, in the pleura, and along the branches of the pulmonary artery and veins. 

 Within the lung numerous pulmonary lymph-glands [lymphoglandulae pulmonales] are found 

 chiefly at the places of branching of the larger bronchi [lymphoglandulae bronchiales[. Scat- 

 tered along the latter, as well as associated with the branches of the pulmonary artery and 

 vein, are found masses of lymphoid tissue. Deposits of carbonaceous matter in the lymphoid 

 structures of the lung are present, except in earh' infancy ; the amount increases with age. 



Nerves. — The vagus and sympathetic contribute to form the pulmonary plexuses in front 

 and behind the root of the lung, from which branches go to accompany bronchial arteries; a 

 smaller number accompany the air-tubes (see p. 957). 



Variations. — Congenital absence of one or both lungs has been observed. Variations in the 

 lobes are not uncommon — four for the right and three for the left lung has been recorded. An 

 infracardiac lobe, as found in certain mammals, sometimes occurs; an infracardiac bronchus is, 

 however, constant in man. More or less complete fusion of the middle and upper lobes of the 

 right lung is not rare. The lungs may be symmetrical, with two lobes each, the apical bronchus 

 of the right springing from the first ventral bronchus, as is normal for the left lung (Waldeyer, 

 Narath) ; or the lungs may have three lobes each, the apical bronchus of the left arising from the 

 main bronchus. The apical bronchus of the right lung may arise from the trachea, an origin 

 that is normal in the hog and other artiodactyls. 



Development of the lungs and trachea. — The first indication of the trachea and lungs 

 appears in embryos of about 32 mm. as a trough-like groove in the ventral wall of the upper 

 part of the oesophagus, communicating above with the pharynx. Later the groove becomes 

 constricted off from the oesophagus, the constriction extending from below upward, so that a 

 tube is formed which opens into the pharynx above. The lower end of this tube soon becomes 

 bilobed, and the lobes, elongating, give rise to additional lobes, of which there are primarily 

 three in the right side and two in the left. The upper unpaired portion of the tube becomes the 

 trachea, while the lobed lower portion gives rise to the bronchi and lungs, the complicated struc- 

 ture of the latter being produced by oft-repeated branchings of the bronchi. 



THORACIC CAVITY 



Thoracic cavity [cavum thoracis] is the term used to denote the space included 

 by the walls of the thorax and occupied by the thoracic viscera. These are, on 

 each side, the lung, surrounded by the pleural cavity, and in the middle the 

 pericardium and heart, great vessels, trachea and oesophagus, all closely associated 

 and forming a dividing wall, the mediastinal septum, standing between the right 

 and left sides of the thoracic space. 



The limits of the thoracic space are given by the skeletal parts of the thorax 

 together with the ligaments involved in the articulations and the muscles and 

 membranes interposed between the bones. The arched diaphragm forms the 

 inferior limit; and the barrier presented by the scalene muscles and the cervical 

 fascia makes the superior boundary, which, it is to be observed, lies above the 

 plane of the superior aperture of the thorax and therefore in the base of the neck. 

 These boundaries are approached by the extension of the pleural cavities; yet 

 there intervenes the parietal layer of the plem-al sac which is connected with 

 the thoracic walls by loose connective tissue, the endothoracic fascia [fascia 

 endothoracica]. 



The form of the thoracic space departs from the external contour of the thorax 

 chiefly through the projection into it of the ridge made by the succession of centra 

 of the thoracic spine, and by the presence on either side of the latter of the broad, 

 deep pulmonary sulcus. On account of these features a transverse section of the 

 thoracic space is somewhat heart-shaped, but, however, much compressed antero- 

 posteriorly (fig. 993). 



The arch of the diaphragm on the right side rises to the level of the spinous process of the 

 seventh thoracic vertebra; on the left, to the level of the eighth thoracic spinous process. At 

 its circumference the diaphragm is in contact to a variable extent above its origin with the inner 

 surfaces of the costal arches. In the lower part of this zone a connection exists between the 

 muscle and the thoracic wall through a continuation of the endothoracic fascia; in the upper 

 part, the phrenico-costal sinus (see p. 1237) intervenes. The level reached by this deepest 

 part of the pleural cavity is lower than the summit of the peritoneal cavity, so they overlap to a 

 considerable extent. 



