1228 



THE RESPIRATORY SYSTEM 



coat [tunica muscularis]. Contraction of this trachealis muscle, as it is more prop- 

 erly named, causes the ends of the tracheal cartilages to be approximated and 

 the lumen of the wind-pipe to be diminished. 



The structure of the walls of the bronchi is similar to that of the trachea. The 

 right bronchus possesses six to eight cartilages; the left, nine to twelve. 



An inconstant broncho-cesophageal muscle may connect the back of the left bronchus with 

 the gullet. 



Vessels and nerves. — The arteries supplying these air-tubes come from the inferior thyreoid 

 and from the internal mammary by its anterior mediastinal or bronchial branches. Venous 

 radicles come together in the annular ligaments and join lateral veins on either side, which empty 

 the blood into the plexuses of the neighbouring thyreoid veins. 



Lymph-vessels are abundant, and are disposed in two sets, one in the mucosa, another in 

 the submucosa. They drain into the tracheal, bronchial and oesophageal lymph-glands. 

 Nerves are provided by the vagus direct, by the inferior laryngeal, and by the sympathetic. 



THE LUNGS 



The lungs [pulmones], the essential organs of respiration, are constructed in 

 such a way as to permit the blood to come into close relation with the air (fig. 

 992). Their genetic connection with the entodermal canal has already been indi- 

 cated (see also p. 1099). In plan of structure the lung has been compared with 



Fig. 992. — Schematic Section of a Lobule of the Lung showing the Relation of the 

 Blood-vessels to the Air-spaces. (After Miller, from the 'Reference Handbook of the 

 Medical Sciences.') 



b.r. Respiratory bronchiole, d.al. Alveolar duct; a second alveolar duct is shown cut off. 

 a,a. Atria, s.al. Alveolar saccule, a.p. Alveolus, art. Pulmonary artery with its branches 

 to the atria and saccules, v. Pulmonary vein with its tributaries from the pleura (1), the 

 alveolar duct (2), and the place where the respiratory bronchiole divides into the two alveolar 

 ducts (3). 



a gland, since it is composed of a tree-like system of tubes terminating in expanded 

 spaces. Closely associated with the system of tubes are certain blood-vessels, 

 some of which take part in nourishing the organ, others participate in its special 

 mechanism. 



The lungs are two in number, and lie one on either side of the thoracic cavity, 

 separated by a i)artition known as the mediastinum (figs. 993, 997, 1000). Serous 

 membranes covering tlie latter riglit and h'ft are j)arts of two closed sacs, the 

 pleurae, eacli oi' which is reflected about a lung and the neighbouring chest-wall 

 after the manner of serous membranes in general. The space enclosetl within the 

 sac-walls is the pleural cavity, genetically a subdivision of the ca^lom. 



Form (figs. 994, 998;. — The lung is pyramidal or conical in form, with the base 

 [basis pulmonis] below and resting on the diai)}iragm, and with apex [apex pul- 

 monis] al)ove, in the root of the neck. Two surfaces, costal and mediastinal, are 

 described. The broad convex costal surface [fades costalis] is directed against 



