954 ^ MANUAL OF ANATOMY 



valves. It is to be noted that the puknonary arteries carry venous 

 blood, whilst the pulmonary veins carry arterial blood. 



The bronchial arteries convey arterial blood to the lungs for the 

 nutrition of their component tissues. They will be described in 

 connection with the descending thoracic aorta, with which they are 

 associated. 



The bronchial veins return their blood chiefly into the right 

 azygos and upper left azygos veins respectively. 



Lymphatics.— The lymphatic vessels of each lung are arranged 

 in two sets — superficial and deep. At the hilum these two sets 

 open into the interhronchial glands. The superficial set receives the 

 lymphatics of the visceral or pulmonary pleura. 



Nerves. — These are derived from the anterior and posterior 

 pulmonary plexuses, which are formed by the pneumpgastric 

 nerves, aided by branches from the sympathetic. The nerves 

 penetrate as far as the air-cells, upon the walls of which they are 

 regarded as terminating in arborizations. The anterior and posterior 

 pulmonary plexuses will be found described on p. gG/-. 



Development of the Respiratory Apparatus. 



The respiratory apparatus consists of the larynx, trachea, and lungs. The 

 earliest indication of it is a median longitudinal groove on the inner aspect 

 of the ventral wall of the oesophageal part of the fore-gut. This groove is 

 called the pulmonary or laryngo-tracheal groove, and it produces an evagina- 

 tion of the ventral wall of the oesophagus. It consists of entoderm, derived 

 from that of the fore-gut, and it is covered by splanchnic mesoderm. This 

 groove gradually deepens, and by the fusion of its lips a longitudinal diver- 

 ticulum of the ventral wall of the oesophageal part of the fore-gut is formed, 

 called the pulmonary or laryngo-tracheal diverticulum. It lies on the ventral 

 aspect of the oesophagus, from which it becomes gradually separated in a 

 caudo-cephalic direction. At the cephalic end, however, the separation is 

 arrested, and a communication (superior laryngeal aperture) persists between 

 this part of the pulmonary diverticulum and the pharyngo-oesophageal tube. 

 The pulmonary diverticulum consists of (i) entoderm, derived from that of 

 the fore-gut, and (2) mesoderm, which invests it. 



The condition of matters now is that there are two tubes, dorsal or pharyngo- 

 CESOphageal, and ventral or pulmonary (laryngo-tracheal), which communicate 

 freely cephalicwards. 



Larynx. — The larynx is developed from the cephalic or proximal part of 

 the pulmonary or laryngo-tracheal diverticulum. (see the Larynx). 



Trachea. — The trachea is developed from the caudal or distal part of the 

 pulmonary or laryngo-tracheal diverticulum, the cartilaginous rings, con- 

 nective tissue and muscular tissue of the trachea being developed from the 

 mcsodermic investment of the primitive diverticulum. 



Lungs. — The rudiments of the lungs appear as two lung-buds, right and 

 left, which are formed by the bifurcation of the pulmonary diverticulum at 

 its caudal end. Like the diverticulum itself, these buds consist of entoderm, 

 invested by mesoderm. The entoderm of each lung-bud and of its various 

 ramifications furnishes all the epithelial elements, bronchial and alveolar, of 

 the corresponding lung. The mesoderm of the bud and of its various ramifi- 

 cations gives rise to the bloodvessels, connective and muscular tissues, and 

 cartilages of the bronchial tubes, as well as to the visceral pleura. The pedicles 

 of the lung-buds give rise to the bronchi. The right lung-bud gives off three 

 processes or vesicles, and the left lung-bud gives off two processes, and in this 



