THE RESPIRATORY SYSTEM 257 



ciliated stratified columnar epithelium. Below this is a submucous 

 connective tissue containing many mucous glands derived from the 

 mucous layer. Between the cartilage and the mucosa is a layer of 

 circular muscle fibers. 



Nasal Passages. Air is taken in and expired through the nasal 

 passages. The external orifices are the external nares and the openings 

 into the pharynx are the choanae. The paired nasal passages are sepa- 

 rated from one another by the nasal septum and the median plates of the 

 maxillary and vomer bones, and from the cavity of the mouth by maxillary 

 and palatine bones. They are hned with a ciliated columnar epithelium 

 containing many mucus-secreting goblet cells. 



Diaphragm. Air is drawn into the lungs under atmospheric pressure 

 as the result of the contraction of the muscles of the diaphragm and ribs. 

 Their contraction raises the rib-basket and flattens the dome-shaped dia- 

 phragm. As a result, the size of the pleuroperitoneal cavity is increased. 

 To fill the enlarged space thus formed, air enters the lungs and inflates 

 them to the size of the chest cavity. The diaphragm is a -muscular 

 partition which divides the cavity of the chest from that of the abdomen 

 and which occurs only in man and other mammals. Lacking a diaphragm, 

 the amphibians must swallow their air. The phrenic nerve, a branch of 

 the cervical plexus of nerves, innervates the diaphragm. 



Development of the Lungs. During the fourth week of the human 

 embryo a laryngo-tracheal groove is formed in the floor of the pharynx 

 immediately behind the fourth gill pouch. Externally this groove appears 

 as a ridge which is bordered on either side by a groove or furrow. By the 

 approximation of these paired lateral grooves and their union in the median 

 plane, the lung anlage is separated from the pharynx, except anteriorly 

 where connexion with the pharynx is retained. The posterior blind end of 

 the diverticulum swells to form the lung anlage while the less expanded 

 anterior portion becomes the larynx and trachea (Fig. 235). The lung 

 anlage later divides into two lateral buds which, by successive subdivision, 

 gradually assume the adult structure. (Fig. 239) 



The cartilages which support the larynx correspond exactly with 

 those which in aquatic vertebrates support the fourth and fifth branchial 

 arches. The muscles of these arches form the laryngeal muscles. Vocal 

 cords appear during the eleventh week. 



Beginning with the fifth week, the paired lung-buds branch in the 

 manner of a compound tubular gland. In this way, the entire lining of 

 the lungs is derived from the pharyngeal endoderm. The connective 

 tissue develops from the surrounding mesenchyma. The splanchnic 

 mesoderm forms the serosa, which covers the lungs and lines the chest 

 cavity. As the two lungs enlarge, they push laterally into the body-cavity 

 and by their ventral extension nearly surround the heart, from which they 



