342 



COMPARATIVE ANATOMY 



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 lined with a ciHated columnar epithelium 

 containing many mucus-secreting goblet cells. 



Fig. 286. — Stages in the development of the trachea, bronchi and lungs in the pig. 

 The pulmonary arteries are shown in black; the veins are cross hatched. Ep, bud of 

 eparterial bronchus. (From Patten's "Embryology of the Pig," after Flint.) 



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

 the cervical plexus of nerves, innervates the diaphragm. 



Development of the Ltmgs. During the fourth week of development 

 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 approxima- 

 tion of these paired lateral grooves and their union in the median plane, 

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

 cormexion with the pharynx is retained. The posterior blind end of the 



