. Comparative anatomy. Anatomy, Comparative. 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-s
. Comparative anatomy. Anatomy, Comparative. 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 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 wher
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