216 STRUCTURE OF THE VERTEBRATES 



pressing the mouth and raising the tongue. This method is only 

 slightly modified in the primitive lung breathing urodeles; but 

 in the frogs the air is drawn into the mouth cavity through 

 the nares which open immediately behind the teeth, and the air 

 is forced through the glottis with the floor of the mouth. Rep- 

 tiles, with the exception of the Chelonia, and Crocodilia, depend 

 almost entirely upon the ribs as the respiratory mechanism. The 

 contraction of the costal muscles forces the air from the lungs; 

 and as the muscles relax the glottis opens and atmospheric pres- 

 sure fills the lungs again. The crocodiles and alligators develop 

 a muscular diaphragm which separates the pleural cavities from 

 the abdominal cavity and is an aid in respiration. 



The diaphragm is highly developed in the mammals. It is a 

 dome-shaped band of muscle, the convex side being toward the 

 pleural cavities. As the costal muscles contract, pulling the ribs 

 downward and inward, the diaphragm relaxes and the air is 

 forced from the lungs. The reverse process greatly increases the 

 size of the chest, and air rushes in when the glottis is opened. 

 In the human, at least, diaphragmatic breathing has largely 

 replaced costal breathing. 



4. Development of the Larynx. The larynx is the supporting 

 mechanism of the glottis and upper end of the trachea. During 

 evolution the development of the lar3^nx has been correlated with 

 the development of cartilages supporting the trachea and bron- 

 chi. The cartilages of the trachea are incomplete dorsally. Those 

 surrounding the lower portion where the trachea bifurcates, and 

 also those surrounding the bronchi, are complete annular car- 

 tilages. The tracheal and laryngeal cartilages differ in origin, the 

 latter being modified from the visceral (branchial) cartilages 

 of the embryo. It will be recalled that the first and second 

 visceral arches form the jaws and hyoid apparatus in all verte- 

 brates above the cyclostomes, the posterior (branchial) cartilages 

 supporting the gills. In the dipnoan and crossopterygian fish, 

 where the swim bladder is modified into a respiratory organ, the 

 opening of the trachea is unsupported; but with the reduction of 

 the number of gill openings and gill cartilages in the amphibia, 

 the larynx begins its development. The seventh visceral (fifth 

 branchial) cartilages are the first to become modified. These 

 form the arytenoid cartilages on either side of the glottis. 



