xn. J THE EXCRETORY ORGAXS. 467 



Analogous motions of the abdominal muscles and ribs 

 alternately dilate and contract the body-cavity, and so fill 

 and empty the lungs in Saurians and Serpents. 



In Chelonians, where the ribs are immovably fixed, and the 

 ventral side of the body is covered with a solid exo-skeleton 

 (the plastron), inhalation is effected by forcing air down the 

 windpipe by an action analogous to swallowing. The mouth 

 being filled with air, the posterior nares and all possible 

 exits are closed, except that leading to the lungs ; and down 

 that exit air is forced by a contraction of the muscles of the 

 throat. Exhalation is effected by means of muscular con- 

 tractions at the most post-axial part of the abdominal cavity. 



In Batrachians (which are devoid of ribs, or have but very 

 short ones) the respiratory movements are similarly effected, 

 air being swallowed by contractions of the muscles of the 

 throat, and expelled by those of the abdominal muscles. 

 Thus to suffocate a Frog it is sufficient to keep its mouth 

 open. 



4. The lungs of. man communicate with the mouth and so 

 with the external air, by means of a single channel, the wind- 

 pipe or TRACHEA ; which divides at its lower end into two 

 branches, termed BRONCHI, one bronchus going to each lung. 



The trachea is a straight, permanently open tube, com- 

 mencing above at the larynx (shortly to be described), which 

 is, as it were, its expanded upper end. It descends, in the 

 middle line, in front of the oesophagus, to opposite the third 

 dorsal vertebra, where it is crossed in front by the arch of the 

 aorta, and bifurcates, dividing into the right and left bronchus. 



The trachea is formed of membrane strengthened by a 

 series of from sixteen to twenty horizontal cartilaginous rings, 

 which are not however complete behind, so that the hinder 

 wall of the tube, being entirely membranous, is flattened. 

 The tube is lined with mucous membrane, and its wall con- 

 tains muscular fibres and cellular tissue, with glands, vessels, 

 and nerves. 



The bronchi have the same structure as the trachea, except 

 that the cartilaginous rings are shorter and narrower. There 

 are from six to eight in the right bronchus ; from nine to 

 twelve in the left. 



Within the lung the bronchi divide and subdivide, like the 

 branches, branchlets, and twigs of a tree. These successively 

 smaller and smaller tubes are kept distended by successively 

 smaller and more delicate cartilages, which are no longer in 

 the form of rings incomplete behind, but are irregularly-shaped 

 H H 2 



