LUNGS 275 



internal skeletal element (pessulus) which bears a semilunar mem- 

 brane on its lower sufTace. In many birds this type of syrinx is 

 often asymmetrical (fig. 297) and is expanded into a (usually) bony 

 resonating vesicle. In the tracheal type of syrinx the lateral portions 

 of the last tracheal rings disappear and the membrane which closes 

 the gap forms the vibratile part. In the bronchial syrinx the mem- 

 branes occur between two successive rings of each bronchus, each 

 ring being concave toward its fellow. By a shortening of the bron- 

 chial wall these membranes are forced as folds into the tube. In all 

 types of syrinx there are muscles attached to trachea and bronchi, 

 which, by moving these parts, alter the tension of the folds, thus 

 changing the note. 



In the mammals the trachea is elongate (shortest in the whales 

 and sirenians, dividing in the latter immediately behind the cricoid 

 into the two bronchi), and the cartilage rings are usually incomplete 

 dorsally, the gaps being closed by membrane. This structure allows 

 the tube to remain open under ordinary conditions and yet allows it 

 to give when food is passing down the oesophagus, just dorsal to it. 

 In the cetacea and sirenia the tracheal cartilages are sometimes 

 spirally arranged. 



Lungs 



The morphology of the lungs may be understood by following 

 their development in the mammals and then describing their modi- 

 fications in the various classes of vertebrates. As stated above the 

 lungs arise as a diverticulum (fig. 298, A) on the ventral side of the 

 pharynx which quickly divides into two sacs, the anlagen of the two 

 lungs. These are gradually pushed posteriorly toward the body 

 cavity, still retaining their connexion with the pharynx by the air 

 duct, and each consisting of an enlarged terminal vesicle connected 

 by a slender portion (the beginning of the primary bronchus) with 

 the undivided tracheal portion. With continued growth each ter- 

 minal vesicle divides again and again, the result being a number of 

 rounded vesicles connected with the primary bronchi by slender 

 tubes, the secondary bronchi (fig. 298, B). By a continuation of this 

 process tertiary and other bronchi are outlined, and also slender tubes, 

 the bronchioles, to be described later, which connect the terminal 

 vesicles with the ultimate bronchi. Next, the inner wall of each 

 vesicle becomes divided into small chambers, the alveoli, the whole 



