30 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



trachea. Here the cartilage rings are modified and coalesced 

 into a tympanic chamber, inside of which are vibratile membranes 

 which take the place of vocal cords, while muscles running from 

 trachea to bronchi alter the tension of the tympanic walls. 



The lungs are all but universally paired. In Ceratodiis the 

 single sac is without trace of separation into halves ; while in 

 some elongate vertebrates (snakes, caecilians) one lung is very 

 small, the other attaining great development. 



The lungs in the lower amphibia are but simple sacs with 

 smooth internal walls ; but in the frogs the internal surface be- 

 comes folded so that a number of cham- 

 bers, the infundibula, are formed, the walls 

 of which in turn are thrown into a number 

 of hemispherical cups (alveoli) lined with 

 pavement epithelium. In the walls of the 

 alveoli run capillary blood-vessels. The in- 

 fundibula open into a large central space 

 connected with the bronchus, and which 

 may be compared directly with the bron- 

 chioles (ififra) of higher forms. 



In many reptiles the conditions are 

 much as in the amphibia, some (snakes) 

 ha\'ing the distal portion of the lung with- 

 out infundibular sacs, others having these 

 complications of the surface extending 

 throughout the organ. In others the bron- 

 chus divides into secondary bronchi as it 

 enters the limgs, each of which is continued as a bronchiole ; 

 or we may have several bronchioles radiating from a single 

 bronchus (Alligator, Heloderma). In some forms (Chatnclcotf) 

 the bronchioles may connect with each other distally, a matter 

 of interest in connection with the parabronchi of the birds 

 (Jnfrd). 



The mammalian lung may be regarded as a complex of lungs 

 like those of a frog. The primary bronchus runs through the 

 lung, giving off on either side secondary bronchi, which in turn 

 bear tertiary bronchi. Each of these latter connects with small 

 tubes, the bronchioles, which lead to infundibula, as in the am- 



FlG. 33. Diagram of 

 lung of frog. Around 

 the margin are shown the 

 infundibula, the walls of 

 which are folded into 

 alveoli. 



