trachea 



bronchial bud 



pulmonary artery, 

 apical bronchus 



tracheal bifurcation 



upper 



right bronchus 



middle lobe 



lower lobe 



upper 



lobe 



left bronchus 



primordium of visceral pleura 



B 



lower lobe 



mesenchymal primordium of 

 stroma of lung 



8 mm 



20 mm 



Figure 9-29. Tfiree stages in the development of the lungs of o human. (After Patten, 1946) 



origin on the pubes, this muscular sheet fans out and in- 

 serts on the free posterior margin of the liver. It draws the 

 liver and heart mass backward, thus increasing the thoracic 

 volume. A similar mechanism is observed in the turtle and in 

 the lizard Tupinambis. Most lizards appear to respire by 

 means of body-wall movements only. 



Birds The lung of a bird ( Figure 9-30) is an extremely com- 

 pact organ broadly attached dorsally to the body wall as in 

 the turtle. From the lungs five pairs of air sacs extend out 

 through the remainder of the body cavity. These are the 

 cervical, clavicular, anterior and posterior thoracic, and 

 abdominal air sacs. (In the chameleon small sacs grow out 

 from the lung proper and suggest the origin of the air sacs 

 of the bird.) The air sacs of the bird extend into the large 

 bones of the body and even into the vertebrae. 



The volume of the body cavity is altered by movements 

 of the ribs acting on the sternum and through the abdommal 

 muscles. In flight, contraction of the pectoral muscles act- 

 ing on the wings compresses the body. The lungs are sep- 

 arated from the rest of the body cavity by a membrane 

 having the same associations as the pleuroperitoneal por- 

 tion of the mammalian diaphragm. This membrane has 

 several slips of muscle from the internal oblique series insert- 

 ing on it, and these can act to pull the membrane backward 

 and thus increase the pulmonary volume. 



Air is drawn in with expansion of the body volume and 

 it passes through the central cavity of the lung, the meso- 

 bronchus. The mesobronchus opens into a series of dorsal 

 and ventral bronchi which branch outward to all parts of 

 the lung. These are connected by numerous parabronchi, 



each of which is enclosed by a zone of capillary loops in 

 which the gaseous exchange occurs. The capillary loops of 

 one parabronchus open into those of the surrounding para- 

 bronchi. The mesobronchus leads back directly into the 

 posterior thoracic and abdominal air sacs, while the other 

 air sacs are connected with the ventral bronchi. With the 

 exception of the cervical, each air sac has several recurrent 

 bronchi which lead into the dorsal (or ventral) system of 

 bronchi. 



The flow of air through the lungs is not understood, but 

 it is generally conceded that the air in the respiratory capU- 

 laries is essentially fresh and changed widi each respiratory 

 movement. The air in the sacs may be quite stale when the 

 bird is at rest but is fresh when the bird flies. 



Amphibions Among the salamanders the lungs are gener- 

 ally elongate, corresponding to the body shape (Figure 9-11). 

 The effect of body shape is indicated also in the case of the 

 snake. The central cavity is usually large and the alveolar 

 tissues relatively thin, subdividing into progressively smaller 

 spaces by distinct septa. In Cryptobranchus the lungs are quite 

 small as compared with those oi Nectmus. In a few salaman- 

 ders lungs are lacking (some species oi Ambyslorm, Rhyacotriton, 

 Salamandnna, Desmognathus, Leurognathus, and the family 

 Plethodontidae). The body surface is used for respiratory ex- 

 change in aquatic forms and in some of the lungless terrestrial 



types. 



The lungs of the frog are ovoid sacs, whose walls are 

 divided into progressively smaller spaces by septa. The 

 lungs are supported by a mesentery attached to the dorsal 

 mesentery of the gut. The posterior end of the lung projects 



THE RESPIRATORY SYSTEM 



281 



