212 



ANIMALS 



pocket in the ventral wall of the digestive tract in the region 

 of the pharynx. This pocket divides into two branches which 

 develop into the right and left lungs. Each branch divides 

 many times so that a very complicated system of tubes is 

 formed. The air tubes are thin walled and a network of blood 

 capillaries closely surrounds them. 



481. Inspiration in the Amphibia and a few Reptiles is a 

 process analogous to swallowing. But in most Reptiles and in 



Vp 



FIG. 118. Three early stages in the development of a mammalian lung. In 

 B the alimentary canal is shown extending upward directly above the letter B. 

 Ep, I, and II, the bronchial tubes. Ap, pulmonary artery; Vp, pulmonary 

 veins. (McMurrich, after His.) 



Birds and Mammals the air is forced into the lungs by atmos- 

 pheric pressure upon muscular expansion of the thoracic cavity. 

 The latter is brought about by elevation of the ribs and, in 

 Mammals, by depression of the dome-shaped diaphragm. 



482. The course which the blood takes may or may not have 

 a fixed relation to the respiratory organs. In Worms and 

 Crustacea some blood is continually being oxygenated, and this, 

 mingling with the rest, is sufficient for the needs of the animal. 

 In Fishes, all the blood passing through the heart is forced 

 through the gills and then passes on to the tissues of the body. 



