STRUCTURE OF THE VERTEBRATES 



215 



into the humeri can be demonstrated by cutting the humerus of 

 a pigeon and blowing smoke into the trachea. In addition to 

 lightening the animal, the mechanism tends to lessen the total 

 amount of air left in the lungs. Therefore the air, which is 

 normally only slightly depleted of its oxygen content, passes 

 across the vascular ducts at inspiration and expiration as it is 

 forced into and from the air sacs. 



]\Iammals have complexly lobulated lungs, with innumerable 

 bronchioles and terminal alveoli. The structures have evolved 

 so far that there is no sign of a central air cavity in any of the 

 lobes. The idea of structure can best be gained by a description 

 of the development of mammalian lungs. The human lungs be- 

 gin in embryos of about 3.5 millimeters as a ventral groove along 



'■^Lung 

 bud 



■Esophagus 



-Secondary 

 bronchi 



Fig. 126. Embryology of the Human Lung. The secondary bronchi divide 

 minutely, each ending in an alveolus. (Adapted from His). 



the esophagus. The posterior portion of the groove soon becomes 

 separated from the floor of the gut and forms a blind tube open- 

 ing into the pharynx. This opening is retained as the glottis. 

 The posterior end divides into two lobes, the distal ends be- 

 coming vesicular. At this stage there is a trachea and two 

 bronchi. The latter continue to divide, the early divisions forming 

 the larger lobes of the lung. The bronchi continue to divide into 

 smaller and smaller tubes until microscopic bronchioles are 

 formed. Each bronchiole terminates in a spheroidal alveolus. The 

 endodermal organ is enveloped and bound together by meso- 

 dermal tissues. Capillaries from the pulmonary arteries ramify 

 around the smaller tubes. 



3. INIechaxism of Breathixg. Most amphibian tadpoles take 

 the air into the mouth and force it into the trachea by com- 



