410 GENERATION. 



nium in large quantity, but its function is connected exclu- 

 sively with excretion. 1 



Development of the Respiratory System. 



On the anterior surface of the membranous tube which 

 becomes the oesophagus, an elevation appears, which soon 

 presents an opening into the oesophagus, the projection form- 

 ing, at this time, a single, hollow cul-de-sac. This opening 

 becomes the rima glottidis, and the single tube with which it 

 is connected is developed into the trachea. At the lower ex- 

 tremity of this tube, a bifurcation appears, terminating first 

 in one, and afterward, in several culs-de-sac. The bifurcated 

 tube constitutes, after the lungs are developed, the primitive 

 bronchi, at the extremities of which are the branches of the 



FIG. 45. 



Formation of the bronchial ramifications and of the pulmonary cells. A, B, development 

 of the lungs, after Rathke ; C, D, histological development of the lungs, after J Miiller. 

 (LoNGET, Traite de physiologie, Paris, 1869, tome iii., p. 90T.) 



bronchial tree. As the bronchi branch and subdivide, they 

 extend downward into what becomes eventually the cavity of 

 the thorax. The pulmonary vesicles, according to Burdach, 

 are developed before the trachea. The lungs contain no 

 air at any period of intra- uterine life, and receive but 

 a small quantity of blood ; but, at birth, they become dis- 

 tended with air, are increased thereby in volume, and receive 

 all the blood from the right ventricle. This process of devel- 

 opment is illustrated in Fig. 45. According to Burdach, the 



1 See vol. iii., Excretion, p. 267, et seq. 



