LARYNX, TRACHEA AND LUNGS 



167 



The Lungs. — Soon after the lung anlages or stem buds are formed (in 5 mm. 

 embryos), the right bronchial bud becomes larger and is directed more caudally 

 (Fig. 171). At 7 mm. the stem bronchi give rise to two bronchial buds on the 

 right side, to one on the left. The smaller bronchial bud on the right side is the 

 apical bud. The right and left chief buds, known as ventral bronchi, soon bifur- 

 cate. There are thus formed three bronchial rami on the right side, two on the 

 left, and these correspond to the primitive lobes of the lungs (Fig. 174). 



On the left side, an apical bud is interpreted as being derived from tlie first ventral bron- 

 chus (Fig. 174). It develops later and remains small so that a lobe corresponding to the 

 upper lobe of the right lung is not developed (Narath). The upper lobe of the left lung thus 

 •would correspond to the upper and middle lobes of the right lung. 



Mediastinum 



Pleuro-peritoneal membrane- 

 Visceral pleura 

 Coronary ligament 



Inferior vena cava 



Parietal pleura 



Pleural cavity 

 Pleuro-peritoneal membrane 



Esophagus 

 Sinusoids of liver 



Ductus venosus 



Falciform ligament , . -, ,,. ,, , .... , 



' ' ~II all of ummtical cord 



Fig. 175. — Transverse section through the lungs and pleural cavities of a 10 mm. human embryo. X 23. 



The bronchial anlages continue to branch in such a way that the stem bud 

 is retained as the main bronchial stem (Fig. 174). That is, the branching is mono- 

 podia!, not dichotomous, lateral buds being given off from the stem bud proximal 

 to its growing tip. Only in the later stages of development has dichotomous 

 branching of the bronchi and the formation of two equal buds been described. 

 Such buds, formed dichotomously, do not remain of equal size (FHnt, Amer. 

 Jour. Anat., vol. 6, 1906). 



The entodermal anlages of the lungs and trachea are developed in a median 

 mass of mesenchyme dorsal and cranial to the peritoneal cavity. This tissue 

 forms a broad mesentery termed the mediastinum (Fig. 175). The right and left 



