LARYNX, TRACHEA AND LUNGS 



175 



thyreoid cartilage is formed as two lateral plates, each of which has two centers of chondrifi- 

 cation. These plates grow ventrad and fuse in the median line. 



The anlages of the cricoid and arytenoid cartilages are at first continuous. Later, sepa- 

 rate cartilage centers develop for the arytenoids. The cricoid is at first incomplete dorsad 

 but eventually forms a complete ring. The cricoid may therefore be regarded as a modified 

 tracheal ring. The corniculate cartilages are portions of the arytenoid cartilages and separate 

 from them. The cuneiform cartilages are derived from the cartilage of the epiglottis. 



The Tracheal Tube. — This gradually elongates during development and its 

 columnar epithelium becomes ciliated. Muscle fibers and the anlages of the 

 cartilaginous rings appear at 17 mm. The glands develop as ingrowths of the 

 epithelium during the last five months of pregnancy. 



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

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



c) 



Fig. 168. — Dorsal and ventral views of the lunRs from a human embryo of five weeks (Merkel). Ap, 

 apical bronchus; Di, D2, etc., dorsal; Vi , ¥2, etc., ventral bronchi; Jc, infracardial bronchus. 



(Fig. 165). 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 other buds, right and left, are known as ventral bronchi. 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. 168). 



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

 chus. It develops later and remains small so that a lobe corresponding to the upper lobe of 

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

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



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

 becomes the main bronchial stem (Fig. 168). That is, the branching is mono- 

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

 in the later stages of development has dichotomous branching of the bronchi and 



