LARYNX, TRACHEA AND LUNGS 



i6s 



5 mm. the respiratory apparatus consists of the laryngeal groove and ridge, the 

 tubular trachea, and the two lung buds (Fig. 165 D). 



The Larynx. — In embryos of 5 to 6 mm. the oral end of the laryngeal groove 

 is bounded on either side by two rounded prominences, the arytenoid swellings, 

 which, continuous orally with a transverse ridge, form the furcula of His (Fig. 

 157 B). The transverse ridge becomes the epiglottis, and, as we saw in connec- 

 tion with the development of the tongue, it is derived from the third and fourth 

 branchial arches. In embryos of 15 mm. the arytenoid swelhngs are bent near 

 the middle. Their caudal portions become parallel, while their cephahc portions 



Trachea, 

 Respiratory anlage 



Esophagus 



Trachea 



Lung bud 

 Esophagus 



Apical hud 

 Primary bronchus 



Ventral hud 

 Esophagus 



Trachea 



Bronchus 



Ventral bud 



Fig. 171. — Diagrams of stages in the early development of the trachea and lungs of human embryos 

 (based on reconstructions by Bremer, Broman, Grosser, and Narath). X about SO. A, 2.5 mm ; B, 

 4 mm.; C, stage B in side view; D, 5 mm.; E, 7 mm. 



diverge nearly at right angles (Fig. 172). The opening into the larynx thus be- 

 comes T-shaped and ends blindly, as the laryngeal epithelium has fused. In 40 

 mm. fetuses (C R) this fusion is dissolved, the arytenoid swellings are withdrawn 

 from contact with the epiglottis, and the entrance to the larynx becomes oval in 

 form (Fig. 173). At 27 mm. the ventricles of the larynx appear and at 37 mm. 

 (C R) their margins indicate the position of the vocal cords. The epithelium of 

 the vocal cords is without cilia. The elastic and muscle fibers of the cords are 

 developed by the fifth month. 



At the end of the sixth week the cartilaginous skeleton of the larynx is indicated by sur- 

 rounding condensations of mesenchyme. The cartilage of the epiglottis appears relatively 



