5G2 THE HUMAN EMBRYO. 



9. The Lungs, 



On the fifteenth day (Fig. 237), a swelling is present on the 

 floor of the pharynx, opposite the first, second, and third branchial 

 arches ; and along the middle of this swelling, or furcula, FL, 

 there runs a longitudinal groove. 



By the sixteenth day this groove is much more pronounced, 

 and its posterior end leads into a short blind pocket- 

 By the end of the third week the pocket has become much 

 deeper, extending backwards, ventral to the oesophagus and 

 independently of this ; and its hinder end is split into right and 

 left lobes (cf. Fig. 238, LG). These lobes are the rudiments of 

 the lungs ; the tube leading to them is the trachea ; and the 

 slit-like opening, or groove, on the floor of the pharynx is the 

 future glottis. 



During the fourth week the lungs grow rapidly, extending 

 back alongside the oesophagus, and dorsal to the heart (Figs. 

 216 and 233, Lg) ; their distal ends are enlarged and are 

 commencing to divide into lobes. The right lung has three 

 terminal buds or lobes, and the left lung two ; these buds form- 

 ing the rudiments of the five lobes of the adult lungs. 



During the fifth week the lungs continue to increase rapidly ; 

 the main lobes elongate greatly, and give rise to secondary and 

 tertiary buds, which end in slightly expanded ampullaB (Figs. 

 235 and 236, Lg). 



The further development of the lungs consists in a continua- 

 tion of the process of budding, by which new tubules and 

 ampullae arise from the older ones, either by dichotomous 

 division, or, as in the. later stages, by lateral branching. The 

 air cells appear, as closely set pouchings of the walls of the 

 ampullae, which are not recognisable until the time of birth. 



The trachea is at first short, but rapidly elongates during 

 the fifth and following weeks. The larynx first becomes evident, 

 as a dilatation of the anterior part of the trachea, towards the 

 end of the fifth week (Fig. 234). The vocal cords, and the 

 ventricles of the larynx, are not formed until about the fourth 

 month. 



The anterior, median part of the furcula becomes the epi- 

 glottis (Fig. 233, Mp) ; while the lateral ridges give rise to the 

 ary-epiglottic folds and the arytenoid cartilages. The thyroid 

 cartilage is said by Callender, and by His, to be formed from the 



