DEVELOPMENT OF THE FACE. 825 



DEVELOPMENT OF THE RESPIRATORY SYSTEM. 



On the anterior surface of the membranous tube which becomes the 

 03sophagus, an elevation appears, which soon presents an opening into the 

 03sophagus, the projection forming 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 extremity of this 

 tube, a bifurcation appears, termi- 

 nating first in one and afterward 

 in several culs-de-sao. The bi- 

 furcated tube constitutes, after 

 the lungs are developed, the prim- 

 itive bronchia, at the extremities 

 of which are the branches of the 

 bronchial tree. As the bronchia 

 branch and subdivide, they extend 



, FIG. 306. Formation of the bronchial ramifications 



downward into what becomes 



eventually the cavity of the tho- 

 rax. The pulmonary vesicles are 



developed before the trachea (Burdach). The lungs contain no air at any 

 period of mtraiiterine life and receive but a small quantity of blood ; but 

 at birth they become distended with air, are increased thereby in volume and 

 receive all the blood from the right ventricle. This process of development 

 is illustrated in Fig. 306. The lungs appear, in the human embryon, during 

 the sixth week. The two portions into which the original bud is bifurcated 

 constitute the true pulmonary structure, and the formation of the trachea 

 and bronchial tubes occurs afterward and is secondary. 



DEVELOPMENT OF THE FACE. 



The anterior portion of the embryon remains open in front long after the 

 medullary plates have met at the back and enclosed the neural canal. The 

 common cavity of the thorax and abdomen is closed by the growth of the 

 visceral plates, which meet in front. At the time that the visceral plates are 

 closing over the thorax and abdomen, four distinct, tongue-like projections 

 appear, one above the other, by the sides of the neck. These are called the 

 visceral arches, and the slits between them are called the visceral clefts. The 

 first three arches, enumerating them from above downward, correspond, in 

 their origin, to the three primitive cerebral vesicles. The fourth arch which 

 is not enumerated by some authors, who recognize but three arches corre- 

 sponds to the superior cervical vertebrae. Of these four arches, the first is the 

 most important, as its development, in connection with that of the frontal 

 process, forms the face and the malleus and incus of the middle ear. The 

 second arch forms the lesser cornua of the hyoid bone, the stapes and the 

 styloid ligament. The third arch forms the body and the greater cornua of 

 the hyoid. The fourth arch forms the larynx. The first cleft, situated be- 

 tween the first and the second arch, is finally closed in front, but an opening 



