923 GENERATION. 



Development of the Respiratory System. 



On the anterior surface of the membranous tube which becomes the oesophagus, an 

 elevation appears, which soon presents an opening into the oesophagus, the projection 

 forming, at this time, a single, hollow cul-de-sac. This opening becomes the rima glotti- 

 dis, and the single tube with which it is connected is developed into the trachea. At the 



lower extremity of this tube, a bifurca- 

 tion appears, terminating first in one, 

 and afterward, in several culs-de-sac. 

 The bifurcated tube constitutes, after 

 the lungs are developed, the primitive 

 bronchi, at the extremities of which are 

 the branches of the bronchial tree. As 

 the bronchi branch and subdivide, they 

 extend downward into what becomes 

 eventually the cavity of the thorax. 



. Formation of the bronchial ramifications and ., 



e pulmonary cells. A, B, development of the hm(js, The pulmonary vesicles, according to 



S% &&*&!& " e el0llmeni fi " e Bnrdaoh, are developed before the tra- 



chea. The lungs contain no air at any 



period of intra-uterine 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. 302. 

 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. We 

 have indicated the pulmonary structure as branching processes from the bronchial tubes, 

 merely for convenience of description. 



Development of the Face. 



The development of the face in the embryon of mammals is somewhat complex, but 

 it is peculiarly interesting, as its study enables us to comprehend the manner in which 

 various very common malformations of the face and palate are produced. The anterior 

 portion of the embryon, as we have seen in studying the development of the trunk, re- 

 mains 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. These are projecting plates of the 

 intermediate blastodermic layer, which gradually extend forward from the vertebral col- 

 umn. At the same time that the visceral plates are thus 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. 1 The first three arches, enumerating them from above downward, cor- 

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

 is not enumerated by some authors, who recognize but three arches, corresponds to the 

 superior cervical vertebrsD. 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 mal- 

 leus 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, becomes obliterated in front by a deposition of 

 plastic matter, but an opening remains by the side, which forms, externally, the external 



1 These arches correspond to the branchial vascular arches, which will be fully described in connection with the 

 development of the circulatory system. 



