DEVELOPMENT OF THE RESPIRATORY SYSTEM 829 



part of abdominal cavity. Its weight, in proportion to the weight of 

 the body at different ages, is as follows : At the end of the first month, 

 i to 3; at term, i to 18; in the adult, I to 36. Its structure is soft dur- 

 ing the first months. As development advances and as the relative 

 size of the liver diminishes, its tissue becomes more solid. 



The pancreas appears at the left side of the duodenum, by the 

 formation of two ducts leading from the intestine, which branch and 

 develop glandular structure at their extremities. The spleen is devel- 

 oped, about the same time, at the greater curvature of the stomach and 

 becomes distinct during the second month. 



The figures in Plate XVI show very clearly several of the important 

 stages in the development of the nervous system, the vertebrae, the ali- 

 mentary canal, liver and pancreas. 



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 glottidis, and 

 the single tube with which it is 

 connected is developed into the 

 trachea. At the lower extrem- 

 ity of this tube, a bifurcation ap- 

 pears, terminating first in one 



Fig. 230. Formation of the bronchial ramifica- 



and afterward in Several Ctlls- tions and of the pulmonary cells. A, B, development 

 d<> tar The bifurcated tnhp of the lungs, after Rathke ; C, D, historical develop- 



ae-sac. i ne tu De ^ after j Miiller ( Longet ) . 



constitutes, after the lungs are 

 developed, the primitive bronchia, at the extremities of which are the 

 branches of the bronchial tree. As the bronchia branch and subdivide, 

 they extend downward into what becomes afterward the cavity of the 

 thorax. The pulmonary vesicles are developed before the trachea. 

 The lungs contain no air during intra-uterine life and receive but a small 

 quantity of blood ; but at birth they become distended with air, are in- 

 creased thereby in volume and receive all the blood from the right ven- 

 tricle. This process of development is illustrated in Fig. 236. The 

 lungs appear, in the human embryo, 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 (see Plate XVI, Figs, i and 2). 



