650 



RESPIRATORY AND BUOYANCY SYSTEMS 



arise. Fifty to eighty terminal bronchioles have been estimated to be present 

 for each lobule of the human lung (Maximow and Bloom, '42, p. 465). From 

 each of the terminal bronchioles, a varying number of respiratory bronchioles 

 arise, which in turn give origin to the alveolar ducts, and, from the latter, 

 arise the alveolar sacs and alveoli. Each alveolus represents a thin-walled 

 compartment of the alveolar sac (fig. 309G). The exact cellular structure of 

 the terminal air compartments or alveoli is not clear. In the frog lung, a layer 

 of flattened epithelium is present. However, in the lung of the bird and the 

 mammal, this epithelial lining may not be complete, and the wall of the alveolus 

 may be formed, in part at least, by the endothelial cells of the surrounding 

 capillaries (fig. 299A; Palmer, '36; Clements, '38). 



4) Development of the Epiglottis and Voice Box. The epiglottis is the 

 structure which folds over the glottis and thus covers it during deglutition. 

 The glottis is the opening of the trachea into the pharynx. An epiglottis is 

 found only in mammals. It arises as a fold in the pharyngeal floor in the area 

 between the third and fourth visceral arches. It grows upward and backward 

 in front of the developing glottis (fig. 3 lOA-C) . In the meantime, the arytenoid 

 swellings or ridges appear on either side of the glottis. 



The larynx or voice box is an oval-shaped compartment at the anterior end 

 of the trachea in mammals. It is supported by cartilages derived from the 

 visceral arches (Chap. 15). The vocal cords arise as transverse folds along 

 the lateral sides of the laryngeal wall. 



5) Cellular Composition. The epithelial lining of the larynx, trachea, 

 bronchi, etc., is derived from the entodermal outpushing, whereas the sur- 



PLICA PHftRYNGO- EPIGtOTTICA OR 

 LATERAL GLOSSO-EPIGLOTTIC FOLD 



ROOT OF TONGUE 

 EPIGLOTTIS 



Fig. 310. Development of the epiglottis and entrance into the larynx in the human 

 embryo. (Consult also fig. 285.) (All figures slightly modified from Keibel and Mall: 

 Manual of Human Embryology, vol. II, '12, Philadelphia, Lippincott.) (A) About 

 16-mm., crown-rump length, 7 to 8 weeks. (B) About 40-mm., crown-rump length, 

 9 to 10 weeks. (C) Late fetal condition. 



