PANCREAS 



295 



there being from 13,00010 56,000 in the entire pancreas of guinea-pigs 

 (Bensley), the average being twenty-two islands per cubic millimeter. In 

 all stages, both in the guinea-pig and in man, they are usually most numer- 

 ous in the tail of the pancreas, and least numerous in its head (Opie, Johns 

 Hopkins Hosp. Bull., 1900, vol. n, pp. 205-209). 



RESPIRATORY APPARATUS. 



Development. The respiratory apparatus, consisting of the larynx, 

 trachea, bronchi, and lungs, arises as a median ventral outgrowth of the 

 fore-gut, immediately behind the last pharyngeal pouches. It apparently 

 is in no way related to the branchial pouches, but it may correspond with 

 the air-bladder of the bony fishes. At 

 the stage when the lung-bud develops, 

 the fore-gut is laterally flattened, so that 

 its lumen is a dorso-ventral cleft. The 

 lung-bud develops as a pear-shaped swell- 

 ing, directed downward, on the ventral 

 border of the fore-gut; and this swelling 

 becomes split off, from below upward, to 

 form the trachea, which is at first short 

 but which rapidly elongates. The upper 

 end of the trachea, with the cartilages 

 which develop around it, constitutes the 

 larynx. At the lower end of the trachea, 

 the pyriform dilatation spreads out on 

 either side to form the primary bronchi 

 (Fig. 293, A). ^ 



The tracheal and bronchial tubes are lodged in a mass of connective 

 tissue, situated above and behind the pericardial cavity, and since this 

 tissue stands in the middle of the thorax it is known as the mediastinum. 

 It is comparable with a broad mesentery. As the bronchi push out later- 

 ally they occupy right and left folds bulging from the mediastinum, called 

 by Ravn the pulmonary wings (ala pulmonales). Into these the bronchi 

 extend and produce branches after the manner of a gland (Fig. 293, B). 

 The pulmonary wings consist of mesenchyma, covered by the epithelium 

 which lines the body cavity. At first they project into the part of the body 

 cavity which connects the peritoneal with the pericardial cavity; later, by 

 the development of the pleuro-pericardial and pleuro-peritoneal membranes 

 respectively (the latter being a part of the diaphragm) the chamber into 

 which the pulmonary wings project is entirely cut off from the rest of the 

 body cavity. On either side, it forms a pleura! cavity (see Fig. 169, p. 

 175). The epithelium and underlying connective tissue covering the pul- 



FIG. 293. RECONSTRUCTIONS OF " T H s 

 LUNGS OF YOUNG EMBRYOS, SEEN FROM 

 THE VENTRAL SURFACE. (His.) 



A, A younger stage than B; ep, apical 

 bronchus; I. II, primary bronchi. 



