234 



HISTOLOGY. 



The blood and lymphatic vessels and the nerves resemble those of the 

 salivary glands. The capillaries have notably wide meshes so that con- 

 siderable portions of the alveoli are not in contact with them. The nerves 

 end around the blood vessels, ducts and pancreatic cells. They are chiefly 

 nonmedullated sympathetic fibers from the coeliac plexus, associated with 

 scattered nerve cells within the pancreas. Lamellar corpuscles may be 

 found in the connective tissue. 



DEVELOPMENT OF THE RESPIRATORY TRACT. 



The respiratory system, consisting of the larynx, trachea, bronchi, 

 and lungs, arises as a gland-like subdivision of the entodermal tract. 

 Beginning opposite the third or fourth branchial arch, two longitudinal 



grooves develop, one on either side of the 

 embryonic ' pharynx.' They deepen pos- 

 teriorly and unite, thus separating the ven- 

 tral trachea from the dorsal oesophagus. 

 The trachea and oesophagus open ante- 

 riorly into the pharynx of the adult. The 

 anterior end of the trachea, with the epi- 

 glottis, thyreoid, cricoid and other carti- 

 lages which develop in the connective 

 tissue around it, constitutes the larynx. 

 Posteriorly the trachea bifurcates, as seen 

 in the front view of the embryo, Fig. 267, 

 A, and these primary subdivisions or 

 bronchi, further subdivide as shown in B. 

 In side view the right lung of an older em- 

 bryo is shown in Fig. 268; the left lung has 

 been cut away. The entodermal outpocket- 



ings are seen to lie in abundant connective tissue which is invaded by blood 

 vessels from three sources, the pulmonary arches, the left atrium and 

 the thoracic aorta. Some branches which grow from the azygos veins are 

 not shown. 



The pulmonary arches are two arteries, one on either side, extending 

 from the ventral to the dorsal aorta. Approximately midway in its course 

 each sends a branch to the lung of the corresponding side. The part of the 

 arch between this branch and the dorsal aorta is early obliterated on the right 

 side, but on the left it persists until birth as the ductus arteriosus (Fig. 268, 

 d,a.}. After birth it is reduced to a fibrous cord which sometimes retains 

 a minute lumen. The spiral division of the ventral aorta into the proximal 

 parts of the permanent aorta and pulmonary artery, has been referred to 



FIG. 267. RECONSTRUCTIONS OF THE 

 LUNGS or YOUNG EMBRYOS, SEEN 

 PROM THE VENTRAL SURFACE. 



A, A younger stage than B; ep, apical 

 bronchus; I, II, primary bronchi. 

 (His.) 



