THE RESPIRATORY SYSTEM 323 



which is completed rather rapidly on inspiration. The foetal and respiratory 

 cells of the adult lung have the sanae embryonic origin. During the early stages 

 of lung development the mesodermic tissue predominates, but with the rapid 

 growth of the tubules the proportion of the two changes until in the adult 

 lung the mesodermic tissue becomes restricted to the inconspicuous pulmonary 

 framework and the blood-vessels. 



TECHNIC 



(i) The technic for the largest bronchi is the same as for the trachea (technic 

 3, p. 304). The medium size and small bronchi are studied in sections of the 

 lung. 



(2) Lung and Bronchi. — Carefully remove the lungs and trachea (human, 

 dog, or cat) and tie into the trachea a cannula to which a funnel is attached. 

 Distend the lungs moderately (pressure of two to four inches) by pouring in 

 formalin-Miiller's fluid (technic 5, p. 7), and then immerse the whole in the same 

 fixative for twenty-four hours. Cut into small blocks, using a very sharp razor 

 so as not to squeeze the tissue, harden in alcohol, stain thin sections with ha;ma- 

 toxylin-eosin (technic i, p. 20), and mount in balsam or in eosin-glycerin. The 

 larger bronchi are found in sections near the root of the lung. The arrangement 

 of the pulmonary lobules is best seen in sections near and horizontal to the sur- 

 face. Sections perpendicular to and including the surface show the pulmonary 

 pleura. 



(3) Respiratory Epithelium (technic i, p. 78). 



(4) Elastic Tissue of the Lung (technic 3, p. 28). 



(5) Blood-vessels. — For the study of the blood-vessels, especially of the cap- 

 illary networks of the alveoU, sections of injected lung should be made. A fresh 

 lung is injected (page 25) with blue gelatin, through the pulmonary artery. 

 It is then hardened in alcohol, embedded in celloidin, and thick sections are 

 stained with eosin and mounted in balsam. 



General References for Further Study 



Miller, W. S.: Das Lungenlappchen, seine Blut- und Lymphgefasse. 

 Councilman: The Lobule of the Lung and its Relations to the Lymphatics. 

 Kolliker: Handbuch der Gewebelehre des Menschen. 



