128 NORMAL HISTOLOGY. 



outer layer of cells possesses cilia. Among these occur the goblet 

 cells. 



The sub-mucosa is made up of loose connective tissue containing 

 glands, blood-vessels, lymphatics, and nerves. 



The fibrous coat invests the outer surface of the trachea and 

 has embedded in its tissue the incomplete rings of hyaline cartilage. 

 These rings in transverse section appear fusiform in outline, extend 

 over three-fourths, or less, of the circumference, overlap by their 

 edges, and exhibit pcrichondrium, matrix, lacunas, and cells. At- 

 tached to the perichondrlum upon the inner surfaces of the cartilage 

 are transverse bundles of smooth muscle, while across the intervals 

 between the rings are other bundles, the whole contrivance serving 

 to contract the tube. There are but few longitudinal muscular bun- 

 dles. 



Laboratory exercise No. 40. Trachea. Fix with ehromic acid, harden 

 with alcohol, embed in cello idin, and stain with hsematoxylin, method 

 No. 7. Make out the structures of the mucosa, observing especially the 

 layer of ciliated epithelium containing* a few goblet cells, the membraria 

 propria, and the two layers of the tunica propria. Search for glands, 

 blood-vessels, etc., in the sub-mucosa. Study the form and structure of 

 the hyaline cartilage in the fibrous layer. Note the disposition of the 

 bundles of smooth muscle. Demonstrate the loose areolar tissue which 

 unites the fibrous coat to adjacent structures. Drawings. 



The bronchi. These do not differ materially in structure from 

 the trachea. As the bronchial tubes decrease in size there are cer- 

 tain modifications in structure, such as: (1) The epithelium be- 

 comes reduced until in the smallest tubes there is but a single layer 

 of ciliated cells. (2) The elastic tissue disappears from the mu- 

 cosa and is replaced by a smooth muscle layer that corresponds to 

 the muscularis mucosae. (3) The cartilage gradually decreases and 

 totally disappears in the terminal bronchioles. 



THE LUNGS. 



The lungs are derived chiefly from the mesoderm. They resem- 

 ble in structure racemose glands. Their nerve supply is received 

 from the central and sympathetic systems. 



