53 



RESPIRATORY ORGANS LARYNX, TRACHEA, AND 



LUNGS. 



THE epiglottis has already been examined (p. 18). The preparation may be referred to 

 again. Its anterior surface is covered by stratified epithelium, and so is the posterior, but here 

 the submucous coat contains adenoid tissue, and a large number of mucous glands (p. 54), 

 which not unfrequently are lodged in depressions of the yellow elastic or reticular cartilage, 

 which constitutes the basis of the organ. Not unfrequently taste-bulbs are met with amongst 

 the epithelium on the posterior surface. 



The arytenoid cartilages consist of yellow fibre-cartilage, and sections of them show tran- 

 sition stages from ordinary hyaline to yellow fibro-cartilage (p. 19). The submucous tissue 

 contains many mucous glands. 



If desired the vocal cords may be hardened in chromic acid and spirit mixture, and vertical 

 sections made in the ordinary way. 



TRACHEA AND LUNGS. 



PREPARATION. Kill an etherised cat by bleeding, open the thorax, and allow the lungs 

 to collapse. Tie a cannula in the trachea, and with a syringe distend the trachea and lungs 

 with a mixture of chromic acid and spirit (p. xxxi). Suspend the lungs in a large quantity of 

 the same fluid. Change the fluid at the end of twenty-four hours ; after three days cut the 

 trachea and lungs into small pieces, and place them in fresh hardening fluid for a week or 

 ten days, and then, after washing them thoroughly, transfer them to alcohol until sections are 

 required. Preserve a piece of human trachea in the same way. Distend a piece of human 

 lung and preserve it as above. Make transverse and longitudinal sections of the trachea, 

 transverse sections through a bronchus as it enters the lung, and sections through the 

 pleura and subjacent lung, and also across a bronchus within the lung-substance. Preserve 

 the sections in preservative fluid (p. xl) until they are required. 



Sections of such an organ as the lung, with its numerous open spaces, ought always to be 

 made after steeping in gum, which fills the spaces ; and after freezing the organ is, as it were, 

 solid, and the gum can easily be dissolved out by water. Nothing is so good as a freezing 

 microtome for making sections of lung. 



The lungs are covered externally by a serous membrane, "ti\& pleura. Its epithelial cover- 

 ing can easily be demonstrated by the silver process (p. xlv) on the lungs of a cat just killed. 

 After silvering, a superficial slice is taken and mounted in dammar or glycerine. If the lung 

 be silvered while it is distended ad maximum, i.e. in a state of inspiration, the cells appear 

 as polygonal squames ; if it be collapsed, i.e. in a state of expiration, the cells appear slightly 

 cubical ; showing that the cells are soft and accommodate themselves during life to the changes 

 of size in the air-vesicles during the respiratory acts. 



