WITH THE MICROSCOPE. 137 



mistaken for epithelium, fig. 217, pi. XXXIII. In the lungs of the 

 frog and newt there are, however, distinct epithelial cells. 



Trachea and Bronchial Tubes. The mucous membrane of the 

 trachea and bronchial tubes must be examined in the recent state by 

 cutting thin sections with a very sharp knife. 



Beneath this mucous membrane is an abundant plexus of lymphatic 

 vessels. In many cases these contain lymph corpuscles and fatty 

 matter in a granular state, so that their arrangement may be easily 

 made out. The lymphatics upon the surface of the lung, immediately 

 beneath the pleura, may also sometimes be very clearly demonstrated. 

 I have one specimen in which these lymphatics are completely dis- 

 tended with large oil globules and granular matter, so that the position 

 of their valves is rendered very distinct, and the smallest branches can 

 be followed into the intervals between the lobules of the lung. In 

 this specimen the tubes certainly form a network, but in many situa- 

 tions appearances are observed which lead to the conclusion that 

 these tubes also commence in cascal extremities. 



In examining the ciliated epithelium of the air passages, it is only 

 necessary to scrape the surface gently, and, if necessary, the prepara- 

 tion may be moistened with a little serum, as water would very soon 

 stop the motion. 



Organs of Secretion. 



237. Salivary Glands and Pancreas. The investigation of the 

 salivary glands and pancreas scarcely requires any special remarks. 

 The best idea of their structure is obtained by subjecting one of the 

 smallest labial or buccal glands, and Brunner's glands to examination. 

 The ultimate follicles and epithelium are very easily demonstrated in 

 specimens which have been soaked for some time in glycerine. It is 

 often troublesome to .trace the continuity of the duct with the follicles, 

 in consequence [of some of the latter covering its terminal portion. 

 The ducts of the salivary gland and pancreas may sometimes be 

 injected, and it is advantageous to subject the organs to firm pressure 

 for some time previously, so as to absorb as much as possible of the 

 fluid they contain, and favour the entrance of the injection. Good 

 sections may often be obtained from specimens which have been 

 hardened in alcohol and soda. The arrangement of the capillaries is 

 easily made out in specimens injected with vermilion, chromate of 

 lead, or transparent injection. If the vessels are injected with gelatine 

 only, very instructive sections may be made. In such investigations, 

 however, it is necessary to make a vast number of sections and 



