THE LUNGS 131 



Fig. 88 shows a single alveolus, the vessels of which have been 

 injected with a solution of colored gelatin. The alveolus has been 

 divided through the middle, and shows as a cup -shaped cavity. 

 The fibrous marginal walls are indicated, with their tortuoua_ 

 capillaries. The epithelial cells lining the bottom are obscured by 

 the opaque capillaries, and shown only between the loops. It is 

 probable that these cells cover the plexus completely, as they line 

 the alveoli. 



We now encounter an obstacle which will frequently be met in 

 our study of organs. It consists of the difficulty in recognizing in 

 sections the plan of structure which we have learned is peculiar to 

 the organ under consideration. For example: A lung has been 

 compared to a tree. The bronchi are the representatives of the 

 branches, and the air -sacs of the fruit. Well, we make a section 

 from human lung it matters little as to the direction with every 

 possible care, and the image in the field of the microscope resem- 

 bles a fragment of ragged lace more nearly than anything else! 

 The arrangement of the tubes and alveoli of the lung has been 

 determined by filling the cavities with melted wax, which, when 

 cold, and the tissue destroyed by acid, gives a perfect mould of the 

 organ. A section gives us but a single plane, and this fact must be 

 always borne in mind. 



PRACTICAL DEMONSTRATION 



With a very sharp razor, cut centimeter cubes from pig's lung. Select 

 portions free from large bronchi, with the pleura on one side at least, and 

 harden with strong alcohol. Human lung, as fresh as possible, may be treated 

 in the same manner. The epithelium of the alveoli shows best in young lung. 

 Lung must be made very hard, or thin sections cannot be cut. If the ordinary 

 ninety-five per cent, alcohol does not harden sufficiently, the process may be 

 completed by transferring the tissue for twenty-four hours to absolute alcohol. 

 The celloidin process is well adapted to this structure. 



Stain the sections with borax -carmine, or hsematoxylin and eosin. Mount 

 in balsam. 



SECTION OF LUNG OF PIG (Vide Fig. 89) 



OBSERVE : 

 (L.) 



1. The large scalloped openings, A, A, transversely divided 

 infundibula. 



2. The divided alveoli, B, B, so sectioned as to cut off both 



