136 HOW TO WORK 



a cell for a newt or other animal may be made upon the same plan. 

 The circulation of the blood in the capillary vessels of a mammalian 

 animal may be studied in the thin membrane forming the wing of a 

 young bat. 



The examination of the moving objects alluded to in this section, 

 should be conducted with the aid of the binocular. 



Examination of Organs of Respiration. 



286. L,ung. There is not much difficulty in demonstrating the 

 different tissues of which the lung is composed. Small pieces may 

 be cut off, and spread out upon the glass slide in the usual way ; the 

 preparation being moistened with water or serum. The addition of a 

 little acetic acid causes the yellow elastic tissue to become very 

 distinct. The boundaries and arrangement of the air-cells may also 

 be readily shown. 



No opinion with reference to the nature of the walls of the air- 

 cells can be arrived at, unless injected as well as uninjected specimens 

 are examined. The twisted and shrunken capillaries of the recent 

 lung containing a few blood corpuscles, produce an appearance which 

 is very likely to give rise to erroneous inferences with regard to the 

 disposition and coverings of these vessels. Either the Prussian blue 

 or carmine injecting fluid may be employed. A most instructive 

 preparation of the lung, however, is made by injecting the vessels 

 with tolerably thick transparent gelatine, which transudes through 

 their walls, and fills the air-cells. After the lung has been thoroughly 

 injected, it is set aside to get cool. Thin slices may be examined, 

 and the vessels will be seen in situ apparently bare, and uncovered 

 oy epithelium. (Physiological Anatomy, Todd and Bowman, page 

 393. Mr. Rainey in the Medico-Chirurgical Transactions, 

 vol. XXXII. 1849, P a S e 47-) 



Much difference of opinion has been expressed with reference to 

 the existence of epithelium in the air-cells of the lung. I have lately 

 carefully examined healthy human lungs which have been prepared 

 in various ways, and have completely failed to demonstrate the 

 presence of such a structure in the healthy adult, or in the lungs of 

 several mammalian animals. I have never seen such appearances as 

 are represented in many drawings, showing this epithelium so dis- 

 tinctly, that one would be led to conclude that it was to be seen 

 without the slightest difficulty. In the foetus and young child, how- 

 ever, cells are found in the air-cells, but it seems to me very doubtful 

 if these take any part in the function of respiration. The masses of 

 germinal matter (nuclei) of the capillary walls may be very easily 



