5 8 PRACTICAL HISTOLOGY. 



required, cut the lung into pieces, and make sections by freezing. Expose the sections to the 

 action of light, until they become brownish in colour. Stain a section with picrocarmine for 

 twenty-four hours, and mount it in glycerine ; and another with logwood for a few minutes, 

 and mount it in dammar. 



EXAMINATION (L). Observe the same general arrangement of the air-vesicles as already 

 described. Select an air-vesicle where a portion of its wall is seen on the flat, and examine. 



(H). Observe the silver lines, indicating the existence of epithelium (squamous), lining the 

 vesicle. These lines bound polygonal areas. Between these larger and clear areas notice 

 small brown granular areas or polyhedral cells, in groups of two or three, some of them 

 with a nucleus stained red or blue. These are young epithelial cells, and are thicker than 

 the squames or placoids. Transition forms exist between them and the clear squames. 

 During a maximum distension of the lung they yield and become flattened out. In a well- 

 distended alveolus, narrow angular apertures, especially in the cement lines, are to be seen. 

 These are the so-called pseudostomata, or small apertures which lead into the lymph-canalicular 

 system of the alveolar wall. (Indicate the squames and small granular cells in PI. XL, Fig. 4.) 



BLOOD-VESSELS OF THE LUNGS. 



The branches of the pulmonary artery break up into capillaries, which are distributed on 

 the walls of the air-vesicles. At the root of the lung the branches of the pulmonary artery 

 and vein and bronchus (p. 55) are found together, the bronchus in the middle, with a blood- 

 vessel on each side of it, within the lung the pulmonary vein pursues a separate course. 

 The bronchial arteries enter the lung at its root, and run in the adventitia of the large bronchi, 

 which they supply with blood. Sections of them will be found near the nerve-trunks at the 

 root of the lung (p. 55). 



PEEPAEATION. Make sections of a lung whose blood-vessels pulmonary artery, or 

 vein, or both have been injected with a carmine gelatine (p. li) or Prussian blue (p. li) mass, 

 and mount them in dammar. 



EXAMINATION (L). Notice the extremely dense plexus of capillaries on the walls of 

 each air-vesicle. Trace a small arteriole from a large branch of the pulmonary artery, and 

 note that it may supply two or three adjacent alveoli ; while the efferent vein usually passes 

 off at the other side. (Indicate the arrangement of the capillaries in PL XL, Fig. 5.) 



(H). Study the capillaries ; note the dense network of more or less wavy capillaries, and 

 especially on a septum between two adjacent alveoli. A twisted capillary may be seen lying 

 at one time in one alveolus, at another in the adjoining alveoli. The space between the 

 capillaries varies according as the lung has been kept distended or not. In preparations which 

 have been placed in alcohol, the air-vesicles shrink considerably, and hence the alveolar 

 capillaries appear relatively close to each other. The large arterial and venous branches lie 

 in the interlobular septa, which are continuous with the adventitia. 



LYMPHATICS OF THE LUNG. 



These are very numerous, and form three systems : (i) the subpleural lymphatics occur 

 in the deep layer of the pleura, and communicate with the pleural cavity on the one hand, 

 and have a direct connection with the lymphatic canalicular system, which lies in the alveolar 

 wall, and which communicates with the pseudo-stomata of the air-vesicles. Branches of 

 this system pass through the interlobular septa to reach (2) the peri-vascular lymphatics 



