56 PRACTICAL HISTOLOGY. 



the mucous glands, whose acini lie between it and the cartilages, though they are most nume- 

 rous where no cartilage exists. Perhaps a lymphatic cord may be found in the submucous 

 coat. Observe the pulmonary blood-vessels and compare the relative thickness of their coats 

 {Indicate the general arrangement in PI. X., Fig. 3.) 



(H). The epithelium and glands have exactly the same structure as in the trachea. The 

 cut ends of the elastic fibres, stained bright yellow, are most numerous where the mucous mem- 

 brane is raised as a fold. Study the fibrous connective tissue mixed with elastic fibres which 

 exists outside and around the cartilages and blood-vessels. Select a transverse section of a 

 nerve, if possible with a ganglion in it. The ganglia and large branches of nerves will be 

 found outside the cartilages, and usually accompanied by one or more small blood-vessels 

 (bronchial). Observe the ganglionic cells, large oval corpuscles with a distinct nucleus and 

 nucleolus, and between or around them the cut ends of nerve-fibres medullated and non- 

 medullated, the whole surrounded by a fibrous sheath. (Indicate a small part of the wall of a 

 bronchus in PI. XL, Fig. i.) 



Vertical Section through the Pleura and the Subjacent Vesicular Tissue. Stain with logwood 

 and mount in dammar. 



EXAMINATION (L). Observe the pleura, made up of fibrous tissue stained blue, and note 

 its normal thickness. From its under surface it sends into the lung, at regular intervals, fine 

 processes of connective tissue the interlobular septa i.e. processes between adjoining lobules. 

 It may be possible to trace the connection of these septa with the connective tissue around a 

 bronchus. They contain many lymphatics, and are often pigmented from the presence of 

 charcoal or soot, especially in the human lung. They are of great importance pathologically, 

 and this connection of the pleural connective tissue with that which enters the lung at its root 

 is also important, e.g. in the production of dilatation of the bronchi and other pathological 

 processes. Here, as in other organs it is important to bear in mind the connection of the con- 

 nective tissue of the capsule with that in the interior of the organ. Observe now, the air- 

 vesicles, cut in every direction, but notice that those placed most superficially, i.e. next the 

 pleura, are -somewhat pyramidal or conical in shape, and have their bases directed towards the 

 pleura ; the air-vesicles elsewhere are polygonal in shape, and may or may not have a base to 

 them, which depends on the line of section. They are separated from each other by a very 

 small quantity of connective tissue, and by elastic fibres, which form a network round each 

 air-vesicle. Here and there a transverse section of a small bronchus may be found. It is 

 recognised by the cubical epithelium lining it, and by the presence of cartilage or glands in 

 its walls. In the vesicular structure search for a transverse section of an infundibulum. The 

 nuclei seen in the air-vesicles belong to the squames lining them, and those forming the 

 capillaries in their walls. Search for a small bronchus opened into longitudinally, and observe 

 its dichotomous division and its expansion into air- vesicles. (Indicate the pleura, and the shape 

 of the air-vesicles, in PL XL, Fig. 2.) 



(H). Observe thef/eura, which consists of two layers ; the superficial one contains a con- 

 siderable number of elastic fibres, and the lower, or sub-pleura, consists of looser connective 

 tissue, which is continued into the lung between the lobules as interlobular septa. In it may 

 be seen sections of the pleural blood-vessels and lymphatics, the latter appearing as fine slits. 

 The endothelium on the surface of the pleura is not visible. 



Observe an air-vesicle. Note its outline rounded or polyhedral. If its thin edge be seen 

 in profile, observe the squamous epithelium lining it, though it is better seen where a portion 

 of the wall of an alveolus is flat in the field. In the alveolar wall trace the branching elastic 



