SECT. 178.] EPITHELIUM OF THE AIR-VESICLES. 387 



ment-epithelium without cilia, which forms a single layer, and 

 jests immediately on the fibrous coat. Its cells measure c/005"' 

 to o*oo7'" in diameter, and 0-003'" to °' 00 4" m thickness; they 

 are pale, granular, and polygonal, in disease sometimes containing 

 fat-granules. A regular desquamation of this epithelium has been 

 stated to take place by some authors, but with as little evidence 

 lure as in the trachea and bronchi. It is true, that single cells ol 

 this epithelium may become mingled with the bronchial mucus, 

 whether by accident or by disease, and, after death, the epithelium 

 of the air-cells in man is frequently found free inside the air-cells 

 and in the finest bronchia ; but the epithelium may be observed in 

 situ in some alveoli in almost every human lung, while, in newly 

 killed animals, there is not the slightest difficulty in observing this 

 membrane in its normal position. 



The interlobular connective tissue of the lung is scanty, even be- 

 tween the secondary lobules, and between the primary or ultimate 

 lobules it exists in exceedingly small quantity. It consists of ordi- 

 nary connective tissue with fine elastic fibres, and contains, in the 

 adult, more or less of a blackish pigment, in the form of small 

 granules, disposed irregularly and in masses ; crystals of pigment 

 may also be found, and these are never included in cells. The walls 

 of the alveoli themselves also very frequently contain this pigment, 

 which, when it is deposited regularly and in small quantities, allows 

 the contours of the secondary lobules, and frequently even those 

 of the primary, to stand out very beautifully. 



The existence of an epithelium in the air-cells has recently been disputed 

 by Rainey, as well as by Jfandl (J/icr. Anat , ii. p. 327), and Ecker. On the 

 other hand, Radclyffe Hall, and Brittan have observed it in reptiles, mam- 

 malian animals, and in man ; Black (Monthly Journal, 1853, p. 2) and Williams 

 also confirm its existence in man. 



§ 178. Vessels and Nerves of the Lungs. — The lungs stand quite 

 by themselves in the disposition of their blood-vessels, possessing 

 two complete systems of vessels which are, for the most part, 

 wholly distinct from each other. They are, the system of the 

 bronchial vessels, for the nourishment of certain parts of their 

 structure, and the system of pulmonary vessels, for the performance 

 of the special functions of the organs. The branches of the pul- 

 monary artery generally follow the bronchia, and lie above and in 

 front of these tubes. They also divide in a dichotomous manner, 

 but more frequently than the bronchia, and they therefore diminish 

 more rapidly in calibre. At length, arrived at the secondary 



c c 2 



