PARENCHYMA OF THE LUNGS. 363 



sively increase in size with age. 1 The air-cells are sur- 

 rounded by a great number of elastic fibres, which do not 

 form distinct bundles for each cell, but anastomose freely 

 with each other, so that the same fibres belong to two or 

 more. This structure is peculiar to the parenchyma of the 

 lungs, and gives these organs their great distensibility and 

 elasticity, properties which play an important part in ex- 

 pelling the air from the chest, as a consequence simply of 

 cessation of the action of the inspiratory muscles. Inter- 

 woven with these elastic fibres is the richest plexus of 

 capillary blood-vessels found in the economy. The vessels 

 are larger than the capillaries in other situations, and the 

 plexus is so close that the spaces between them are narrower 

 than the vessels themselves. When distended, the blood-ves- 

 sels form the greater part of the walls of the cells. 



There is some difference of opinion among anatomists with 

 regard to the lining of the air-cells. Some are of the opinion, 

 with Eainey and Mandl, that the mucous membrane, and 

 even the epithelium, cease in the small bronchial tubes, and 

 the blood-vessels in the cells are uncovered. The presence of 

 pavement epithelium has been demonstrated, however, in the 

 cells, but the scales are detached soon after death, and cannot 

 always be observed. All who contend for the existence of a 

 mucous membrane admit that it is of excessive tenuity. 

 Robin, Kolliker, and others have demonstrated in the air- 

 cells very thin scales of pavement epithelium, from -^Vo * 

 ^oVo- of an inch in diameter, which are applied directly to the 

 walls of the blood-vessels.' 2 The epithelium here does not 

 seem to be regularly desquamated, as in other situations. 

 Examination of injected specimens shows that the blood-ves- 

 sels are so situated between the cells, that the blood in the 

 greater part of their circumference is exposed to the action 

 of the air. 



1 MAGENPIE, Memoire sur la Structure du Poumon de VHomme, etc. Journal 

 de Physiologic, 1821, tome i., p. 78. 



2 KOLLIKER, Manual of Human Microscopic Anatomy, London, 1860, p. 387 ; 

 and POUCHET, Histologie Humaine, Paris, 1864, p. 286. 



