452 MANUAL OF HISTOLOGY. 



Complete collapse or distension of the air cells, however, never takes 

 place under normal conditions in the lung. The possibility of this is pre- 

 cluded by the situation of the organ. This, namely, is hermetically sealed 

 up within the cavity of the chest. On account of its distensibility, there- 

 fore, it follows all the motions of the thorax in inspiration, most accurately 

 lying in contact with every part of the internal surface of the latter. 

 Then, on account of its elastic nature, and aided by the muscles of its 

 air passages, it contracts with every expiration as much as is admitted of 

 by the walls of the chest. It never, however, goes so far as entire collapse, 

 which is only reached naturally when the cavity of the thorax is laid 

 open, on which it at once takes place. 



If we now inquire into the texture of this elastic alveolus, which is 

 constantly expanding and contracting during life, we will find a few 

 points elucidated by fig. 433. The walls of the air vesicles, as continua- 

 tions of the finest bronchial ramifications, present for our consideration, 

 in the first place, an extremely delicate membrane of connective-tissue, 

 measuring about 0*0023 mm., and less, in thickness. At the right side 

 of the plate a portion of this may be seen in the large central alveolus. 

 This very fine membrane serves to connect the crowded capillaries of the 

 walls, and probably clothes the surfaces of the latter. We require, llow- 

 ever, further research on this point before it can be established. 



This membrane of the air cells is then covered externally by a greater 

 or smaller number of elastic fibres, varying greatly as to thickness. They 

 present themselves either scattered or in groups. The strongest fibres are 

 to be seen in the interalveolar septa, especially between adjacent vesicles, 

 closely packed together. The remaining portions of the alveolus are 

 poorer in them than the entrance, and especially the fundus. Here are 

 to be seen, scattered at wide intervals, the most delicate elastic elements, 

 measuring perhaps 0*0011 mm., and appearing like reticular connections 

 between the air vesicles. The limiting membrane, on the other hand, 

 does not appear to possess many nuclei, most of those which are to be 

 seen probably belonging to the capillaries or epithelia. 



These primary lobules of the lung, best studied in the infant, the 

 structural relations often becoming very indistinct in the adult, enter 

 again into the formation of the secondary lobuli, connected together 

 through the medium of connective-tissue. The diameter of these latter 

 may be roughly estimated at 1 or 2 mm. They are more distinctly seen 

 in the adult than in the infant, in the form, of polygonal fields on the 

 surface of the organ, marked out by the deposit between them of black 

 pigment. 



By the aggregation of these lobuli the larger lobes are gradually formed, 

 the consideration of which belongs to descriptive anatomy. 



It is a curious feature in the existence of the interstitial connective- 

 tissue of the lung, that there is usually a certain amount (frequently very 

 large) of black pigment deposited in it. The walls of the air vesicles, also, 

 may likewise be affected in the same way; besides which, molecules of 

 this substance are met with in the protoplasmic bodies of the smaller 

 epithelial cells of the respiratory tubes, and in certain rounded corpuscles 

 connected with the mucus of the part. We have already referred to the 

 pigmentation of the bronchial glands (p. 418). 



It was for a long time supposed that this pigmentation resulted from a 

 deposit here of true melanin. But from the fact of its not being present 

 in the lungs of wild animals, while in man, living in an atmosphere of 



