LUNGS OF MAMMALIA. 



575 



447 



less acute angles ; and, after the fourth or further division, ac- 

 cording to the size of the lobule, they maintain an ultimate dia- 

 meter of about -j^th of an inch : then the cylindrical form is 

 lost, and the air-tube becomes an intercellular passage, beset with 

 dilatations, or ( air-cells,' aggregated at the periphery of the lobule 

 into groups. The ciliate 

 mucous membrane termi- 

 nates abruptly, where the 

 bronchial tube becomes, as 

 at a and b, fig. 446, an 

 intercellular passage ; but 

 formifaction shows its re- 

 sults, as f nuclei' and e pave- 

 ment cells ' upon the free 

 surface of the air-cells. The 

 intercellular passages inter- 

 communicate, as in fig. 445, 

 a 9 the bronchial ramifica- 

 tions, ib. b, c, do not : in 

 fig. 446 is shown the abrupt 

 transition from the terminal 

 bronchial tube, a, to the 

 intercellular passage, with 

 its appended air-cells, b, e. 



The openings of the air- 

 cells are strengthened or de- 

 fined by fibres of yellow elastic tissue, fig. 447, minute filaments 

 of which have been traced over the wall of the cell. The branches 

 of the pulmonary artery accompany those of the bronchi to the in- 

 tercellular passages, as at fig. 448, a, and are there resolved into 

 the arterioles, b, b, encompassing the orifices of the air-cells, where 

 they pass into the capillary network, d, e ; whence the aerated 

 or arterialised blood is received into the beginning of the pul- 

 monary vein c. 



On a general comparison of the lung-structure in the two warm- 

 blooded classes, it may be affirmed, of mammals, that the secondary 

 and tertiary bronchi, instead of a ' central ' hold a ' peripheral ' 

 course ; have arborescent, not pinnatifid divisions ; and more gra- 

 dually decrease in size : moreover they terminate in cells on the 

 parietes of which the pulmonary capillaries offer only one side to 

 the respiratory medium, instead of being wholly immersed in the 

 extrabronchial air, as in birds. 



In the Ornithorhynchus the tracheal tube, fig. 308, m, is wide; 



Pulmonary cells, showing trabecular ' fibres, and epithelia 

 precipitate: magn. cclxviii". 



