STRUCTURE OF LUNGS. 321 



their walls are studded with the openings of innumerable air cells, 

 there being, however, no terminal vascular air cavities as in the 

 mammalian lung. 



The respiratory apparatus of mammals consists of (1) vascular 

 sacs filled with air, known as the lung alveoli ; (2) channels by 

 which these sacs are ventilated the air-passages; (3) motor ar- 

 rangements, which carry on the ventilation of the lungs the 

 thorax. 



1. The lungs are made up of innumerable minute cavities 

 (alveoli), with thin septa springing from the inner surface so as 



FIG. 145. 



Section of small portion of Lung in which are seen a bronchial tube with 

 its plicated lining mucous membrane in the centre, and the large blood- 

 vessels at the sides cut across. Loose areolar tissue and numerous lymphatics 

 surround the large vessels and separate them from the lung tissue. 



to divide the space into several compartments or air-cells. Each 

 of these cavities forms a dilatation on the terminal twig of a 

 branching bronchus, and may be regarded as an elementary lung. 

 The aggregate of these cavities, and the branches of the air-pas- 

 sages and vessels distributed to them make up the structure of the 

 lung. 



The walls of the cavities are formed chiefly of fine elastic fibres, 

 and the surface is lined with exceptionally delicate and thin celled 



27 



