326 MANUAL OF PHYSIOLOGY. 



reach large air cavities, and their walls are studded with the 

 openings of innumerable air cells, there being, however, no ter- 

 minal vascular air cavities as in the mammalian lung. 



The respiratory apparatus of mammals consists of (1) vascular 

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

 which these sacks are ventilated the air passages ; (3) motor 

 arrangements, 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 raucous 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 passages 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 

 epithelium. Supported in the delicate frame-work of elastic and 



