STRUCTURE OF THE LUNGS OF BIRDS. 



69 



hexagonal form on section in the bird, whilst their voluminous 

 walls contain the true respiratory tissue, and constitute the 

 principal portion of the entire lung. They run parallel and 

 in close contiguity to each other ; at first in a straight, but 

 subsequently in a sinuous or zigzag direction, whilst they 

 frequently communicate by anastomosing branches. Their 

 internal free surface is circular on section, and is characterized 

 by the presence of strong membranous annular ridges, which 



Fig. 134. 



Fig. 134. Section of two pulmonary pipes or canals from the Goose. 

 " The air cavities of that on the right side tensely filled with dark- 

 coloured injection. In that on the left the pulmonary vessels are 

 injected. 



are continuous with each other by transverse anastomoses, 

 succeed each other at tolerably regular distances, and are con- 

 nected by numerous longitudinal thin intermediate septa. A 

 honeycombed surface is thus produced, the floor of which is 

 formed by the parenchyma composing the voluminous wall of 

 the air pipes. From the base of each of these alveoli several 



