STRUCTURE OF THE LUNGS OF BIRDS. 71 



comes flatter towards the extremities of the bronchia. The 

 nutrition of the membranous walls of these tubes is eifected by 

 means of a plexus of capillary vessels with elongated meshes, 

 which is chiefly distributed in the internal fibrous layer. 



Since from all those parts of the bronchia of the Bird that are 

 immediately adjacent to the parenchyma of the lung the pul- 

 monary canals are given off rectangularly and in close prox- 

 imity, the bronchial wall must necessarily lose its membranous 

 character, and become reduced to a trellis- work. The trabeculse 

 of this consist of compact bands of smooth muscular tissue 

 intermingled with a few elastic fibres, lying in a matrix of 

 fibrous connective tissue, which last expands to form the inter- 

 nal fibrous layer, supporting an abundant capillary plexus and 

 a flattened columnar and ciliated epithelium. From these 

 muscular trabeculse surrounding the orifices of the pulmonary 

 canals, finer fasciculi of a similar character are continued into 

 the pipes themselves, and here form the thickened free borders of 

 the above-mentioned transverse annular ridges, which, like the 

 anastomosing and less strongly projecting longitudinal septa, 

 are elsewhere composed of fibrous connective tissue and of 

 delicate elastic fibres alone. 



The matrix of the thick, spongy, external pulmonary-tube- 

 walls is composed of a very delicately fibrous and quite homo- 

 geneous connective tissue with fine elastic plexuses, which 

 supports the close capillary plexus destined to effect the inter- 

 change of gases in respiration. 



This respiratory capillary system is developed from the 

 branches of the pulmonary artery running to the extremity of 

 the pulmonary canals, and passes from thence by minute ter- 

 minal branches into the parenchyma, becoming ultimately con- 

 tinuous with the rootlets of the pulmonary vein, which pursue 

 a similar but opposite course. The capillaries* that often 

 slightly project into the lumen of the pulmonary canals, but 

 are always closely invested by the matrix of connective tissue, 

 or, in other words, are more or less completely imbedded in the 



* Rainey maintained (Medico-Chirurgical Transactions, 1849, p. 50) that 

 the capillaries projected freely into the lumen of the pulmonary canals 

 without receiving support from any connective tissue. 



