LUNGS OF MAMMALIA, 



573 



of which, in most mammals, do not coalesce, but either overlap, 

 meet, or, more commonly, fail to meet by about one- fourth, or less, 

 of their circumference, fig. 453, b. The slit or interval, which is 

 usually at the back, or gular surface, of the windpipe, is completed 

 by a musculo-membranous sheet. The hoops themselves are 

 connected together by a 



elastic 



443 



Transverse section of trachea through a cartilaginous 

 hoop, e. cclxviii. 



strong elastic membrane 

 occupying their intervals 

 and also extended over 

 both their outer and inner 

 surfaces. The entire tube 

 is invested by loose areolar 

 tissue, and is lined by a 

 mucous membrane with a 

 ciliated free surface. 



The tracheal cartilage, 

 fig. 443, e, consists of a 

 fibrous basis, charged with 

 nucleate cells. Unstriped 

 muscular fibres extend be- 

 tween the ends of the hoop, 

 having their attachment to 

 the inner surface, some 

 short way from the end itself, as at k, fig. 443, others pass ob- 

 liquely between contiguous hoops. On the inner surface of the 

 tracheal cartilages* and muscles is a stratum of elastic, chiefly 

 longitudinal, fibres, ib. i : their 



XXA. 



fasciculi are most conspicu- 

 ous, extending in a serpentine 

 course along the back part of 

 the tube. The mucous mem- 

 brane consists of a basilemma, 

 fig. 444, «, and of finer areolar 

 tissue, b, forming a bed of 

 numerous nucleate cells, c, d, 

 the innermost, e, or those next 

 the inner surface of the air- 

 tube, being clavate, and sup- 

 porting on their base, each 

 from about twenty to fifty vibratile cilia, so acting as to direct 

 throat-ward the matters with which they are in contact. The 

 mucus lubricating the ciliate surface and entangling any foreign 

 particles admitted with the air, is the secretion of small, for the 



Section of tracheal ciliate mucous membrane, iuagn 



CCLXVIII. 



