574 



ANATOMY OF VERTEBRATES. 



most part racemose, glands, most conspicuous at the gular part 

 of the wall, fig. 443, I, with longish ducts opening upon the 

 ciliate surface. The trachea bifurcates into the ' bronchi,' which, 

 before they penetrate their respective lung, resemble their trunk- 

 tube in structure : after penetration, or when ' intra-pulmonary,' 

 the incomplete hooped form of cartilage is exchanged for a series 

 of irregular curved pieces, expanded so as to encase the whole 

 circumference of the several bronchial ramifications to near the 

 terminal ones, where the cartilages become thinner, smaller, more 

 remote from one another, and ultimately cease ; when the fibro- 

 membranous walls owe their patency to the expansive force of the 

 contained air. The muscular fibres affect, for the most part, a 

 circular disposition, but some run along the bronchial ramifica- 

 tions, thus serving both to contract the area and diminish the 



446 



445 





Transition of ultimate bronchial branches, b, into 

 intercellular passages, a. cclxviii. 



Section of terminal bronchial tube 

 magn. xi„v'. 



length of the tube. With the longitudinal muscle are blended 

 elastic fibres, and in large proportion in the terminal branches, 

 fig. 447, a } a : the transverse muscles, ib. b, c, have no terminal 

 tendons as in the trachea. 



The ultimate portions of lung to which the bronchi are distri- 

 buted are called < lobules,' on entering which, as in fig. 445, the 

 air-tube divides and subdivides, its branches diverging at less and 



