518 STRUCTURE OF THE TRACHEA AND BRONCHIA. 



They are not alike in their size or form ; some of them are 

 rendered broader than others, by the union of two or three 

 rings with each other, as the uppermost. The lowermost also is 

 broad, and has a form which is accommodated to the bifurca- 

 tion of the tube where it forms the bronchia. Their number 

 varies in different persons, from fifteen to twenty. 



These rings may be considered as forming a part of the first 

 proper coat of the trachea, which is composed of them, and of 

 an elastic membrane that occupies all the interstice between 

 them ; so that the cartilages may be regarded as fixed in this 

 membrane. 



A similar arrangement of rings exists in the great branches 

 of the bronchia ; but after they ramify in the lungs, the carti- 

 lages are no longer in the form of rings : they are irregular in 

 their figures, and are so arranged in the membrane, that they 

 keep the tube completely open. These portions of cartilage do 

 not continue throughout the whole extent of the ramifications ; 

 for they become smaller, and finally disappear, while the mem- 

 branous tube continues without them, ramifying minutely, and 

 probably forming the air-cells of the lungs. 



— At the orifices of the bronchial ramifications, the existence 

 of a semilunar cartilage has been pointed out by Prof. Horner, 

 forming rather more than half of their circumference, and 

 having its concave edge turned upwards. These cartilages 

 appear to be intended to keep the orifices open.* — 



The small bronchial tubes after they become entirely membra- 

 nous are very elastic : the lungs are very elastic also ; and it is pro- 

 bable that their elasticity is derived from the membranous bronchiae. 



On the inside of this coat of the trachea is an arrangement 

 of muscular fibres, which may be called a muscular coat. It is 

 best seen by pealing off or removing the internal coat, to be 

 next described. 



On the membranous part of the trachea, where the cartilagi- 

 nous rings are deficient, these muscular fibres run evidently in 

 a transverse direction : in the spaces between the cartilages 



* Special Anatomy, by W. E. Horner, M. D. Prof. Anat. Univers. Penn- 

 sylvania. 



