THE TRACHEA. 459 



joined in the majority of the rings ; they even overlap in some. These 

 extremities are thin and wide, and sometimes bifurcate and unite with the 

 adjoining rings. 



In the middle part of the trachea, these rings are generally larger than 

 at the origin or termination of the tube. The last ring, in serving as a 

 transition between the trachea and bronchi, presents a more complicated 

 arrangement ; being frequently completed by isolated cartilaginous plates, 

 and is always divided by a median spur or bifurcation — directed towards the 

 interior of the trachea, into two lateral segments, each of which corresponds 

 to a bronchus. 



Ligaments. — The rings of the trachea are united at their borders by 

 intermediate ligaments, which are composed of elastic tissue, and permit the 

 lengthening or shortening of the tube they concur to form. 



Towards the extremities of the arcs, they are confounded with a thin 

 cellular layer that unites these extremities. The first cartilage is received by 

 its anterior border into the cricoid ring, and joined to it by the wide annular 

 ligament mentioned at page 452. Owing to the elasticity of this ligament, 

 the two cartilages it binds together can move one within the other, 

 like two segments of a telescope, and in this way vary the length of the 

 tube. 



Muscular layer. — This layer only covers the superior face of the 

 trachea ; it is formed of pale, rose-coloured, transverse fasciculi, attached 

 by their extremities to the internal face of the cartilages. Its action 

 undoubtedly diminishes the diameter of the trachea, by contracting the arcs 

 composing this cartilaginous tube. (KoUiker has found some longitudinal 

 fibres passing across the transverse ones at the posterior part of the trachea. 

 Leyh describes longitudinal fibres in the anterior wall of the trachea, 

 between the mucous membrane and the cartilaginous rings, and which, he 

 states, diminish the length of the tube.) 



Mucous membrane. — Continuous with that of the larynx, this membrane 

 is prolonged, through the medium of the bronchi, and in becoming modified 

 in character, into the air-cells. Its free or superficial 

 surface is perforated by glandular orifices, and ex- '^' 



hibits longitudinal ridges which are inefiacable by 

 distension ; it is lined with ciliated epithelium. Its 

 deep face is covered with yellow elastic tissue dis- 

 posed in longitudinal fasciculi, and adheres inti- 

 mately either to the face of the cartilages and their 

 intermediate ligaments, or to the posterior muscular 

 layer. 



An essential characteristic which distinguishes 

 this membrane from that lining the larynx, is its ciliated epithelium 

 slight sensibility. ^^°« ™^ tkachea. 



(The tracheal glands, whose orifices are so nume- ^^^^ataT elT.tic fibrfs" 

 rous in the mucous membrane, abound towards the 2, Homogeneous surface 

 posterior part of the tube ; they are small, ovoid layer of the mucous 

 bodies, lying between the muscular and fibrous coats. membrane ; 3, Round 

 Other glands, less in size, are placed between the i„^i^''„3*lg^™^ calfd 

 layers of fibrous tissue imiting the cartilages at the ^""il ""^ ^ ' ' ^ '^ " 

 sides of the trachea. Their secretion is poured out 

 upon the free surface of the mucous membrane, to lubricate and protect it.) 



Vessels and nerves. — The small arteries emanating from the vessels 

 in the vicinity of the trachea — as the carotid and the collateral branches 



