588 



RESPIRATORY APPARATUS IN MAMMALIA. 



TRACHEA, BRONCHI. AND LUNGS OF THE 

 HORSE (VIEWED FROM ABOVE). 



not form perfect rings, being incomplete 

 on the upper side of the trachea. Each 

 is a kind of arc, composed of a cartilaginous 

 plate flattened and curved on itself ; the 

 extremities are turned towards each other, 

 and joined in the majority of the rings ; 

 they even overlap in some. These ex- 

 tremities are thin and wide, and some- 

 times bifurcate and unite with the adjoin- 

 ing 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 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 connec- 

 tive-tissue layer that unites these ex- 

 tremities. The fii-st cartilage is received 

 by its anterior border into the cricoid ring, 

 and joined to it by the wide annular 

 ligament mentioned at p. 531. 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 



A, Pharyngeal cavity, thrown open to show the entrance 

 to the oesophagus, B, and that of the larynx, c ; d, 

 oesophagus ; E, left sac of the stomach ; F, its ri^ht 

 sac; G, duodenum; H, trachea; i, right bronchial 

 ramifications; K, left lung; L, anterior aorta; M, 

 common origin of the bronchial, oesophageal, and 

 first intercostal arteries. 



