28 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



oesophagus. The jointing of this tracheal skeleton permits of 

 fiexibilit}'. In the bronchi there occur only irregular cartilagi- 

 nous elements, which never form rings or semi-rings like those 

 of the trachea. As a rule, the trachea pursues a straight course ; 

 but in certain birds (swans, cranes, birds of paradise, etc.) it 

 becomes extensively convoluted, its windings being either be- 

 tween the sternum and the breast muscles, or within the breast- 

 bone itself. 



At its upper end the trachea becomes widened and special- 

 ized, and is known as the larynx, which, like the trachea, has 



a cartilaginous frame- 

 work. In the lower 

 amphibia this support 

 consists of a pair of 

 cartilages, the ar3?te- 

 noid cartilages, one on 

 either side, to which are 

 added in the higher 

 amphibia a ring carti- 

 lage, the cricoid,^ which 

 reappears in a similar 

 shape in the reptiles. 

 In the birds the larynx 

 is somewhat rudimen- 

 tary, its place being 

 taken by the syrinx to 

 be mentioned below. 

 In the mammals, be- 

 sides the cricoid and 

 arytenoids, there is 

 added as a development 

 an incomplete ring of cartilage farther in front, the thyroid car- 

 tilage. This arises for the most part from the third of the 

 visceral arches, the fourth contributing to a considerable extent. 

 Other and smaller cartilages are also added in the same group, 

 but need no description here. 



1 This may be the product of fusion of the cartilages of the fifth gill arch, a view which 

 receives support from the fact that the muscles of the larynx are innervated by the hypo- 

 glossal nerve. 



Fig. 30. Dorsal (/f ) and ventral (5) views 

 of human laryngeal apparatus. A, arytenoid carti- 

 lage; AH, anterior horn of thyroid; C, cricoid 

 cartilage; £, epiglottis; GC, greater (posterior) 

 horn of hyoid; H, hyoid; LC, lesser (anterior) 

 horn of hyoid ; Z, ligament connecting hyoid and 

 thyroid; PH, posterior horn of thyroid; T^ 

 thyroid cartilage. 



