. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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 tra
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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 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
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