LUNGS AND AIR DUCTS 



273 



even in the embryo, but the thyreoid shows its double origin in its 

 development. 



In the higher mammals the thyreoid cartilage forms a half ring on 

 the ventral side of the anterior end of the larynx, its anterior dorsal 

 angles being produced into cornua connected by ligament with the 

 hyoid (fig. 295). Dorsal to the thyreoid is the glottis with the aryte- 

 noids in its walls. Posterior to it is the ring-shaped cricoid, following 

 which is the trachea. Anterior to the glottis is a fold of the mucous 

 membrane of the pharynx, the epiglottis, supported by an internal 

 cartilage (possibly the fourth branchial arch) which articulates with 

 the anterior margin of the thyreoid. The epiglottis usually stands 

 erect, leaving the glottis open for respiration, 

 but during deglutition it is pulled back into 

 the glottis, thus preventing the entrance of food 

 into the trachea. 



Internally the cavity of the larynx bears 

 a vocal cord on either side. These are folds 

 of the mucous membrane, extending from the 

 thyreoid to the arytenoids, and by movements 

 of these latter cartilages they can be tightened 

 or relaxed, thus altering the pitch of the note 

 caused by their vibration. Anterior to these 

 cords is a pocket, the laryngeal ventricle (sinus 

 of Morgagni) on either side, small in most 

 mammals, but developed in the anthropoid 

 apes to large vocal sacs (in the chimpanzee 

 there is a median vocal sac in addition), 

 which act as resonators, adding to the strength of the voice. 



In the whales and young marsupials the larynx is prolonged so that it pro- 

 jects into the choana behind the soft palate. In the whales (fig. 296) this is an 

 adaptation to the manner of taking food from the water and breathing at the 

 same time. In the young marsupials the milk is forced into the mouth by 

 the muscles of the mammae of the mother and this arrangement prevents 

 strangulation. 



Trachea.— In the tetrapoda the trachea is strengthened by the 

 formation of cartilage in its walls, the beginnings of which are seen 

 in the urodelcs where the fifth branchial arch gives rise to these ele- 

 ments (p. 270). Their arrangement varies considerably in the 

 urodeles and cjecilians, being sometimes scattered pieces, sometimes 

 regularly arranged and even united in the lateral walls (fig. 291), 

 18 



Fig. 296. — Larynx of 

 Xiphius cavirostris (after 

 Gegenbaur) from side 

 showing the prolongation 

 of epiglottis (e) and aryte- 

 noid (c) which project 

 into the choana; c, cri- 

 coid; th, thyreoid. 



