AIR-TUBES AND LARYNX 375 



a median pouch projects between the epiglottis and thyroid 

 cartilage (or more rarely between the latter and the cricoid), and 

 may come to lie partially within the body of the hyoid, which (e.g. 

 in Mycetes, Fig. 281) 1 is swollen to form a large bony chamber. 

 The folds of mucous membrane bounding the laryngeal pouches 

 anteriorly are spoken of as false vocal cords ; these are not present 

 in all Mammals. True vocal cords may also be wanting, being 

 replaced functionally by a ridge of the mucous membrane which 

 is only slightly vibratory and may be temporarily strengthened by 

 the contraction of the thyro-arytenoid muscle (e.g. Monotremes, 

 Cetaceans, Monkeys). 



The epiglottis, which consists mainly of elastic fibre-cartilage, 

 has close relations to the soft palate, extending upwards from 

 the anterior border of the thyroid cartilage in front of the glotcis, 

 which it covers when pressed backwards : it is often, when at rest, 

 embraced more or less firmly by the soft palate in such a way that 

 its distal end lies in the passage of the posterior nostrils (naso- 

 pharyngeal chamber), so that respiration and feeding can go on 

 independently of one another. 2 



An interesting adaptation for the method of lactation is seen 

 in the larynx of Marsupial embryos, in which it and the epiglottis 

 become greatly elongated and are firmly embraced by the soft 

 palate, so that it cannot be moved from this position. Thus 

 respiration can go on freely while the milk is pumped down 

 the oesophagus on either side of the larynx by the action of the 

 compressor mamma? muscle of the mother. In Cetacea (more 

 particularly the Odontoceti), a similar arrangement occurs, and is 

 here adapted for the aquatic life of the animal. In all Mammals 

 a similar position of the larynx is seen in the embryo if not in the 

 adult. 



The length of the trachea is proportional to that of the neck : 

 when the latter is short, the two bronchi may arise close behind 

 the cricoid cartilage (Sirenia) : except in Bradypus, in which it 

 takes a peculiar course, it lies close to the ventral side of the 

 gullet. The cartilaginous rings are usually incomplete dorsally, 

 where their open ends are connected by a membrane enclosing 

 smooth muscle-fibres. 3 In the bronchi, the rings are only excep- 

 tionally closed on the dorsal side, and are usually present for a 

 considerable distance along the intrapulmonary bronchi. 



1 Outgrowths from various parts of the laryngeal mucous membrane may 

 occur in various Mammals (e.g. Cetacea, Ungulates). 



2 The phylogenetic history of the epiglottis is unknown. It was probably 

 originally a paired structure, and the small cartilages of Wrisberg are apparently 

 derived from it. 



3 In the Cetacea they are incomplete ventrally, and in certain Marsupials 

 and Rodents, as well as in Phoca and Lemur, they form complete rings. 



