•288 COMPARATIVE ANATOMY 



above/ and serving as a point of origin and insertion for important 

 intrinsic and extrinsic muscles. 



The vocal cords extend between the thyroid and the arytenoids, 

 and the mucous membrane above them becomes involuted to form 

 the laryngeal pouches. In Anthropoids and certain other Monkeys 

 {e.g., Mycetes) these may reach such a large size that they serve 

 as resonance cavities, and lie partially -within the body of the 

 hyoid, which is swollen to form a large bony chamber (Fig. 234). 

 The folds of mucous membrane bounding the laryngeal pouches 

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

 in all Mammals. 



The epiglottis, which consists of elastic fibro-cartilage, stands in 

 close relation to the soft palate, extending upwards from the anterior 

 border of the larynx, in front of the glottis : 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.^ 



An interesting adaptation for the method of lactation is seen in the 

 larynx of Marsupial embryos, in which it, together with the epiglottis, 

 becomes greatly elongated and is firmly embraced by the soft palate, so that 

 it cannot be moved from this position. Thus respiration can go on freely 

 while the milk passes down the oesophagus on either side of the larynx. 

 In Cetecea {e.cj., Phoctena), a similar arrangement occurs, and is here 

 adapted for the aquatic life of the animal. Probably in all Mammals a 

 similar position of the larynx is seen in the embryo. 



The Lungs proper. 



Dipnoi. — In Ceratodus the lung is a wide unpaired sac, without 

 any trace of a dividing septum : in other Dipnoans it is dis- 

 tinctly paired throughout the greater part of its length, the anterior 

 unpaired portion being largely filled up by spongy trabeculae. 



The lung extends through the whole length of the body-cavity, 

 and is covered by peritoneum on the ventral surface only; the 

 lining mucous membrane forms bands and networks similar to those 

 seen in the air-bladder of many Fishes {e.g., Lepidosteus, Fig. 225). 



Amphibia. — The lungs of Proteus and Necturus (Fig 235), 

 though paired throughout, remain at a lower stage than those 

 of the Dipnoi, inasmuch as their internal surface is perfectly smooth, 

 and has, therefore, a much smaller superficial extent. They 



^ The cricoid may be complete or incomplete ventrally, and its dorsal portion 

 usually becomes raised to form a broad plate with which the arytenoids are articu- 

 lated (Figs. 2,33 and 234). 



^ The epiglottis was probably originally a paired structure, consisting of 

 hyaline cartilage, and it is possible that the small cartilages of Wrisberg and 

 Santorini present in the larynx in addition to the more important cartilages de- 

 scribed above may be specialisations of part of the same structure. 



