LUNGS. 257 



The epiglottis serves as a protection to the aperture of the 

 glottis, and its form varies much ; occasionally it may undergo 

 degeneration. The thyroid cartilage, which, as already mentioned 

 (see p. 255), is probably to be derived from the visceral skeleton, 

 is originally paired (Monotremes). In higher types, it forms a 

 cartilaginous capsule which encloses the cricoid and arytenoids x 

 on the ventral side. The thyroid serves as a point of origin and 

 insertion for important muscles which stretch the vocal cords. 



The vocal cords extend between the thyroid and the ary- 

 tenoids, and the mucous membrane above them becomes in- 

 voluted to form the laryngeal pouches. In Anthropoids, and 

 certain other Monkeys, these may reach such a large size that they 

 serve as resonance cavities, and come to lie partially within 

 the body of the hyoid, which is swollen to form a large bony 

 chamber (Fig. 208, D, 1, 2, 3}? 



The folds of mucous membrane bounding the pouches of the 

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

 present in all Mammals. 



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

 of Marsupial embryos, in which it becomes elongated so as to extend up- 

 wards into the internal nostrils, where it is firmly embraced by the soft palate. 

 Thus respiration can go on freely while the milk passes down the oesophagus on 

 either side of the larynx. In Cetacea (e.g. Phocrena) a similar arrangement is 

 observable, and is here adapted for the aquatic life of the animal. In many 

 other Mammals the epiglottis is embraced by the soft palate, so that feeding and 

 respiration can go on without interfering with one another. 



The Lungs in a more Restricted Sense. 



Dipnoi. In Ceratodus, the lungs form a wide unpaired 

 sac, without any trace of a dividing septum, while in other Dipnoi 

 they are distinctly paired posteriorly, though single anteriorly. 



They extend through the whole length of the body-cavity, and 

 are covered by the peritoneum only on the ventral surface : the 

 mucous membrane lining them forms bands and networks similar 

 to those seen in the air-bladders of many Fishes (e.g. Lepidosteus). 3 



Amphibia. The lungs of Menobranchus and Proteus 

 remain at a lower stage of development than those of the Dipnoi, 



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

 usually becomes raised to form a broad plate which articulates with the arytenoids 

 (Fig. 208, Or, Cr l , Co). Each of the latter often gives rise to an outgrowth an- 

 teriorly, which, becoming separated from it, forms a cartilage of Santorini. 

 A further independent cartilage (cartilage of Wrisberg) is sometimes present 

 in the aryepiglottidean fold. 



2 In the Gorilla the resonance vesicle extends above the sterno-cleido-mastoideus, 

 and reaches backwards as far as the shoulder and pectoralis major. 



3 It is worthy of remark that Lepidosteus, like many other Fishes, comes to 

 the surface and appears to swallow air, but it cannot be stated whether its air-bladder 

 has any important respiratory function until the relations of the blood-vessels are 

 known. 



S 



