314 HISTORY OF THE HUMAN BODY 



equal size, which embrace the trachea and protect it frorr 

 collapse. These are deficient behind, where the trachea comes 

 in contact with the oesophagus, as a provision to allow the 

 passage of a large mouthful, but are strongly developed in 

 front and serve to keep the trachea distended. The most an- 

 terior of these rings is much heavier than any of the others 

 and is probably formed by the fusion of several of them. It 

 is known as the cricoid cartilage and is topographically con- 

 sidered a part of the larynx. The tracheal rings must have 

 been developed in some way from the tracheal pieces that 

 segmented off from the lateral cartilages, but the manner of 

 their formation is not known. Similar rings occur in the 

 trachea of the Gymnophiona, the rare Order of subterranean 

 amphibians, but whether these are homologous wittTthose of 

 the reptiles and birds has not yet been determined. 



There is but little variation in laryngeal form among the 

 representatives of the Sauropsida, and this in spite of the great 

 differentiation of voice in the case of the birds, since in these 

 the voice is produced, not by the larynx, but by a special organ, 

 the syrinx, or lower larynx, situated at the forking of the 

 bronchi and not found outside of the group of birds. 



In mammals a conspicuous addition is seen in the thyreoid 

 cartilage. The origin of this piece is not apparent in pla- 

 cental mammals, in which it appears as an extensive shield, 

 covering the ventral surface of the entire organ, but in the 

 more primitive monotremes, instead of the single shield-like 

 piece, there are two pairs of narrow bars which from their 

 origin and their similarity to the more anterior ones, as well 

 as from their mode of development, are clearly seen to be 

 branchial arches, evidently the 2nd and 3rd (Fig. 88). This 

 leaves only the first arch, which in this Order unites with the 

 true hyoid arch to form the hyoid complex (" hyoid bone" 

 of human anatomy), to which it contributes its posterior 

 cornua, the thyreo-hyals. The cricoid cartilage is much as in 

 Sauropsida and is manifestly the result of the consolidation 

 of certain of the upper tracheal rings. 



The development of the lungs is mainly along the lines of 



