DEVELOPMENT OF THE HUMAN LARYNX 59 



does the muscle extend as far posteriorly in its insertion on the 

 thyreoid. So that it would appear that the pars obliqua is a later 

 manifestation, and owes its development to a more complete 

 growth along the inferior cornu, along which it inserts more ex- 

 tensively later. 



The m. cricoarytaenoideus posterior is a large conspicuous muscle 

 as seen in fig. 36 of the wax model. It is in its adult position, 

 arising from the medial inferior part of the lamina cartilaginis 

 cricoideae and converging upward to insert on the apex of the pro- 

 cessus muscularis cartilaginis arytaenoideae. It is closely applied 

 to the cricoid cartilage as can be seen from a study of the sections 

 and extends somewhat more lateralward on the cricoid cartilage 

 than in the adult. Otherwise it conforms perfectly with the adult 

 type. 



The ni. cricoarytaenoideus lateralis is a considerable muscle 

 mass on the lateral surface of the cricoid cartilage, which from a 

 study of the sections alone, would seem to merge in with the mass 

 that by nature of position and relation should become the thyreo- 

 arytaenoideus muscle, but careful graphic reconstruction reveals 

 the picture seen in fig. 35 where the independence of the muscle 

 is plainly evident. Its fibrillation is not as extensive as that of the 

 cricoarytaenoideus posterior. Its origin on the upper lateral 

 surface of the cricoid and its insertion on the processus muscu- 

 laris cartilaginis arytaenoideae is well marked. 



M. thyreoarytaenoideus is likewise well marked at this stage and 

 fig. 35 shows its relation to the median surface of the thyreoid 

 and its insertion on the processus muscularis cartilaginis arytaen- 

 oideae. Undoubtedly some of its fibres and those of the cricoary- 

 taenoideus laterahs interlace, just as they do in the adult, but the 

 origin of the majority of the fibres from the thyreoid cartilage is 

 distinct enough to justify a clear differentiation of the two muscle 

 masses. Fiirbringer ('75), maintains that the secondary inter- 

 lacement of these muscles as exists in adults becomes apparent 

 at 48 mm., and does not believe that the sphincter formation of 

 adults is related to the primary sphincter. I have not studied 

 embryos later than this stage, so that I am in no position to ques- 

 tion the accuracy of this statement; yet I can hardly see why it 



