DEVELOPMENT OF THE HUMAN LARYNX 39 



THE MUSCLES 



The figs. 4 and 5 which are intended to illustrate, among other 

 things, some of the musculature of the larynx, may give a wrong 

 impression, which possibility therefore, I hasten to avoid. There 

 is by no means the fibrillation in these rimscles, as shown in fig. 6. 

 The muscles, with the exception of the circoartyaenoideus poste- 

 rior, which is faithfully drawn — are merely condensed mesenchyme, 

 with the barest suggestions of fibrillation, and the character given 

 them in the drawings are merely for the sake of clarity. 



M. cricoarytaenoideus posterior is very well developed, as shown 

 in fig. 8, of large size, and well advanced to fairly complete fibril- 

 lation. The muscle is somewhat more laterally situated than in 

 the adult, but otherwise conforms closely to the later stages. It 

 is plentifully innervated by the recurrent laryngeal nerve. 



M. cricoarytaenoideus lateralis (fig. 4) though not nearly so pre- 

 cisely outlined, this muscle mass, nevertheless, assumes consider- 

 able proportions at this stage. It is difficult, as it was in the case 

 of the 10.5 mm. embryo, to decide whether the premuscle tissue 

 represents only the cricoarytaenoideus lateralis, or whether it 

 also includes what is to become the thyreoarytaenoideus m. No 

 fibers originating from the mesial surface of the lateral portions 

 of the thyreoid cartilage were found, not even a slight trend of the 

 condensed mesenchyma. So that the muscle may not appear at 

 all till later in development. The thyreoepyglotticus and aryepi- 

 glotticus are both absent. 



The inter arty aenoideus (fig. 6) has made its appearance, but is 

 partly attached to the insertion fibers of the circoarytaenoideus 

 posterior. It is fibrillated. 



The cricothyreoideus muscle (fig. 5) can be made out at this time, 

 sending its fibers from the ventrolateral portion of the cricoid to 

 the mesial surface of the thyreoid. It cannot be entirely separated 

 from the cricoarytaenoideus lateralis mass, but its innervation by 

 the superior laryngeal nerve helps in the identification. 



