DEVELOPMENT OF THE HUMAN LARYNX 61 



In comparison with muscular development elsewhere in the 

 body, and even in the neighborhood of the larynx, it is noticed that 

 the laryngeal musculature is backward, especially in the richness 

 and thickness of fibrillation; but I think that a study of the vari- 

 ous illustrations will justify the statement that in a 20 mm, human 

 embryo sufficient differentiation has occurred to admit of definite 

 recognition of all the extrinsic and intrinsic muscles of the larynx. 

 Considerable emphasis has been laid by most authorities on the 

 sphincter formation of the larynx and pharnyx musculature; the 

 continuity of the two, one sphincter within another, as it were. 

 Notably the excellent contributions of Strazzer, Fiirbringer and 

 Kanthack lay stress on this. The existence of such a structure, 

 is suggested in the earlier stages, but that this sphincter formation 

 and horizontal fibre arrangement is the predominate feature at the 

 20 mm. stage, is I feel, not true, but capable of explanation. A 

 study of cross sections, certainly inclines to such an impression; 

 and fig. 30 suggests this especially; but reconstructions at this 

 period of development do not bear this out, nor do the study of 

 frontal sections as in embryo 19.5. mm.; and these studies surely 

 suggest that the muscles are too clearly differentiated at this time 

 to exemplify a sphincter arrangement, except in the same sense 

 as such a general structure exists in the adult. Of course recon- 

 structions, expecially wax models are by no means infallible, and 

 not nearly as accurate as, for instance, the injection method for 

 the studj'^ of blood vascular development; but I do consider the 

 results of these reconstructions, both wax and graphic, to be well 

 inside the limits of error. 



THE NERVES 



A'', laryngeus superior (figs. 33, 34, and 36) is very distinct, of 

 considerable proportion, and easily made out. It divides into 

 two portions, the ra7nus externus and internus at about the level 

 of the superior cornu of the hyoid. Ramus externus is by no 

 means small in calibre, as seen in figs. 34 and 36, it descends al- 

 most vertically upon the outside of the muscle constrictor pharyn- 

 gis inferior to the m. cricothyreoideus, in whose substance it can 



