256 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



the foramen ovale starts to close and the ductus arteriosus 

 begins then to contract, and within the 1st month becomes 

 reduced to a fibrous cord. The ductus arteriosus represents the 

 dorsal segment of the 5th left arch ; the corresponding part of 

 the right 5th arch disappears soon after it is formed. 



The Larynx. — The larynx is developed in the fiopr of the 

 pharynx out of the basal part of the pulmonary diverticulum. 

 The origin of the upper aperture of the larynx has been already 

 described (page 250). It is probable that the thyroid cartilage 

 represents the skeletal parts of the 4th and 5th visceral arches, 

 but this cannot be regarded as settled. There is no interval to 

 be recognised between the hyoid bone and thyroid cartilage in 

 the earlier stages of development. By some, the epiglottis is 

 believed to be a derivative of the fifth arch. 



Only in man and the higher anthropoids are the true vocal 

 cords covered by stratified epithelium ; but all the muscles of the 

 human larynx are represented in the larynx of the ape, but in a 

 less specialized condition. 



left. cms. 



__ right cms. 



ensiform 



Fig. 210.— Diagram of the Diaphragm to show the Parts formed by each of the five 



Elements. 



Occasionally the saccule of the larynx, a development from the 

 apex of the ventricle, may protrude through the thyro-hyoid 

 membrane, thus giving rise to an air cyst in the neck. Such 



