1072 



SPLANCHNOLOGY 





into contact with one another and with the back of the epiglottis, and the entrance 

 to the larynx assumes the form of a T-shaped cleft, the margins of the cleft adhere 

 to one another and the laryngeal entrance is for a time occluded. The mesodermal 

 wall of the tube becomes condensed to form the cartilages of the larynx and trachea. 

 The arytenoid swellings are differentiated into the arytenoid and corniculate car- 

 tilages, and the folds joining them to the epiglottis form the aryepiglottic folds 

 in which the cuneiform cartilages are developed as derivatives of the epiglottis. 

 The thyroid cartilage appears as two lateral plates, each chondrified from two 

 centers and united in the mid-ventral line by membrane in which an additional 

 center of chondrification develops. The cricoid cartilage arises from two cartil- 

 aginous centers, which soon unite ventrally and gradually extend and ultimately 

 fuse on the dorsal aspect of the tube. 



J. Ernest Frazer 1 has made an important investigation on the development of the larynx 

 and the following are his main conclusions: 



The opening of the pulmonary diverticulum lies between the two fifth arch masses and behind 

 a "central mass" in the middle line the proximal end of the diverticulum is compressed between 

 the fifth arch masses. The fifth arch is joined by the fourth to form a "lateral mass" on each 

 side of the opening, and these "lateral masses" grow forward and overlap the central mass and 

 so form a secondary transverse cavity, which is really a part of the cavity of the phaiynx. The 

 two parts of the cavity of the larynx are separated in the adult by a line drawn back along the 

 vocal fold and then upward along the border of the arytenoid eminence to the interarytenoid 

 notch. The arytenoid and cricoid are developed in the fifth arch mass. The thyroid is primarily 

 a fourth arch derivative, and if it has a fifth arch element this is a later addition. The epiglottis 

 is derived from the "central mass," and has a third arch element in its oral and upper aspect; 

 the arch value of the "central mass" is doubtful. 



FIG. 948. Lung buds from a human embryo of 

 about four weeks, showing commencing tabulations. 

 (His.) 



FIG. 949. Lungs of a human embryo more 

 advanced in development. (His.) 



The right and left lung buds grow out behind the ducts of Cuvier, and are at 

 first symmetrical, but their ends soon become lobulated, three lobules appearing 

 on the right, and two on the left; these subdivisions are the early indications of the 

 corresponding lobes of the lungs (Figs. 948, 949). The buds undergo further sub- 

 division and ramification, and ultimately end in minute expanded extremities 

 the infundibula of the lung. After the sixth month the air-sacs begin to make their 

 appearance on the infundibula in the form of minute pouches. The pulmonary 

 arteries are derived from the sixth aortic arches. During the course of their 

 development the lungs migrate in a caudal direction, so that by the time of birth 

 the bifurcation of the trachea is opposite the fourth thoracic vertebra. As the 

 lungs grow they project into that part of the celom which will ultimately form the 

 pleural cavities, and the superficial layer of the mesoderm enveloping the lung 

 rudiment expands on the growing lung and is converted into the pulmonary pleura. 



THE LARYNX. 



The larynx or organ of voice is placed at the upper part of the air passage. 

 It is situated between the trachea and the root of the tongue, at the upper and 



Journal of Anatomy and Physiology, vol. xliv. 



