Henry Gray (18251861). Anatomy of the Human Body. 1918.
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 cartilages, 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 cartilaginous 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 linethe 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 pharynx. 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 lobulations. (His.) (See enlarged image)
FIG. 949 Lungs of a human embryo more advanced in development. (His.) (See enlarged image)
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 subdivision and ramification, and ultimately end in minute expanded extremitiesthe 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.
1a. 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
Note 1. Journal of Anatomy and Physiology, vol. xliv. [back]