THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 365 



cartilage in the lateral hyothyreoid ligament. The anlagen of the arytenoid 

 cartilages develop in the arytenoid ridges as condensations of the mesenchyme, 

 which later are converted into true cartilage (Fig. 323). The apex and vocal 

 process of each arytenoid become elastic, the main body becomes hyalin. 

 The corniculate cartilages (cartilages of Santorini) are split off from the cephalic 

 ends of the arytenoids and are of the elastic variety. The cricoid cartilage, 

 like the others, is preceded by a condensation of mesenchyme. Chondrifica- 

 tion begins on each side and then progresses around dorsally and ventrally until 

 a complete hyalin ring is formed. From its developmental resemblance to the 

 tracheal rings, the cricoid is sometimes regarded as the most cephalic of that 

 series. The epiglottic cartilage develops in the epiglottic ridge as two sepa- 

 rate pieces which subsequently fuse. It is of the elastic variety. The cuneiform 

 cartilages (cartilages of Wrisberg) are split off from the two pieces of the epi- 

 glottic, and are of the elastic type. 



Attempts have been made to determine which branchial arches are represented by the 

 laryngeal cartilages. It seems quite definitely settled that the thyreoid is derived in part, at 

 least, from the fourth arch. There is much doubt as regards the others, for there is great 

 difficulty in determining their derivation in the human embryo, since the arches disappear 

 at such an early stage. Furthermore, some of these cartilages may represent arches which 

 are present in lower forms but do not appear in the higher Mammals. 



The larynx is situated much farther cranially in the foetus and in the new- 

 born child than in the adult. In a five months foetus it extends into the naso- 

 pharyngeal cavity, whence it migrates caudally to its adult position. The 

 laryngeal skeleton becomes ossified during postnatal life. Ossification begins 

 in the thyreoid and cricoid cartilages at the age of eighteen to twenty years, 

 and in the arytenoids a few years later. Three centers appear in the thyreoid 

 one on each side near the inferior cornu and one in the medial line between 

 the two wings. In the cricoid, ossification begins near the upper border on 

 each side, in the arytenoids at the lower borders. Ossification usually begins 

 earlier and proceeds more rapidly in the male than in the female. 



As an example of the explanation which Embryology offers of certain peculiarities of 

 structure in the adult, the case of the recurrent laryngeal nerve may be cited. The heart and 

 aortic arches are primarily situated in the cervical region. At that time a branch of the 

 vagus on each side, passes behind the fourth aortic arch to reach the larynx. As the 

 heart and arches recede into the thorax, the nerve is pulled caudally between its origin and 

 termination, so that in the adult the left nerve bends around the arch of the aorta and the 

 right around the subclavian artery. 



The Trachea. 



The portion of the original tube between the larynx and the tw T o caudal out- 

 growths which form the bronchi and lungs, develops into the trachea. It lies 

 ventral to the oesophagus and is surrounded by mesodermal tissue which is 



