1100 THE KESPIEATOEY SYSTEM. 



The Larynx. The rudiment of the larynx appears, at the cranial or pharyngeal 

 end of the primitive respiratory tube, about the twenty-fifth day, and before the trachea 

 separates off from the O3sophagus in the form of two lateral swellings the aryteenoid 

 swellings, which lie caudal to the fourth visceral pouches, and possibly represent 

 rudimentary fifth branchial arches (Kallius). The arytsenoid swellings are connected 

 by a ventral median ridge which intervenes between the ventral ends of the third visceral 

 arches. At this period the site of the future larynx is represented at the pharyngeal 

 end of the respiratory tube by a U-shaped ridge which surrounds the tube cranially 

 and laterally, and is known as the furcula. 



The cranial or anterior portion of the furcula forms a median elevation from which 

 the epiglottis is developed, whilst the lateral portions of the furcula the arytoenoid 

 swellings eventually form the ary-epiglottic folds. On the medial side of the latter, 

 about the fourth month, a furrow marks the future site of the ventriculus laryngis 

 [Morgagni], the margins of which later become the vocal folds. 



About the eighth week the cartilaginous framework of the larynx is indicated by 

 mesoblastic condensations of the connective tissue around the now slit-like rima glottidis ; 

 and at the same period the rudimentary arytsenoids, the cricoid and the cartilages of 

 the trachea are all continuous laterally. 



The epiglottic cartilage is developed, as stated, in the anterior portion of the furcula, 

 and chondrifies relatively late. It may possibly represent a rudiment of the cartilage of 

 the sixth branchial arch, and according to Goppert it is at first continuous dorsally with 

 the cuneiform cartilages, which, therefore, are derivatives of the epiglottic cartilage. 



The thyreoid cartilage is laid down in the form of two separate lateral mesoblastic 

 plates, in each of which chondrification proceeds from two centres, ventral and dorsal, 

 which probably represent the cartilages of the fourth and fifth branchial arches. As 

 development proceeds the sheets of cartilage formed from these centres fuse, and eventually 

 extend ventrally to fuse with their fellows of the opposite side, in the median plane. 

 Chondrification is completed comparatively late, and when incomplete it results in the 

 formation of an abnormality the thyreoid foramen. The superior cornu of the thyreoid 

 cartilage is at first continuous with the greater cornu of the os hyoideum, and the remains 

 of this cartilaginous connexion is seen in the presence of the cartilage triticea in the 

 lateral hyothyreoid ligament of the adult. 



The pro-cartilaginous rudiments of the cricoid and arytsenoid cartilages are at first 

 continuous with each other, but later become differentiated by the appearance of separate 

 cartilaginous centres for the arytsenoids, and an incomplete ring, for a time deficient 

 dorsally, for the cricoid. The cricoid thus resembles develop men tally a tracheal ring, 

 with which it probably corresponds morphologically. Chondrification proceeds in the 

 cricoid by two centres, one on each lateral side. These centres unite ventrally, but 

 dorsally fusion does not take place until much later, and is finally completed by an exten- 

 sion of chondrification from the lateral into the dorsal plate. The cricoid thus differs 

 from the tracheal ring, in having its chondrification completed dorsally, whereas this 

 never takes place in the tracheal ring. 



The arytcenoid cartilages are, as stated, at first continuous with the cricoid cartilage 

 by fibrous tissue, but become eventually completely separated from it by the appear- 

 ance of one chondrification centre for each arytsenoid. 



The corniculate cartilages (Santorini) are merely portions of the arytaenoid cartilages 

 separated off by segmentation ; whilst the cuneiform cartilages ( Wrisbergi) are, as previ- 

 ously stated, derivatives of the epiglottic cartilage. 



The Trachea. The trachea is developed from the intermediate portion, of the 

 median longitudinal groove. Originally, both this portion of the primitive respiratory 

 tube and the oesophageal portion of the primitive alimentary canal were of equal length ; 

 but as development proceeds both tubes lengthen, the latter more rapidly than the 

 former, so that eventually the lung rudiments no longer lie on the ventral and lateral 

 sides of the primitive stomach, but come to lie on the cephalic side of that viscus, 

 and are separated from each other by the oesophagus dorsally and the heart and 

 pericardium ventrally. In this way, that is by unequal growth, it comes about that the 

 trachea in the adult is shorter than the oesophagus, though originally both were < 

 equal length. 



The cartilaginous rings of the trachea are developed like the cricoid cartilage, with the 

 difference that in the trachea the process of chondrification does not extend into their dorsal 

 portions, and hence, in the adult, the C-shaped rings of the trachea are deficient dorsally 

 an arrangement which admirably adapts itself to the functional uses of both trachea and 

 oasophagus. 



