154 



NOSE 



nasal and maxillary processes in the roof of the stomodoeum constitute the 

 primitive palate, while the two mesial nasal processes which have meantime fused 

 together, form a broad primitive septum (intermaxillary process) (fig. 194 B). The 

 primitive choanse do not correspond in position to the permanent posterior nares, 

 which are placed much farther back, and are established only in the third month, 

 when the permanent palate has been developed. The nasal sacs extend backwards 

 as the face takes shape and the interocular septum is produced, appearing as 

 narrow clefts in the roof of the primitive mouth. Each is surrounded by 

 mesenchyme in which a cartilaginous nasal capsule is laid down. On the outer 

 side that is in the lateral nasal process the cartilage takes the form of a 

 curved plate (fig. 196), connected behind with the trabecular region of the base of 

 the skull, and ending below in a free margin. The two lateral cartilages join 

 mesially with a septal cartilage (fig. 196), which has developed in the fronto- 

 nasal process as a forward projection of the trabecular region of the base of 

 the skull. The septal, like the lateral cartilage, ends below in a free edge, so 

 that the capsule is open below. The floor of the fossae is completed by the 



FIG. 197. THE KOOF OF THE MOUTH OF A HUMAN EMBBYO ABOUT TWO AND A-HALF MONTHS OLD, 



SHOWING THE DEVELOPMENT OF THE PALATE. (After His.) 



p g., processus globularis ; p.g. 1 , palatal process of process.us globularis ; mx, maxillary process ; 

 mx l , palatal fold of maxillary process. Close to the angle between this and the palatal process of the 

 processus globularis, on each side, the primitive choanae. 



growth from the lower part of the maxillary processes of the palatal folds 

 (fig. 197), which unite with one another and then with the lower end of the septum 

 to form the palate. The palatal folds extend from the line of union of the mesial 

 nasal and maxillary processes backwards on to the wall of the pharynx. They 

 are at first below the level of the dorsum of the tongue, and are directed downwards 

 and inwards (fig. 195). As the tongue sinks between the growing mandibles they 

 are rotated into a horizontal position and meet in the middle line above the tongue. 

 The posterior parts of the folds, however, maintain their original direction (Polzl). 

 We have already seen that the mesial nasal processes, which unite superficially 

 to form the central part of the upper lip and the philtrum, extend backwards in the 

 roof of the stomodceum, and there unite to form the intermaxillary process (fig. 197). 

 This projects farthest back in the middle line, and has two oblique lateral borders, 



1 For suggested explanations of this change of position of the palatal folds, see the following 

 papers : His, W., Abhand. math.-phys. Kl. Sachs. G-es. Wiss. 1901 ; Polzl, Anna, Anat. Hefte, xxvii. 

 1904 ; Schorr, Anat. Anzeiger, xxx. 1907. See also Gb'ppert, Morph. Jahrb. xxxi. 1903, and Anat. Anzeiger, 

 xxiii. 1903, on the more general question as to the origin of the secondary palate. 



