NOSE 



151 



obliterated. The cavity is again established after birth ; it is believed that 

 this is due, in part at any rate, to the establishment of respiration. By the 

 expansion of the cavity in various directions its several recesses are formed, and 

 the ossicles and the chorda tympani nerve, which, as we have seen, lie at first in 

 the mesenchyme external to and above the primitive tympanum, come to be 

 enclosed in folds of the mucous membrane within the fully developed cavity; 



DEVELOPMENT OF THE NOSE. 1 



The olfactory organ appears towards the end pf the third week as an area of 

 thickened ectoderm on either side of the fore-brain. By the upgrowth of its margins 

 the area soon becomes depressed below the surface, and the so-called olfactory pit 

 is produced. The depression is at first pyriform in shape, the smaller end running 

 towards the stomodoeum (fig. 192). The mouth of the pit next becomes 

 constricted by the thickening and dra wing-in of its lips ; but at its pointed 



Fit. 192. PBOFILE VIEW or THE HEAD OF A HUMAN 



EMBhYO OF NEARLY FOUR WEEKS. (His.) 



olf, olfactory depression passing posteriorly into a 

 deep pit,' the rudiment of Jacobson's organ ; mx, 

 maxillary process ; tun, raandibular arch ; hy, hyoidean 

 arch ; br l , br-, first and second branchial arches. 



-pr.glol. 



FIG. 193. HEAD OF AN EMBRYO ABOUT 

 TWENTY-NINE DAYS OLD, FROM 



BEFORE. (His.) 



pr.glob., globular extremity of the 

 mesial nasal process. The other 

 letters as in fig. 192. 



(stomodceal) end the circumference is interrupted, the raised margin ending 

 mesially and laterally in the mesial and lateral nasal processes (fig. 193). Between 

 these the pit is continued as a groove or furrow on to the roof of the stomodceum. 

 We have already seen (p. 88) that the lateral nasal processes form the alse nasi, 

 and unite with the maxillary processes, which in turn form the cheeks and outer 

 parts of the upper lip ; also that the mesial nasal processes (processus globulares) 

 unite with one another to form the central part of the upper lip and philtrum, 

 and then unite with the maxillary processes to complete the lip. If now the under 

 aspect of the processes in their first phases be examined, it will be seen that they 

 extend backwards in the roof of the embryonic mouth, being separated by the 

 groove already referred to (fig. 194 B). The groove is not, however, an open fissure 

 communicating with the olfactory pit, but is filled by a raphe of ectoderm produced 

 by the fusion of the opposing surfaces of the several processes. It is not clear 

 whether in man the epithelial raphe is a primary formation an epithelial band 

 between the mesial and lateral nasal processes (Hochstetter) -or is secondarily 



For literature, see Karl Peter, Hertwig II. Th. i. and ii. p. 78 seq. Later references in footnotes. 



