THE ORGANS OF SPECIAL SENSE. 



591 



have acquired a secondary function. In these forms the nose is not only an 

 apparatus for receiving olfactory stimuli, but also serves to convey air to and 

 from the lungs; it is in a sense a respiratory atrium. The sensory epithelium 

 which the olfactory nerves supply is limited to relatively small areas in the supe- 

 rior conchae and nasal septum. Stratified columnar ciliated epithelium lines all 

 other parts of the cavities. 



Studies on the development of the olfactory nerve have led to diverse 

 opinions, but the investigations of His and Disse go to show that the fibers 

 are processes of cells derived from the thickened ectoderm or olfactory placodes. 

 In human embryos of about four weeks some of the cells in the upper part of 

 the nasal fossa become modified to form the neuro-epithelium. From the 



Jacobson's organ 

 Inferior concha 



Jacobson's cartilage 



Palatine process 



Nasal septum 



Nasal cavity 



Oral cavity 



FIG. 510. From a section through the head of a human embryo of 28 mm., showing the nasal 

 septum, the nasal cavities, the oral cavity, and the palatine processes. Peter. 



peripheral pole of each cell a short slender process grows out to the surface of 

 the epithelium. From the opposite pole a slender process (the axone) grows 

 centrally until it penetrates the olfactory lobe, where it ends in contact with the 

 dendrites of the first central neurone of the olfactory tract. Most of these cells 

 remain in the epithelial layer, but a few wander into the subjacent mesoderm 

 and become bipolar cells which resemble the bipolar cells of the embryonic 

 posterior root ganglia (p. 509). Other epithelial cells of the nasal fossa are 

 converted into the sustentacular cells of the olfactory areas. 



Jacobson's organ arises at the beginning of the third month as a small out- 

 pocketing of the epithelium on the lower anterior part of the nasal septum 

 (Fig. 510). This evagination grows backward as a slender sac along the nasal 

 septum for a distance of several millimeters and ends blindly. In the adult 

 the sac degenerates and often disappears. In some of the lower Mammals 

 38 



