146 THE STRUCTURE OF MAN 



incisivi. The latter are sometimes wide, sometimes constricted, 

 and they communicate with the mouth either independently or 

 by a common orifice. In fresh embryos the passage of the canal 

 is to be found open only in exceptional cases ; there are usually 

 two canals present on both the buccal and nasal surfaces of the 

 palate, the former of these are usually the more prolonged. Both 

 pairs are lined with mucous membrane, and, ending blindly, form 

 together an obtuse angle. Traces of the buccal ends of these canals 

 may still be found in some adults in the form of epithelial 

 strands ; as a rule, however, they disappear without leaving any 

 trace, while the upper or nasal portions persist. 



Between the two canals, or their vestiges, which run up from 

 the buccal cavity just behind the inner incisors, there is on the 

 palate a papilla, the so-called papilla palatina incisiva (p.p., 

 Fig. 95). This has been investigated by Merkel, and found to 

 be a sensory organ, but its physiological significance is not under- 

 stood. 



Returning to the actual organ of Jacobson in Man, the 

 epithelial tubes which form its inner lining agree in every 

 respect morphologically with those of certain lower Mammals 

 (e.g. the Rat). The epithelium of the outer wall somewhat 

 resembles that of the regio respiratoria of the nasal cavity, and 

 that of the inner wall, which is almost four times as thick, that 

 of its regio olfactoria. There are no traces, however, of the 

 characteristic filamentous olfactory sense -cells the cells being 

 much more like the supporting cells of the olfactory epithelium. 

 Between them occur short fusiform elements which do nob reach 

 the surface (and may perhaps be incompletely developed olfactory 

 cells). Numerous acinose glands open into the organ. 



Although no nerves have been as yet discovered in the organ 

 in the human adult, in the embryo, as in the lower Mammals, 

 a well-defined branch of the olfactory nerve (n.o., Fig. 89, 1) runs 

 to it. 



All things considered, the organ of Jacobson in Man has 

 certainly all the characteristics of a vestigial structure. This is 

 seen not only in its inconstant occurrence7 in its frequent one- 

 sided development, and in its degeneration, which commences 

 even during fcetal life, but in its histological structure (Merkel, 

 Schwink, Chiarugi). In Anthropoids it is still further reduced. 



[This organ attains its fullest morphological development 

 in the Monotremes (Ornithorhynchus) (Symington).] 



