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 
(eg. 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 not 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 (70., 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 occurrence, in its frequent one- 
sided development, and in its degeneration, which commences 
even during foetal 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). | 
