



ORGAN OF JACOBSON AND OLFACTORY NERVE 155 



ith which the palatal folds meet to complete the palate in front. Between them 

 openings persist for a time the ducts of Stenson, which correspond to the 

 permanent passages from the mouth to the nose in lower mammals. Though 

 obliterated during embryonic life in man, the ducts are represented by strands of 

 tissue occupying the foramina of the same name in the bony palate. 



The hard palate is formed by the extension of bony plates into the membranous 

 folds. Posteriorly these are absent, and muscular tissue extends into the folds, 

 giving rise to the soft palate and uvula. The palato-pharyngeal folds repre- 

 sent the posterior ends of the palatal folds which do not in this region unite 

 with one another. The turbinate processes appear in the second month, long 

 before the palate is completed, as projections from the outer wall composed of 

 a basis of mesenchyme covered by thickened epithelium (fig. 196). In these 

 projections cartilaginous plates are laid down, connected with the nasal capsule, 

 which, growing inwards and becoming curved, form the rudiments of the various 

 conchse. The accessory sinuses are produced by outgrowths from the originally 

 simple furrows separating the primitive turbinate processes. 



The mechanism by which the processes and furrows are formed is variously interpreted. 

 In the first place, it may be definitely stated that the projections are not due to an inpushing 

 of the wall by the cartilaginous strands which become the conchae. The folds are present 

 before cartilage is formed within them (fig. 196). There are two other explanations. The 

 projections may be either free ingrowing folds of the mucous membrane (Killian, Mihalkovics, 

 and others) or they may be elevations left by excavations of the furrows in the outer wall 

 (Legal, Schonemann). According to Schonemann, epithelial ridges grow out in the position of 

 the future furrows, and these are excavated into epithelial pockets. From this point of view, 

 the complexities of the nasal fossae are due to the operation of a single process, the early 

 furrows being produced in the same fashion as the later sinuses. Both factors may be at work 

 simultaneously (Glas). 1 



The organ, of Jacobson, though represented by a vestige merely in the adult 

 human being, is a well-marked structure in the embryo. It appears as a deep 

 pocket at the stomodoeal end of the olfactory pit, and afterwards, when the mouth 

 of the pit is closed in, as a pocket on the lateral aspect of the mesial nasal process 

 (fig. 195). Later, it has the form of a narrow duct, oval in section, running longi- 

 tudinally in the substance of the septum (fig. 196) and opening anteriorly near the 

 upper orifice of Stenson's duct. When the septal cartilage becomes formed, a 

 special curved plate of cartilage is developed which partially encloses the organ 

 and persists in the adult. 



The nostrils are closed for a time by an epithelial plug (fig. 196), the permanent 

 passages being established by a shedding of the central cells in the epithelial mass. 



Olfactory nerve. What was formerly described by anatomists as the 

 olfactory nerve is in reality, as we have already seen, a portion of the cerebral 

 hemisphere cut off to form a hollow stalk, which afterwards (in man) becomes a 

 solid strand, just as does the optic stalk. The distal end of the olfactory stalk 

 lies close to the developing olfactory pit, and the two become connected during 

 the fifth week by nerve-fibres. During the fourth week the lining of the olfactory 

 pit undergoes histogenetic changes comparable to those seen in the wall of the 

 neural tube. According to His, a group of cells becomes detached from the 

 epithelium, and constitutes a ganglion resembling a spinal ganglion. The cells 

 become bipolar, and their processes, establishing a connexion on the one hand 

 with the olfactory epithelium, and on the other with the brain, form the olfactory 

 nerve-fibres. According to Disse (for the bird), the nerve-fibre-producing elements 

 remain in the epithelium and themselves become the olfactory cells, which are thus 

 directly connected with the brain by single central processes. This view of the 



1 Glas Anat. Hefte, B. xxv. 1904. 



