THE OEGANS OF THE OUTER GERM-LAYER. 517 



primitive oral cavity is divided into two storeys, one above the other. 

 One, the upper part, becomes associated with the organ of smell, to 

 the enlargement of which it contributes ; it is distinguished from the 

 space that arose from the original olfactory pit, or the olfactory 

 labyrinth, as naso-pharyngeal jJO'Ssage. This opens behind into the 

 pharynx by means of the posterior nares. The lower part becomes 

 the secondary oral cavity. The partition that has been formed from 

 the maxillary process is the palate, which later, when the develop- 

 ment of the bones of the head can be traced, is differentiated into the 

 hard and the soft palate. 



A small portion of the palatal fissure, which in young embryos 

 traverses the palate from in front backward and unites oral and 

 nasal cavities (fig. 290 *), is preserved in most Vertebrates and con- 

 stitutes the ductus Tuisopalatimis or Stenson's diict. A probe may be 

 passed through it from the nasal to the oral cavity. In Man the 

 duct of Stbnson is closed during embryonic life ; there is preserved, 

 however, in the palatal process of the bony maxilla at the correspond- 

 ing place a vacuity, the canalis incisivus, occupied by connective 

 tissue, blood-vessels, and nerves. 



Where the ducts of Stenson are present, there are found in their 

 vicinity the organs of Jacobson, concerning which the statement has 

 already been made that they are established very early as special 

 depressions of the two olfactory pits. In Man this organ is converted 

 into a narrow tube, which lies a little above the canalis incisivus and 

 " pursues a straight course backward and slightly upward close to 

 the cartilaginous partition, ending blindly " (Schwalbe). In Mam- 

 mals the organ is more highly developed (figs. 290, 291 J) ; it is 

 enveloped in a special cartilaginous capsule (Jacobson's cartilage, 

 jk) and receives a special branch of the olfactory nerve, which ter- 

 minates in a sensory epithelium, which agrees with that of the 

 regio olfactoria. Frequently {e.g., in Ruminantia) it opens into the 

 beginning of Stenson's canal, which in this case remains open as 

 a communication between nasal and oral cavities. 



I cited the formation of folds as the second meang of increasing the 

 internal surface of the organ of smell. These are developed in 

 Mammals (figs. 290, 291) and in Man on the lateral walls of the 

 nasal chambers ; they run parallel to one another from in front 

 backward ; their free margins grow downward, and in consequence of 

 the forms which they assume are called the three nasal turbinated 

 processes, while the spaces between them are designated as upper, 

 middle, and lower nasal passages. From the cartilaginous cranial 



