1262 A MANUAL OF ANATOMY 



by the lateral nasal process) unites with the globular process, the nasal 

 groove is obliterated, and the olfactory pit is cut off from the stomodaeum. 

 By the development of the hard palate, on either side, the ectodermic covering 

 of the nasal groove disappears. The posterior part of this covering con- 

 stitutes the naso-stomodaal membrane. When this membrane ruptures, on 

 either side, a dorsal aperture, called the primitive posterior naris, is formed, 

 likewise on either side, which estabUshes a fresh communication between the 

 olfactory pit (blind end) and the stomodaeum on the cephalic side of the 

 corresponding palatal shelf. 



By the formation of the hard palate the nasal fossae are almost entirely 

 shut off from the permanent mouth, and by the formation of the septum 

 nasi the fossae are entirely separated from each other. The communications 

 which originally existed between the olfactory pits and the stomodaeum become 

 considerably curtailed as the hard palate is being formed, and they are per- 

 manently represented, in man, by the canals of Stensen, in the region of the 

 anterior palatine fossa. In certain animals — e.g., ruminantia — there is a 

 wide passage on either side of the median line, between the ventral part of 

 the hard palate and each nasal fossa, which passage leads to the organ of 

 Jacobson. 



Septum Nasi. — ^The septal cartilage of the nose is derived from the eth- 

 moidal division of the basal sheet of cartilage, called the chondrocranium, 

 and the vomer is developed in the mesoderm which invests the posterior and 

 inferior parts of the septal cartilage. The perpendicular plate ol the ethmoid 

 is developed from the ethmoidal division of the chondrocranium, as is also 

 the cribriform plate. 



The superior, middle, and inferior spongy bones, or conches, appear as folds 

 of ectoderm which project from the outer wall of the nasal fossa. These folds 

 contain mesoderm, and in this mesoderm cartilage, pertaining to the car- 

 tilaginous nasal capsule, is formed. The cartilage undergoes ossification, 

 and gives rise to the three conchae, the upper two forming part of the lateral 

 mass of the ethmoid, whilst the inferior concha persists as an independent 

 bone, usually spoken of as the inferior turbinate bone. 



Air-Sinuses. — ^The frontal, ethmoidal, sphenoidal, and maxillary air-sinuses 

 are developed as evaginations of the nasal mucous membrane, which extend 

 into spaces formed by absorption within the respective bones. The mastoid 

 cells are developed as evaginations of the tympanic mucous membrane, which 

 is continuous with the nasal mucous membrane through the Eustachian tube 

 and naso-pharynx. 



Olfactory Organ. — The true olfactory organ is situated in the upper part, 

 or olfactory region, of each nasal fossa. The olfactory epithelium, which covers 

 the mucous membrane of this region, is developed from the upper part of the 

 corresponding olfactory pit. 



External Nose. — The dorsum and tip of the nose, and the columella nasi 

 are developed from the portion of the mesial nasal process which Ues between 

 the globular processes. The upper and lower lateral nasal cartilages are derived 

 from the cartilaginous core of the corresponding lateral nasal process. The 

 anterior naris of each side represents, as stated, the external orifice of the 

 corresponding olfactory pit. 



Organ ol Jacobson. — Jacobson's organ is rudimentary in man, 

 but is Well developed in ruminantia. It takes the form of a blind 

 pouch, Which is situated in the lower and anterior part of the septum 

 nasi on either side. The minute orifice of the pouch lies above the 

 recess or depression of mucous membrane which projects slightly 

 into the upper end of Stensen's canal. The pouch extends upwards 

 and backwards in the septum nasi for a very short distance, and 

 terminates in a blind extremity. The cartilage of Jacobson lies 

 underneath it. Jacobson's organ does not seem to perform any 

 function in man. In those animals, however, in which it is Well 

 developed, e.g., ruminantia, it receives two nerves, one of Whicli 



