THE NASAL CAVITIES AND OLFACTORY STRUCTURES. 23 



(Elliot Smith). In man many of these parts are merely vestigial. 

 The higher olfactory centre has been located in the hippocampal 

 gyrus, but no evidence has been given showing any connection 

 between the callosal gyrus and this sense. In animals with a 

 highly developed olfactory sense (carnivora, etc.) these parts of the 

 brain which form the rhinencephalon are well developed. The 

 fornix in its greater part, and the longitudinal striae as association 

 tracts, connect the brain areas which are subservient to the sense 

 of smell (see Fig. 18). 



It is important, from a clinical point of view, to remember that 

 the olfactory nerves are surrounded by prolongations of the arach- 

 noid membrane and subarachnoid spaces, and through these spaces 

 infections may spread from the nasal cavities to the meninges. 

 Further, the olfactory tracts rest on the edges of the small wings 

 of the sphenoid, and may be injured in falls on the forehead. 



The Nasal Cavities. — The separation of the nasal cavities from 

 the stomodaeum by the downgrowth of the mesial and lateral 

 nasal processes, and the ingrowth of the horizontal plates of the 

 maxillary processes, has been already described (p. 3). These 

 processes also form and bound the anterior and posterior nares. 



Development of the Nasal Air Sinuses. — The manner in 

 which the nasal mucous membrane pushes its way through the 

 lateral nasal cartilage into the maxillary process to form the 

 Antrum of Highmore has been already described (page 12). 

 The other air sinuses — the frontal, lachrymo-ethmoidal, anterior, 

 middle and posterior ethmoidal, and sphenoidal sinuses — six in 

 all, arise in the same way as the antrum but at a much later date. 

 Although they begin to bud out about the 3rd year, they assume 

 their active growth in the earlier years of puberty, and reach 

 their full size before the 30 th year. 



At birth, the lateral mass of the ethmoid is a thin plate, 

 carrying the superior and middle turbinate processes, which 

 almost fill the nasal cavity (Fig. 7). The entire ethmoid is narrow, 

 and hence the proximity of the eyes in children. Beneath the 

 middle turbinate is a thumbnail-like impression — the hiatus semi- 

 lunaris (Fig. 19). The antrum buds out near its posterior end, 

 arid the point at which the bud starts becomes its opening. The 

 uncinate process of the lateral mass of the ethmoid forms the 

 prominent lower margin of the hiatus. 



