138 THE NOSE. 



the bulla ethraoidalis, the hiatus semilunaris, and the apertures of the air-cells 

 which open into this meatus. 



The middle meatus, which lies beneath the middle turbinal, is roughly triangular 

 in shape, with the base of the triangle directed forwards. It expands in front of 

 the middle turbinal into a nearly smooth-walled chamber which communicates 

 through the vestibule with the anterior nares ; this chamber is known as the atrium 

 of the middle meatus. On the outer wall above the atrium a low ridge (figs. 154, 

 155) may usually be detected passing downwards and forwards from the anterior 

 attachment of the middle turbinal. This ridge (agger nasi of H. Meyer), which 

 is seen also in the macerated bone (fig. 153), is the rudiment of a well-developed 

 turbinal (nasoturbinal), which is met with in most mammals (Schwalbe). It is 

 usually better marked in the foetus and new-born child, in which it is seen to be 

 continued below and behind into the imcinate process of the ethmoid, which forms the 

 lower and anterior boundary of the hiatus semilunaris (Seydel). The groove above 

 the agger nasi leads to the olfactory part of the nose, and is termed sulcus olfactorius. 



In the lower meatus, which lies between the inferior turbinal and the floor of the 

 fossa, is the inferior orifice of the nasal duct which is defended by one or two folds of 

 membrane : when there are two, the folds are often adapted so accurately together 

 as to prevent air from passing up from the cavity of the nose to the lachrymal sac. 



The roof of the nasal fossa is divided into three parts, viz. : nasal, in front ; 

 efhmoidal, in the middle, and sphenoidal, behind, corresponding with the bones of 

 the same name. The roof is at the front formed only by the conjunction of the 

 septum and lateral walls, and is quite narrow, but it is broader near the choanae. 

 Above and behind the upper turbinal is a diverticulum of the nasal fossa, the 

 spheno-ethmoidal recess, which communicates posteriorly with the sphenoidal sinus 

 (fig. 154). The floor of the fossa is broader than the roof. In it in front is the 

 incisor foramen, but this is in the recent state generally closed (fig. 154). Some- 

 times, however, a narrow funnel-shaped tube of mucous membrane (nasopalatine 

 canal, canal of Stensen, or canalis incisivus) passes obliquely downwards from each 

 nasal fossa for a short distance towards the front of the hard palate. Yesalius, 

 Stensen, and Santorini believed that these tubes of membrane opened generally into 

 the roof of the mouth by small apertures close behind the central incisor teeth. 

 Haller, Scarpa, and Jacobson found the canals in man usually closed, and often 

 difficult of detection, and these statements have been confirmed by most modern 

 investigators. The canal is a remnant of the wide communication between the 

 nasal and buccal cavities found at an early period of foetal life, being in man usually 

 obliterated, at least in its lower part, before birth, although persistent in many 

 animals. It is long represented below by a solid column of epithelium cells 

 continuous with the epithelium of the palate, and above by a narrow tube lined 

 with ciliated epithelium, opening into the floor of the nasal fossa but closed below. 



The part of the inferior meatus which lies behind the incisor canal, together 

 with the space immediately behind the posterior end of the superior and middle 

 turbinals as far back as the orifice of the Eustachian tube, belongs to the primitive 

 buccal cavity of the foetus, having become separated from the permanent mouth by 

 the growth of the palate (see Yol. I., p. 97). It is known as the naso-pharyngeal 

 part (ductus naso-pharyngeus), and its mucous membrane is marked off behind from 

 that of the pharynx by a prominence which is termed the naso-pharyngeal fold. 



The anterior and lowermost part of the nasal cavity is termed the vestibule 

 (fig. 157). This is bounded laterally by and corresponds in extent with the ala and 

 the cartilage of the aperture. It is lined by skin and is furnished with hairs and 

 with sebaceous and sudoriferous glands. The hairs are large and numerous just 

 within the orifice of the anterior nares (vibrissae), but over the rest of the. vestibule. 



