1206 THE RESPIRATORY SYSTEM 



The attached margins of the middle and inferior conchse are both arched, the convexities 

 being upward. The highest point of the convexity is near the middle of the attached margin 

 in the inferior concha and lies about 17 mm. above the floor of the nose; the anterior end of this 

 concha is approximately 25-35 mm. distant from the apex of the nose (Kalhus). 



From the anterior end of the middle concha a slight variable elevation of the mucous mem- 

 brane of the nose extends forward and downward. This, the agger nasi, which is regarded as 

 of constant occurrence in the new-born, appears to be a rudimentary representative of the naso- 

 turbinale of mammals (Schwalbe). 



Below the agger nasi a broad depression of the lateral wall, the atrium meatus medii, leads 

 posteriorly beneath the anterior free margin of the middle concha to the middle meatus, while 

 above the agger, between it and the roof of the nasal cavity, the slight olfactory groove [sulcus 

 olfactorius] ascends upon the lateral wall to the olfactory region. In this region, above the 

 superior concha, is a corner of the nasal cavity of interest on account of the sphenoidal sinus 

 opening into it: this is the spheno-ethmoidal recess [recessus spheno-ethmoidalis]. 



Variation in the number and position of the openings into the meatuses is of practical inter- 

 est. An accessory mouth of the maxillary sinus is rather frequently met with, especially in old 

 people; it lies most commonly behind the hiatus semilunaris. The infundibulum ethmoidale 

 may open independently of the hiatus semilunaris at a spot beneath the anterior end of the at- 

 tached margin of the middle concha. In the inferior meatus the mouth of the naso-lacrimal 

 duct, which is found 22-25 mm. behind the posterior margin of the nares, may have one or more 

 accessory openings associated with it; these are perforations of the plica lacrimalis. 



CommuDication between the nasal cavity and the nasal part of the pharynx 

 is effected by means of the paired posterior apertures [choanae]. These are oval 

 in form, their height greater than their width. They are located at either side 

 of the posterior edge of the nasal septum and are limited above by the body of 

 the sphenoid, below by the line of junction of the hard and soft palate. 



From the plane of the choana forward a rather constricted portion of the nasal cavity ex- 

 tends for a short distance to reach the level of the posterior ends of the middle and inferior con- 

 chiB. Into this region, which is known as the meatus naso-pharyngeus, open posteriorly the 

 superior, middle and inferior meatuses. Posterior rhinoscopic examination reveals the choanae, 

 the naso-pharyngeal meatus, the posterior extremities of the three conchae and of the meatuses 

 beneath them. 



Dimensions of the nasal cavity.— The length of the floor averages approximately 40 mm., 

 the width 32 mm., the height from floor to lamina cribrosa 47 mm. The length of the lateral 

 wall is about 63 mm. The choana measures 29.8 mm. high and 15.5 mm. broad. The area of 

 the two nares is 2 sq. cm. 



Paranasal sinuses [sinus paranasales] (figs. 964-968). — The location, form and 

 relations of the bony-walled spaces connected with the nasal cavity have been 

 fully described in the section on Osteology. The conditions observed in the 

 living subject differ in certain respects from those present in the macerated skull; 

 the spaces are lined by a mucous membrane which, though affecting but slightly 

 the form of these chambers, modifies considerably the openings by which they 

 communicate with the nasal cavity. These openings permit the entrance and 

 exit of air and to some extent the escape of fluids which may accumulate in the 

 sinuses. While the significance of these spaces is not at present clear it is, how- 

 ever, certain that they function in lightening the weight of the skull, and probable 

 that indirectly they serve in connection with the sense of smell. 



Maxillary sinus (of Highmore*) [sinus maxillaris Highmori] (figs. 965, 966, 967). 



Entrance into the maxillary sinus is offered through the middle part of the hiatus 

 semilunaris, that is, the deep, narrow notch between the ethmoidal bulla and un- 

 cinate process of the ethmoid. Viewed from within the sinus, the opening appears 

 as an oval window in the upper part of the medial wall — a position unfavourable 

 to the dischaige of matter, when the body is in the upright posture. An accessory 

 opening, situated behind the normal ostium, is present in about 10 per cent, of 

 cases. 



Measurements of 90 specimens of the adult sinus maxillaris gave as the average the following 

 (SchaefTcr) : 



Dorsosuperior diagonal 38 mm. 



Ventrosuperior diagonal 38.5 mm. 



Hupcroiiiferior 33 mm. 



Ventrodorsal 34 mm. 



Mediolateral 23 mm. 



Increase in capacity of the maxillary sinus is sometimes observed as the result of more or 

 less exten.sive excavation of the bony processes of the iiiaxilla adjacent to it, viz.: the alveolar, 

 palatal, frontal and zygomatic. On the other hand narrowing of the cavity is encountered, 



* Highmore, Nathaniel: English physician. B. 1613, D. 1685. 



