Skull and Hyoid 



247 



By cutting away the dependent turbinals the underlying mass is left by itself 

 (No. 3), and the groove on its back part, which was partly covered by the upper turbinal, 

 is seen to have an opening in it, while the removal of the middle turbinal has exposed 

 the rounded lower and front part of the mass (bulla ethmoidalis) and, below this, the 

 contracted opening of the antrum and a curved groove (hiatus semilunaris) leading up 

 to the opening of the frontal sinus. The mass whose limits are thus roughly made out 

 is the labyrinth of the lateral mass of the ethmoid, and consists of air cells with thin 

 bony walls. A scheme of the relations between outer 

 wall, labyrinth, and turbinal is shown in Fig. 195. 



But the cavity represented in the labyrinth in 

 this figure is purely schematic ; there are really a 

 number of cavities, separated by walls that are very 

 variable in position individually, but they may be 

 divided into groups. There are three such groups, 

 anterior, middle, and posterior, and the air cells 

 belonging to any one of these groups do not com- 

 municate with those of the other groups, and not 

 always with each other : the cells of each group, 

 however, open together those of the posterior group 

 in the upper meatus (Fig. 194), those of the middle 

 and anterior groups in the middle meatus. The 

 posterior ethmoidal cells are behind, above, and internal 

 to the middle cells, and these are again behind and 

 internal to the anterior group : these last are there- 

 fore the cells exposed on the outer aspect of the lateral 

 mass where the os planum is deficient in front, and are 

 covered in by the lachrymal. 



The general relations of these groups to each 

 other are shown schematically in Fig. 196 : it must 

 be again remembered that each group is not a single 

 cavity, but is composed of a variable number of cells. 

 The schematic position of the three groups is shown 

 on section in the three planes, and it is clear that a 

 transverse section through the front part of the mass 

 will show anterior ethmoidal cells chiefly, with middle 

 cells'perhaps internal to them, and hiatus semilunaris 

 situated between the two groups : a section across the 

 middle of the mass may have only middle cells, with 

 middle turbinal hanging from their inner and lower 

 part ; one further back goes through posterior cells, 

 with the middle group perhaps represented externally 

 and below, separated from the posterior group by the 

 groove of the upper meatus, over which the upper turbinal hangs down from the inner 

 wall of the posterior group. Two sections of the region are also shown (Fig. 197) to 

 compare with the schemes and exhibit the inner subdivisions of these cell-groups. 



The group of posterior cells is placed in front of the sphenoidal turbinate, and the 

 recess (Fig. 194) between them is known as the spheno-ethmoidal recess : the opening of 

 the sphenoidal sinus in the sphenoidal turbinate is thus in the recess. The posterior 

 group may show a partial subdivision by a groove into which one or more of the cells 



roof' of nasal fan. 



FIG. 197. Sections at different 

 levels through the lateral 

 masses of an ethmoid, seen 

 from below. On the right 

 side of the figure the section 

 runs at a lower level ; A., 

 anterior cells on outer side of 

 infundibulum ; M., four cells 

 of middle group, the hinder 

 ones 1 ying outside the posterior 

 group which is represented by 

 the inner cells at and behind 

 the level of P. On the other 

 side the section runs just 

 below the roof and only opens 

 cells of the posterior group, 

 which are very large in this 

 specimen. Observe that the 

 back part of the posterior 

 group may be partially sepa- 

 rated, when the groove be- 

 tween it and the rest of the 

 group becomes the meatus 

 suprema and the fourth turbi- 

 nal hangs from its projecting 

 side. See Fig. 194. 



