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The ethmoid and the pneumatic sinuses.

The pneumatic sinuses.

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divided in several mutually communicating compartments, separated
from each other by bony lamellse of different heights, some
of them so high that the respective compartments are only communicating
through a narrow slit right down at the bottom which
is formed by the arched wall of the brain-cavity. The primary
(o: the highest) bony lamellse reaching down to the bottom are
arranged so that they radiate out into the sinus from the medial
and rostral walls.

From the same interval in which sinus 6' ends there is, a
little more ventrally, a small aperture leading into a quite small
sinus 6 a', completely covered by sinus 6'.

Between the 7. and 8. ectoturbinals and c. 1 cm ventrally to
the lamina cribrosa there is a large oval aperture (5 mm high and
4 mm broad) leading into a very large sinus 7', which through
the caudo-lateral part of the frontale extends into the parietale,
where it expands in an enormous sinus occupying almost the
whole of this bone and already reaching a little down into the
supraoccipitale. Ventrally the suture-plane between the parietale
and the squama temporalis make the boundary between this sinus
and sinus 8'. Medially the sinus extends to the septum sinuum,
caudally to the sinus 6' and expands a little into the right half;
the sinus is here c. 6| cm deep. In spite of the presence of a
system of consolidating bony lamellse it still makes the impression
of forming a large continuous pneumatic cavity extending over a
considerable part of the wall of the cavum cranii which makes
the arched bottom of the sinus. The aperture leads into a rather
high, but quite narrow compartment, reaching caudad downwards
to the sinuses 8' and 9 a'. After having passed these two sinuses
it extends towards the surface, hereby increasing considerably in
depth, and then divides into two sections. The smaller medial one
reaches to the dorsal plane of the skull, caudo-laterally to the
sinus 6', and extends a little downwards into the supraoccipitale.
Through a narrow, short, canal-shaped communication, lying ro-
strally at the very bottom, the medial section continues into the lateral
one. This is very large, only with a smaller part reaching to the
dorsal plane of the skull, whilst for the greater part it lies in the
lateral plane of the skull and is from here extending downwards
into the supraoccipitale. The extension into the last-named bone
makes the distinct impression of taking place through a process
of resorption, as it appears as a row of furrow-shaped depressions
of different depth and separated from each other by projecting
bony edges. The grooves look as if they had been excavated in
the enormously thick (2J cm) spongiosa in the supraoccipitale.
In this sinus a system of consolidating bony lamellae, 9 in all,
has already been developed. They issue from the circumference
of the sinus and from the bottom and reach even to the surface.
They radiate towards the middle of the sinus at a right angle to
the bottom, and this bottom being arched, the caudal bony lamellae
must extend obliquely (dorso-laterally) towards the temporal
plane of the skull (the plane from which the m. temporalis
takes its origin), at such angles that the direction of the lamellae
corresponds to the pulling direction of this muscle. A single one
of these lamellae has a sinuous course. Between some of the lamellae
there are transverse connections, most of which lie close
to the surface and are quite low; a single transverse connection
reaches so deep down into the sinus that there is only a narrow slit-
shaped communication between the neighbouring compartments.

Between the 7. and 8. ectoturbinals, a little ventrally to the
opening into the preceding sinus, there is a little round aperture
leading into a quite small sinus 7a'; it vaults forwards as a thin-
walled "bulla" at the bottom of sinus 8 a' and is completely hidden
by this sinus.

Close below the lamina cribrosa, between the 8. and 9. ectoturbinals
, there is a narrow (c. 2 mm), but high (c. J cm) aperture
of a very large sinus 8', the bottom of which is formed by
the arched wall of the cavum cranii. The aperture leads into a narrow
, canal-shaped passage extending caudad into the frontal bone
(pars temporalis), covered by the sinus 9'. Gaudo-ventrally to
this latter the sinus expands into the frontal bone, where it is
irregularly multilocular; it then continues caudad through the
suture into the squama temporalis where it expands as an extremely
large compartment, lying ventrally to the sinus 7', and
is in circumference almost equal to this latter. This temporal part

of the sinus 8' is through an almost frontally placed high bony
lamella divided into a dorsal and a ventral section, which are
rostrally communicating through an opening of the breadth of a
finger. The dorsal section which reaches up to the sutura squamosa
is a continuation of the frontal part of the sinus and forms
caudally another large continuous compartment which is expanding
into the supraoccipitale in the same way as mentioned for
sinus 7'. The ventral greater part reaches caudad to the suture
between squama and exoccipitale, where it borders on the tympanal
sinus occipitalis. This section which is still a large coherent
compartment, only caudally divided by a few high bony lamellae,
extends into the processus zygomaticus which it pneumatizes in
about its medial half forming in it a rather large compartment.
Owing to this expansion the outer auditory passage must pass
through the sinus in which it presents itself as a projecting tube,
4J cm in length; only ventrally the sinus does not enclose it, the
bone surrounding the passage having here a narrow slit. The ventral
section expands farther medially in the naso-ventral part of
the squama, thereby reaching, ventrally to the cavum cranii, partly
into the ala temporalis of the basisphenoid, partly forwards into
the root of the processus pterygoideus. This part of the sinus is
very irregular, with several small sections in different levels. This
basal part of the sinus besides also extends into the proc. zygomaticus
ventrally to the already mentioned prolongation.

| cm ventrally to the aperture of the preceding sinus there
is the narrow (2 mm) but high (8 mm) aperture of the sinus 8a'.
The greater part of this high but narrow sinus, which is divided
into several smaller compartments, lies in the dorsal part of the
medial orbital wall (the frontale), where it borders caudally on
the sinus 9' and 9 a' and besides on the sinus maxillaris. The
sinus 8 a' also reaches to the dorsal plane of the skull by an
elongated narrow compartment in the rostral part of the frontale,
and into the proc. zygomaticus; medially it borders on the sinus 6'
and the two intermaxillary sinuses.

Between the 9. and 10. ectoturbinals and close below the lamina
cribrosa there is a high, narrow aperture leading into the
rather extensive sinus 9' in the frontal bone, just caudally to the
orbit where it borders on sinus 8 a'. The sinus 9' reaches even
up into the proc. zygomaticus and forwards into the sharp-edged
crista orbito-temporalis; it is divided into some narrow compartments
joining in a little larger one in the ventral part of the
sinus.

\ cm ventrally to the aperture of the preceding sinus there
is the small oval opening of sinus 9a'. This small and narrow
sinus lies in the frontal bone where it is for the greater part
covered by the sinus 8 a'; only a single little part of it reaches
to the surface in the medial orbital wall rostrally to the sinus 9'.

Between the 10. and 11. ectoturbinals, close below the lamina
cribrosa, there is a little rounded opening leading into a small bag,
the sinus 10'; it is completely covered by the sinus 9'.

Almost at the base of the ventral part of the basal lamella
of the 10. ectoturbinal there is a rather large, oval opening; the
appertinent rather small sinus 10a' is irregular and divided into
several smaller sections. It lies in the ventral part of the frontale
(pars orbitalis) and extends down to the alveolus of dp4; here it
borders rostrally on the sinus maxillaris.

The sinus 12', the aperture of which is lying close below the
lamina cribrosa, is a small and narrow sinus in the frontal, off the
middle of the crista orbito-temporalis; it is completely covered by
the sinuses 8' and 9'.

The sinus 19' is a little bag totally covered by the sinus 8'.

The sinus 21'. The rather large, round aperture lies quite
ventrally at the basal lamella of the 21. ectoturbinal. The very
small sinus is found at the bottom of the ventral part of the frontal
bone and extends a little down into the maxillary, to the alveolus.

The aperture of the sinus II' is high (8 mm) and rather broad
(3 mm) and is situated far ventrally to the lamina cribrosa. The
sinus, which lies caudally to the preceding and is still smaller
than it, is partly covered by that sinus and by the sinus 23 a'.

The sinus 23', which is a little larger than a pea, lies in
the rostrodorsal part of the presphenoid, directly under the canal-
shaped foramen opticum.

\ cm ventrally to the aperture of the preceding sinus there