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The ethmoid and the pneumatic sinuses.

The pneumatic sinuses.

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further downwards in the supraoccipitale they are still represented
by large compartments with few bony lamellae. In the caudal wall
a large part of the supraoccipitale is still free, whereas the ventrolateral
part of the exoccipitale is already containing a relatively
large sinus occipitalis; the tympanicum is in full activity becoming
pneumatized. The lateral wall is completely pneumatic and in
the maxillary the sinuses extend right down to the alveoli; in
the orbita and in the temporal region some parts of the sinuses
are already divided into small compartments, whilst dorsally and
especially caudally they still consist of large ones.

In Elephant e (PL 29 fig. 3 and 4) the sinuses has grown
considerably. The skull has begun taking shape, especially in
the cerebral part where it expands strongly arched to each side;
caudally in the midline it is very concave because of its being
developed more in its lateral parts than in the middle. The sinuses
have become higher; the number of small compartments
has increased; in the dorsal and the lateral walls the sinus has
obtained its greatest depth, 7—8 cm. In the occipital plane the
sinus has reached down on each side of the groove to the lig. nuchas
but still appears as a large continuous compartment with few
already wavy bony lamellae; in a few of these lamellae the sinus
has commenced extending into their root. In the intermaxillary
the sinus extends right on to the rostral border mediad of the
alveolus.

In Elephant h the sinuses have expanded into so to say all
the bones, jugale only excepted, and have become further developed
, as described above. In the pars cerebralis they have reached
their most imposing circumference, with a maximum depth of 18
cm in the dorsal wall and 10 cm in the lateral. In the occipital
plane the sinuses extend right down to the edge of the foramen
magnum but are indeed very low here. The intermaxillare has
grown considerably in length, but not so the sinuses, wherefore
almost all the rostral half is free.

As we wished to conserve the right half of the skull of the
Elephant Chang, an idea of the pneumaticity could only be obtained
by means of the sagittal section (PI. 20), the shape of the
skull and the transverse sections of the left half (PL 41). The sinuses
have grown immensely in height and breadth, both in the
pars faciei (intermaxillare and the hard palate) and — especially
— in the pars cerebralis, where the sinuses have in the dorsal
wall obtained a maximum depth of 26 cm; also the ventral wall
has become strongly thickened, especially in the rostral part; in
the basioccipitale the sinuses cease at 4 cm from the border of
the foramen magnum. In Elephant h the sinuses in the last
named bone extend considerably further in caudal direction; in
Chang the pneumatizing of this bone has not kept up with its
growth in height. Of special interest is the median part of the
occipital plane reaching from the ventral border of the groove for
the lig. nuchas to the dorsal border of the foramen magnum.
In Elephant h, where the distance from the ligament fossa to the
foramen magnum measures 9 cm, the sinuses extend down to
foramen magnum although not being very deep here. In the only
slightly younger Elephant g, which is of about the same size, the
pneumatic sinuses do not extend below the groove for the nuchal
ligament; the margin above the foramen magnum is rather thick
(closely above the foramen magnum it is 2|—3 cm thick), spongy.
In the skull k the same conditions are apparently present. In Chang
the pneumatic cavities are continued ca. 4 cm below the ligament-
fossa, but there is a distance of 14 cm from here to the foramen
magnum which is without pneumatic cavities, and here the cranial
wall is, as said above, very thin. According to this description
there seems to be a certain variation in the distribution of the
pneumatic sinuses in this part of the skull in older Elephants.
It is hardly probable that the pneumatic sinuses should redraw
from the foramen magnum when once they had reached so far.

The pneumatic sinuses are developed from the cavum nasi
and from the cavum tympani.

The tympanal pneumatizing is partly restricted to the tympanicum
, where it commences by forming cellulce tympanicce,
some of which increase so much in growth that they must be
designated as sinuses; they expand in the wall of the tympanicum
proper. Already in Elephant c, in which there are still
many cellulae to be found, this formation of sinuses has commenced
. In Elephant h the formation of sinuses is carried on,
and the sinuses are already so deep, that a system of consolidating
bony lamellae (1—2 cm high) is shaped and arranged in the form
of a radiation in rostrad direction. Finally a sinus occipitalis is
developed caudally from the cavum tympani, expanding in the
lateral part of the exoccipitale. It is already present in Elephant c
and has obtained a rather considerable size in Elephant h.

The most extensive part of the pneumatizing process originates
from the nasal cavity. A general view of this pneumatizing,
which is acquired by examinating the figs, in PL 43, 44 and 45,
yields above all the impression of a very considerable variation;
only very few sinuses may be designated as constant, namely the
sinus maxillaris and the sinus nasalis. As for the other sinuses
there is, even in the two halves of the same skull, a great variation
in the number of sinuses, in the figures of the sinuses appearing
in each system, and — last but not least — in the extension of each
sinus. Similar conditions are met with in other Mammals, but
not by far to such a degree as in the Elephant. In this latter animal
a sinus, which is in one half of the skull very large, may in the
other half be reduced to a small bulla completely covered by a
neighbouring sinus, f. inst. sinus 7' in Elephant c; the variation
may even by carried so far that a sinus of very great extension
in one skull may have been completely reduced in another, but
replaced by another sinus, f. inst. sinus intermaxillaris inferior in
Elephant c; it is missing in Elephant h, but replaced by the sinus
maxillaris. Altogether, we get the impression that a strong fluctuation
in the details is taking place within the pneumaticity as
a whole. It is as if the sinuses were competing for obtaining a
large volume, the contest however always ending in a definite
result, the final shape of the skull.

As to the details, the pneumatizing developed from the cavum
nasi consists of a system of sinuses developed from the ethmoid,
of a sinus maxillaris, and finally of some sinuses specific to the
Elephant. These sinuses together pneumatize by far the greater
part of the wall surrounding the cavum cranii — with the only
exception of the part of the exoccipitale which is occupied by the
sinus occipitalis — and obtain here their most enormous extension;
they further pneumatize the whole of the pars faciei, where they
may reach far forwards in the intermaxillare.

The ethmoidal pneumatizing occupies first of all the pars cerebralis
but also proceeds to the pars faciei especially in the medial
wall of the orbit, in Elephant h even far forwards into the intermaxillare
(the whole of the pars ascendens). This pneumatizing
consists of a system of sinuses the apertures of which are found
on the wall of the ethmoid in the intervals between the lines
of origin of the basal lamellae (textfigure). It is well worth
noticing how small these apertures are even in the old Elephant
Chang; the diameter of the round openings varies from 3 to 6
mm; the oval or longitudinal openings are 2—4 mm in breadth,
and 4—10 mm in height (the last-named measure however only
in a single case). It is only a relatively scanty number of sinuses
which are refound in the systems of the three examined skulls
(PL 45 figs. 1—3); a great variation is occurring both in the
number of the sinuses, in their figures and not least in their
extension. The Elephant c (PL 43 figs. 1—4) is a typical specimen
in this respect; but no matter how the forming of the details of
the system proceeds the pneumatizing of the pars cerebralis is
executed mainly through the presence of a small number (2—3)
of enormous sinuses in the dorsal, the caudal and the lateral walls,
whilst in the pars faciei there are many smaller or even quite
small sinuses. Special attention must be called to the large sinuses
in which a consolidating system of a particular kind has
taken place. Already in Elephant c the indication of this system
is found in the shape of a series of bony lamellae which radiate
from the periphery of the sinus into this latter and extend from
the floor to the ceiling; these bony lamellae are designated as the
primary ones. In Elephant h the system is fully developed, consisting
of a series of very high lamellae which radiate into the
sinus at a right angle from the floor; the caudal (2.—7.) lamellae
in PL 42 fig. 1 extend obliquely dorso-laterally towards the temporal
plane in a direction corresponding to that of the traction
of m. temporalis. These lamellae are of a singular shape, they are
folded like a piece of corrugated iron, whereby the consolidating