Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
Orbits.
Com para- the scull, the malax* bone attached to the maxillary only,
tive is not united to the zygomatic process of the temporal. In
Anatomy. ]ong_muzz]ecl Edentata,in which the face is conoidal,
the maxillary bones extend to the orbits, and are separat¬
ed by a broad lacrymal bone, while a long palate bone
forms the inner wall of these fossce. The zygomatic arch,
which is interrupted in the ant-eater and pangolin, is com¬
pleted in the Cape ant-eater and the ai-madillo. In the
tapir and rhinoceros the maxillary bone passes beneath
the orbit; and the nasal bones form a sort of vault, which
supports in the fii'st animal the trunk, and in the second
the horn.
In the Cetacea the maxillary and intermaxillary bones
form a sort of flattened beak, distinguished into four paral¬
lel bands, of which the maxillary, which are external, alone
bear teeth in three genei*a, provided with the latter organs.
The nasal fossa is a vertical opening before the cranium,
surrounded before and laterally by the intermaxillary
bones. The maxillary ascend in the same manner, and
cover that part of the frontal bone which forms the orbitar
vault, but do not themselves contribute to the formation
of this cavity. The nasal bones are two minute tubercles
implanted on the frontal bone above the narrow aperture.
The malar is in the shape of a style, suspended by cartilages
beneath the orbit; and the latter cavity is completed be¬
hind by a process of the frontal, which joins the zygoma¬
tic of the temporal bone, and below which the orbitar
and temporal /b.ss<g communicate.
The direction of the ox-bits, the shape of their base or
facial border, and their relation to the temporal/ossa, are
important circumstances in the animal face and cranium.
In the simi(E the angle of the orbitar axes is rather
smaller ; and the shape of the margin, which is quadrilate¬
ral in the jocko, becomes oval in the oui*ang-outang and
American monkeys. The angle of the axes enlarges in
the other Mammalia; and the base or anterior margin
becomes nearly circular in the Zoophaga, Rodentia,
Edentata, and Pachydermata ; but the arch is incom¬
plete behind. In \heRuminants and Solidipeda, however,
in which it is also circular, the border is complete. In
the Cetacea the orbitar vault is semicircular, their axes
are rectilineal, and there is no floox*.
In the human scull the junction of the malar bone
with the frontal and sphenoid completes the orbit extei*-
nally, and prevents it from communicating with the tem¬
poral/ossa ; and the same arrangement is observed in the
simia. In the Carnivora, Rodentia, Edentata, and
Pachydermata, however, in which the malar bone is
united neither to the frontal nor the sphenoid, the orbit
is not only incomplete on the external posterior border,
but communicates freely with the temporal In the
Ruminants is observed an arrangement intermediate be¬
tween that of the Quadrumana and that of the Carni¬
vora. The malar bone, united to the frontal, completes
the orbitar ring; but as it is not united to the sphenoid, it
allows the orbits and temporal fossce. to communicate. The
orbit of the mole is so superficial, that it can scarcely be
said to exist.
Ihe lower jaw of the mammiferous quadruped differs
from that of man chiefly by the following circumstances.
Ihe triangular flat surface which constitutes the chin, and
which is most distinct in the Caucasan race, begins to be¬
come faint in the negro, and is altogether lost in the monkey
tribe. In the ourang-outang, indeed, the animal character
of the lower jaw appears distinct in the vertical convexity
ot the anterior arch of the jaw, and the retreating of its
lower margin. In the lower Quadrumana the anterior
maxillary arch is still more retreating, and the maxillary
rami form a more acute and elongated angle. These
animal characters are still more conspicuous in the Car-
79
The lower
jaw.
NivoRA,most Of the Pachydermata, Ruminants, Soli- Compara-
dungula, and Rodentia. The ascending ramus also tive
becomes short in the Pachydermata and several of the Anatomy.
Cetacea, more so in the Zoophaga, and is almost extinct
in several of the Rodentia, for instance the paca, beaver,
and porcupine, and the armadillo, ant-eater, and duck¬
bill, among the Edentata. In the Zoophaga, however
in which the prehensile and masticatory muscles are large
and powerful, the ramus becomes broad, and its coronoid
process is extensive. The angle which the ramus forms
with the body of the jaw, and which is almost right in the
adult human subject, becomes obtuse in the lower ani¬
mals, nearly at the same rate at which the ramus dis¬
appears ; and indeed the transition of the angle into a
straight line implies the disappearance of the ramus. This,
therefore, is the character in the Edentata and Ceta¬
cea, in which there is neither ramus nor coronoid process,
after these parts have been seen for the last time in the
amphibious Mammalia.
When the mammiferous cranium is considered gene¬
rally, and the relative direction and proportion of the
cranial and facial part of the head examined, we recog¬
nise more distinctly the characters by which the lower
orders of that class are distinguished from man. This
character consists in the position of the occipital bone and
hole, the position and direction of the facial bones in re¬
lation to the frontal, the elongation of the former, and
large size which they present in relation to the cranial.
In the human subject, it has been already observed, the
position of the occipital bone is oblique and horizontal,
and the plane of the occipital hole is horizontal, while its
position is anterior. In most quadrupeds, while the bone
assumes a vertical position, the hole becomes posterior,
and its plane vertical or oblique, in proportion as the face
is elongated. The plane of the occipital hole forms with
that of the horizontal a considerable angle, which Dauben-
ton undertook to determine, by drawing one line through
the plane of the aperture, and another from its posterior
margin through the lower edge of the orbit. [Mem. de
VAcad, des Sciences de Paris, 1764, p. 568.) In the horse
this angle is about 90°, while in the ourang-outang it is
only 37°, and in the lemur 47°. In other respects, however,
it furnishes an imperfect result, since in most quadrupeds
which differ very much it ranges between 80° and 90°.
The direction of the face in relation to that of the Camperian
cranium, determined according to the method of Camper, line and
furnishes more accurate results. While in the human angle-
subject it varies, according to the races, from 70° to 80°, ‘
in the ourang-outang it is only 65°; in the American
and long-tailed monkeys about 60° ; in the macaca and
baboon about 45°; and, lastly, in the mandrillo, the most
vicious and ferocious of the monkey tribe, only 30°. In
some species in which the ear is elevated and the guttu¬
ral fossa deep, for instance in the pongo and alouate or
howler, the small size of this angle does not indicate pro¬
portional elongation of muzzle; and to rectify this incon¬
venience, it is requisite to draw the basilar line of the
facial angle parallel to the base of the nostrils. With
this modification, however, the Camperian line admits of
correct application to the human race and Quadrumana
only, in which the frontal sinuses are small and not promi¬
nent. In quadrupeds, for instance the carnivora, several
of the ruminants, and in the elephant, the frontal sinuses
are so large and prominent as to affect the results given
by the facial angle very materially. In other orders,
again, for instance the Rodentia and the morse, the nose
occupies so much space that the cranium is inclined
backwards without its walls being free before; and it is
impossible to know where the facial line passes. These,
therefore, must be measured by the inner surface. Lastly,