Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29464/378/
370 
MONOTREMATA. 
by a squamous modification of the coronal 
suture. The part described in this Article 
as a lamelliform portion of the petrous bone, 
(fig. 169, e,) which extends upon the lateral 
and part of the posterior region of the skull, is 
regarded by the Editors* of the Leçons d’Ana- 
tomie Comparée, Ed. 1837, as the squamous 
portion of the temporal ; and the flat oblong 
bone, (fig. 169, B,) which forms part of the la¬ 
teral wall of the cranial cavity and the posterior 
half of the zygomatic arch, and which supports 
the articular surface for the lower jaw, is thought 
to be the malar bone. But when we consider 
the low development or total disappearance of 
the malar bone in the skull of the Insectivora 
generally, as in Echinops and Centetes among 
the Feræ, and as in the edentate Manis and 
Myrmecophaga, it is unlikely that the malar 
bone should attain so superior a size and fulfil 
such important functions in the Monotrema- 
tous Edentata, in which its condition, according 
to the above views of the editors of the Leçons 
d’Anat. Comparée, would be unique in the 
mammiferous class. It appears to me to be 
more reasonable to regard the malar bone as 
either altogether absent in the Echidna, as it is 
in the Manis, and the zygomatic arch as being 
completed in the Echidna by a greater exten¬ 
sion of the zygomatic processes of the tem¬ 
poral and superior maxillary bones; or else to 
suppose that these are actually united, at an 
earlier period, by a separate intervening jugal 
style, which, however, I have not been more 
successful in finding than the Continuators of 
Cuvier. With respect to the great develop¬ 
ment which the petrous bone, according to my 
view, must present in the Echidna, it may be 
observed that this bone forms part of the occi¬ 
pital region of the skull in most Marsupials,- 
and also contributes as large a proportion to 
the lateral parietes of the skull in certain Ro¬ 
dents, as the Helamys, as it is here described 
to do in the Echidna. 
The side of the cranium is principally formed 
by the largely developed petrous bone (fig. 170, 
e) and the great ala of the sphenoid, which meet 
and are joined in the interspace separating the 
parietal from the squamo-temporal bone, by a 
nearly vertical harmonia half an inch long; 
this harmonia is crossed at nearly right angles 
by a deep groove, which in some parts sinks 
through the wall of the cranium and exposes 
its cavity. The groove or canal first runs be¬ 
tween the squamous and petrous elements of 
the temporal bone, and is a conspicuous fea¬ 
ture in the skull of the Ornithorhynchus. 
The lower part of the side of the skull of the 
Echidna is closed by the squamous element ofi 
the temporal, which overlaps a large portion 
of the petrous bone, and by a small portion of 
the sphenoid : it is represented detached from 
the skull at fig. 169, B. The lower boundary 
of the squamo-temporal forms a straight line, 
from which the glenoid surface for the lower 
jaw (f) is extended inwards at a right angle, 
upon'the base of the skull; the anterior part 
* The very able anatomists, MM. Laurillard and 
Duvernoy. 
is continued forwards, protecting the temporal 
fossa by a thin vertical plate of bone, and then 
diminishes to a slender, straight, styliform, 
zygomatic process which rests obliquely on a 
corresponding process of the superior maxil¬ 
lary bone. 
The tympanic cavity is shallow, and exca¬ 
vated in the basal part of the petrous bone, 
where it is widely open in the macerated skull : 
it is figured closed by the tympanic bone and 
membrane at g,fig-170, and exposed by their 
removal at e" fig. 170. The plane of the mem- 
brana tympani is horizontal, and its exter¬ 
nal surface looks nearly downwards. Three- 
fourths of its circumference are implanted in 
the groove of the very slender incomplete hoop 
formed by the detached tympanic bone, which 
is figured with the anchylosed malleus at c,fig. 
169. The petrous bone is continued from the 
tympanic fossa forwards and inwards, in the 
form of a broad and nearly horizontal process, 
(fig. 170, el,) to the pterygoid plate of the sphe¬ 
noid, (it, i',) which is also horizontal. The pe¬ 
trous and pterygoid plates are joined by an 
oblique harmonia, and both contribute to extend 
the bony palate backwards. The palatal pro¬ 
cess of the petrous bone is abruptly terminated 
behind by the Eustachian groove (fig. 170*). 
The frontal bone (fig. 169, h) in the cranium 
here described was divided by a median frontal 
suture, toothless like the rest ; the angle be¬ 
tween the superior and the orbital plates is 
rounded off; the orbital plate joins the great 
ala of the sphenoid by a deeply sinuous suture. 
The anterior part of the frontal is largely over¬ 
lapped by the bases of the nasal bones, which 
encroach upon the interorbital space. 
The nasal bones (fig. 169, n) receive the upper 
edge of the superior maxillary bone into a groove 
at their outer margin, and articulate anteriorly 
with the intermaxillaries (fig. 169, o); but these 
meet above the nasal canal in front of the nasal 
bones for an extent of about three lines, and 
thus exclusively form the boundary of the single, 
oval, and terminal external nostril. The lower 
or palatal process of the intermaxillary extends 
backwards in the form of a long and slender 
pointed process which is wedged into a fissure 
of the superior maxillary bone. 
The anterior palatal or incisive foramen is a 
single large elongated fissure extending from 
the narrow anterior symphysis of the inter¬ 
maxillaries backwards, for some distance, be¬ 
tween the palatal processes of the maxillaries. 
At the back part of the bony palate a narrow 
fissure extends forwards between the pterygoid 
bones, and the intermediate extent of the 
bony palate is entire, or presents only a few 
minute perforations. The palatal bones, if 
originally distinct, soon become confluent with 
the maxillaries. There was no separate osseous 
style representing the malar bone between the 
zygomatic processes of the maxillary and tem¬ 
poral bones in the skull here described* 
The zygomatic process of the superior max- 
* Cuvier says, “ Entre ces deux apophyses est 
un très-petit filet qui represent le jugal.” Cuvier, 
Ossem. Foss. 4to., vol. v,, p. 145.
        

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