Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/950/
TEMPORO-MAXILLARY ARTICULATION. 
940 
manner of trace to indicate where the joint 
once was. In fact this anchylosis had existed 
for fifty years before the death of the patient. 
No record is preserved of the cause of it. 
The joint of one side only is anchylosed, but 
that of the other side is much altered in form. 
An abnormal tubercle of bone projects down¬ 
wards from the middle of the glenoid cavity 
and is received into an abnormal excavation 
or alveolus in the middle of the condyle. The 
lower jaw is much wasted in size, and has lost 
all its teeth save the two front incisors. The 
upper jaw bones are thin and light. 
The motion of the lower jaw is often lost 
owing to an affection not immediately con¬ 
nected with the joint itself. When, as often 
happens in scarlatina, cancrum oris, &c. there 
is extensive sloughing of the inside of the 
cheeks, the cicatrices resulting from the heal¬ 
ing of the great wounds contract, and form 
bands, extending from the upper to the lower 
jaw, so strong and unyielding that the muscles 
which open the mouth are unable to antago¬ 
nise them. 
Comparative Anatomy.—If a palaeonto¬ 
logist were asked what fragment of a verte¬ 
brate skeleton, speaking generally for all ver- 
tebrata, would give him most information as 
to the status and affinities of the animal to 
which it belonged, he would most probably 
answer — the articular portion of the lower 
jaw or the articulation that receives it. Of 
the convex and concave surfaces which go to 
form this articulation, in all the mammalia 
the convexity is on the inferior maxilla, and 
the concavity on the squamosal bone, whilst 
in the three other vertebrate classes the re¬ 
verse is invariably the case — the concavity is 
on the inferior maxilla, the convexity on the 
bone that articulates with it. 
The under jaw does not articulate with the 
same, or to speak more accurately, with the 
homologous bone in all vertebrate animals. 
In all the mammalia it articulates, as in man, 
with the squamous element of the temporal 
— the squamosal bone. In birds, reptiles, and 
osseous fishes it articulates with bones which 
are clearly the special homologues of the tym¬ 
panic ring of the human subject. In cartila¬ 
ginous fishes its articulation is with the ptery¬ 
goid bone, the homologue of the human 
internal pterygoid plate. The Lepidosiren, 
in which so many other characters of the 
osseous and cartilaginous fishes are so curi¬ 
ously blended together, in strict accord with 
this circumstance, presents an instance of the 
pterygoid and tympanic bones contributing 
each a part — the former the inner, the latter 
the outer part, of the articular surface for the 
reception of the lower jaw.* 
It is well worth while to stop here and 
review what is stated in the two preceding 
paragraphs. What is said is, really, this ; — 
every animal that sucklesitsyoung has a convex 
articular surface to its lower jaw, whilst every 
vertebrate that lays eggs has a concave sur¬ 
face. Or this—every vertebrate animal that 
* Owen. 
has hair upon it, that has a diaphragm, or an 
epiglottis, has a convex articular surface to 
its lower maxilla, whilst all vertebrates that 
are destitute of these have a concave surface. 
Or, again, all animals that suckle their young, 
and have diaphragms, hair, and epiglottides, 
present their squamosal bones for the articu¬ 
lation of their inferior maxillae, whilst all in 
which the possession of these characters is 
negatived present for this articulation their 
tympanic, or, rarely, their pterygoid bones. 
Can any physiological reason be assigned for 
this ? Can any final purpose, holding good in 
all, or in the majority of, instances, be shown 
to be served by this difference ? I think none 
can. One cannot conceive but that it is a 
matter of perfect indifference whether the 
convexity is on this bone or that. Let us look 
once more to the facts. The bat that flies, 
but not the swallow, the whale that swims 
but not the cod-fish, the camel that walks the 
desert, but not the ostrich, the carnivorous 
lion, seal, and weasel, but not the eagle, 
penguin, crocodile, and shark, have convex 
articulations to their lower jaw and present 
to them their squamosal bones. Here then 
is a caveat for the physiologist. A character 
found in an animal may have no physiological 
signification,—no relation to external circum¬ 
stances, nor even a functional connexion with, 
or dependence on other characters wherewith 
it coexists, perhaps invariably. It may be due 
to the status only of the animal. Physiolo¬ 
gically independent it may exist in an animal 
only because other independent characters 
co-exist. It may be a Syneilogy, not a Teleo- 
loSV- 
That certain independent characters in¬ 
variably go together, which was so elaborately 
illustrated by Cuvier, is a fact of a high order, 
perhaps the twilight of some great truth. If 
future investigations should prove that truth 
to be progressive developement, towards which 
hypothesis the inquirer is, even now, tempted 
by so many striking facts, as well as by the 
admirable use that can be made of it as a 
scaffold theory, then we should say, and as 
making use of a scaffolding we may say it now, 
that certain characters are attained to at a cer¬ 
tain stage in the chain of development, and, 
therefore, those are found coexisting which are 
proper to the degree of development to which 
the animal has arrived. Such characters I 
have been accustomed to call Syneilogies *, a 
word which at all events has the merit of re¬ 
ferring only to a well known fact, without in¬ 
volving any hypothesis. To the palaeontologist 
this “ correlation of independent characters ” f 
is, of course, invaluable, and for the purpose of 
arranging natural groups in the animal kingdom, 
these, so to speak, useless, or Syneilogical, cha¬ 
racters are immeasurably more valuable than 
those modifications to meet special exigencies 
which are called teleologies. 
Mammalia. — In all mammalia, except man, 
the articular surface on the squamosal bone is 
bounded posteriorly, or, in the rodents, inter- 
* rvv, \6yt>t. 
■j- Cuvier.
        

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