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/lit29465/239/
RADIO-ULNAR 
but it is to the latter only that our attention 
is at present directed. 
The sigmoid cavity of the ulna— is a depres¬ 
sion situated on the outer side of its upper ex¬ 
tremity, and, in respect of its position, it 
might be expressed as an articular facet seated 
on the external margin of the coronoid process. 
In shape it is somewhat quadrilateral ; and is 
concave in both directions, but most so in the 
anterio-posterior, which corresponds to the 
convexity of the head of the radius, and is 
also much the longest surface of the two. 
With trifling individual variations, it usually 
forms about the fourth of a circle. Superiorly, 
it is separated from the greater sigmoid cavity 
by a smooth elevation directed from before 
backwards : anteriorly, inferiorly, and pos¬ 
teriorly, the border of this articular surface 
overhangs the coronoid process of the ulna, 
the concave upper part of its anterior surface, 
and its posterior surface respectively. The 
junction of the two latter sides of its margin 
is marked by a strong ridge, which commences 
the external border of the bone : and, fre¬ 
quently the antero-inferior angle gives off a 
similar prominence ; which, after a short 
course downwards, converges to join the 
preceding. 
Articular cartilage covers these surfaces of 
the radius and ulna. 
The annular or orbicular ligament — is the 
next constituent, and is a strong and some¬ 
what cord-like band of white fibrous tissue, 
which completes the remaining three-fourths 
of the articular circle left unaccounted for by 
bone. Its width is about one third ot an inch, 
its direction is horizontal like that of the sig¬ 
moid cavity. It arises behind from the poste¬ 
rior margin of this surface, and partly from its 
inferior border, uniting beyond these with the 
periosteum covering the surfaces of bone over¬ 
hung by them. In front, it is inserted into 
the anterior margin in a similar manner. 
Above, it receives and is continuous with the 
anterior ligament of the elbow joint ; far¬ 
ther outward, it is also joined by the external 
lateral ligament of the same articulation. Its 
lower border is free around the neck of the 
radius. 
The synovial membrane is a process sent off 
from that which lines the articular surfaces of 
the elbow joint. A cul-de-sac passes down¬ 
wards into the lesser sigmoid cavity, extending 
to its inferior extremity, but around the neck 
of the radius, and between it and the orbicular 
ligament, the remainder of this circular pouch 
has a diminished vertical extent ; sufficient, 
however, to allow it to pass under the or¬ 
bicular ligament, and appear from beneath its 
lower border. 
The movement of the head of the radius at 
this articulation is one of simple rotation 
around its own axis ; since the articular sur¬ 
faces in contact with it together form a circle, 
in which its only movement can be a revo¬ 
lution. And, as above stated, about three- 
fourths of this circle is formed by ligament; 
the remainder by bone. But in addition to 
this chief provision for the limitation and di- 
ARTICUL ATION S. 229 
rection of motion, the convex radial tuberosity 
of the humerus forms a kind of pivot, which 
is received into the cavity which occupies the 
upper surface of the radius, and, no doubt, 
steadies and assists the movement by tending 
still more to define the axis of this part of the 
bone. The articulation of the atlas with the 
odontoid process of the axis, offers many 
analogies to this of the radius and ulna both 
in the structure of the joint and in the re¬ 
sulting movements. 
(2.) The lower radio-ulnar articulation— is, 
in many respects, the reverse of the preceding ; 
since instead of presenting a cylindrical ex¬ 
tremity of the radius revolving within a con¬ 
cave facet of the ulna, the latter bone itself 
offers a rounded termination, on and around 
the outer side of which the radius plays by a 
concave articular surface. The constituents of 
the joint are, the surfaces of the radius and ulna 
just alluded to ; a fibro-cartilage which, with a 
kind of imperfect ligamentous capsule, forms 
the means of union of the bones ; and a syno¬ 
vial membrane interposed between their ar¬ 
ticular surfaces. 
The loiver extremity of the radius — ap 
proaches somewhat to the form called by 
geometricians a parallelopiped. Its largest sur¬ 
faces are the anterior and the posterior : the 
upper is joined and surmounted by the shaft 
of the bone, and the lower enters into the 
formation of the wrist joint. The outer side 
is occupied by the tendons of the muscles 
which extend the thumb : and the inner, 
which looks slightly upwards, articulates with 
the ulna. 
This surface is quadrilateral, and of these 
the two antero-posterior sides are much the 
longest. The upper is nearly straight, the 
lower somewhat concave downwards to adapt 
it to the convex surface of the radio-carpal 
articulation ; and they slightly diverge behind 
so as to make the posterior vertical border 
almost twice as deep as the corresponding 
anterior side. The articular surface itself is 
concave from before backwards, taking a curve 
whose extent is about one fifth of a circle. 
The lower end or head of the ulna — is of 
even smaller size than the upper extremity of 
the radius which was previously described ; 
a condition which is in conformity with its 
slight share in the wrist joint. The base of 
this cylindrical head has a smooth surface 
and is almost circular in shape ; internally 
it offers a depression bounded by the pro¬ 
minent styloid process extending vertically 
downwards; externally, a margin defines its 
separation from the articular facet which oc¬ 
cupies the outer part of the cylinder. 
This convex surface is usually a little longer 
in the horizontal direction than the corre¬ 
sponding radial concavity, forming about a 
fourth of a circle ; but in all other respects it 
is, as it were, moulded to it. Above, its 
margin projects beyond the constricted shaft. 
A layer of articular cartilage covers both 
these surfaces. Ligamentous fibres in sparing 
quantities, and with no very definite direction, 
unite the upper, anterior, and posterior bordei s
        

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