288 THE ARTICULATIONS OF JONES: 
of the acetabular fossa, as well asa complete tubular Investment for the ligamentum 
teres. 
Occasionally the synovial bursa, which is subjacent to the tendon of the ilio- 
psoas muscle, communicates with the interior of the hip-joint through an opening 
in the anterior wall of the capsule (Fig. 222), situated between the pubo-femoral 
ligament and the inner or lower limb of the ilo-femoral ligament. 
Movements at the Hip-Joint.—The movements which occur at the hip-joint are those of 
amultiaxial joint. These are flexion, extension, abduction, adduction, rotation, and circumduction. 
The range of each of these movements is less extensive than in the case of the shoulder-joint, be- 
cause, at the hip, the freedom of movement is subordinated to that stability which is essential alike 
for the maintenance of the erect attitude and for locomotion. When standing at rest in the erect 
attitude the hip-joint occupies the position of extension, and as the weight of the trunk is trans- 
mitted in a perpendicular which falls behind the centres of the hip-joints, both the erect attitude 
and the extended position are maintained to a large extent mechanically, without sustained mus- 
cular action, by means of the tension of the ilio-femoral ligament. Moreover, the tension of this 
ligament is sustained by the pressure of the front of the head and neck of the femur against its 
synovial surface. In this association of parts it is important to note that the articular cartilage 
of the femoral head may be, and in certain races is, prolonged to the front of the femoral neck ; 
and further, that the constant friction does not destroy the synovial lining of the capsule. 
Again, the same mechanism which preserves the erect attitude prevents an excessive degree of 
extension or dorsiflexion. In movement forwards, 7.e. ventral flexion, the front of the thigh 
is approximated to the anterior abdominal wall. The amount of this movement depends upon 
the position of the knee-joint, because when the latter is flexed the thigh may be brought imto 
contact with the abdominal wall, whereas when the knee-joint is straightened (¢.e. extended) 
the tension of the hamstring muscles greatly restricts the amount of flexion at the hip-joint. 
Abduction and adduction are likewise much more restricted than at the shoulder-joint. Abduc- 
tion is brought to a close by the tension of the pubo-femoral band and the lower part of the 
capsule, and, in addition, the upper aspect of the neck of the femur locks against the margin 
of the acetabulum. Excessive adduction is prevented by the tension of the upper band of the 
ilio-femoral ligament and the upper part of the capsule. Rotation or movement in a longi- 
tudinal axis may be either inwards, 7.e. towards the front, or outwards, ze. toward the back. 
In the former the movement is brought to a close by the tension of the ischio-capsular hgament 
and back part of the capsule, aided by the muscles on the back of the joint; im the latter— 
rotation outwards—the chief restraining factor is the outer or upper limb of the ilio-femoral 
ligament. The total amount of rotation is probably less than 60°. 
Circumduction is only slightly less free than at the shoulder, but it is complicated by the 
preservation of the balance upon one foot. 
The value and influence of the ligamentum teres are not easily estimated, because it may be 
absent without causing any known interference with the usefulness of the joint. In the erect attitude 
this ligament lies lax between the lower part of the femoral head and the acetabular fat. In the 
act of walking it is rendered tense at the moment when the pelvis is balanced on the summit of the 
supporting femur. Analysis of this position shows the femur to be adducted, with probably, in 
addition, a small amount of flexion (¢.e. bending forwards) and internal rotation. Again, this 
ligament is said to be tense when the thigh is rotated outwards. The equivalent of this 
movement is doubtless found in the rotation of the pelvis, which occurs in the act of walking. 
at the moment of transition from the toe of the supporting foot to the heel of the advancing 
foot. The interest connected with this ligament is perhaps morphological rather than physio- 
logical. It is believed by some to represent the tendon of a muscle which in birds occupies a 
position external to the joint capsule. 
THE KNEE JOINT. 
The knee-joint (articulatio genu) is the largest articulation in the body, and 
its structure is of a very elaborate nature. The part it plays in maintaining the 
erect attitude materially influences its construction, and special arrangements are 
provided for the mechanical retention of the joint in the extended position, in view 
of the fact that the line of gravity falls in front of the centre of the articulation. 
Its principal axis of movement is in the transverse direction, consequently it belongs 
to the ginglymus or hinge variety of the diarthroses. At the same time a slight 
amount of rotation of the tibia in its long axis is permitted during extreme flexion ; 
but while this fact is of considerable importance in the study of certain accidents 
to which the joint is liable, as well as in the study of its comparative morphology, it 
is not sufficiently pronounced to interfere with its classification as a hinge-joint. 
Articular surfaces pertaining to the femur, tibia, and patella, enter into the 
formation of the knee-joint. The articular surface of the femur extends over a 
large part of both condyles, and may be divided into patellar and tibial portions — 
