HIP-JOINT. 285 
bone through its articular auricular surfaces, and in this position it is suspended by the posterior 
sacro-iliac hgaments, and kept securely in place by the “grip” due to the irregularity of the 
opposed surfaces of the two sacro-iliac articulations. Since the weight of the trunk is trans- 
mitted to the anterior and upper end of this sacral platform, there is a natural tendency for the 
sacrum to revolve upon the transverse axis which passes through its sacro-iliac joints. If this 
were permitted, the promontory of the sacrum would descend downwards and forwards towards 
the pelvic cavity, as really does occur in certain deformities. This revolution or tilting up- 
wards of the forepart of the sacrum is prevented by the action of the great and small sacro- 
sciatic ligaments, extending from the ischial tuberosity to the hinder and lower end of the 
suspended oe of the sacrum. Not only so, but these ligaments, acting on a rigid sacrum, 
tend to hold up the weight upon the sacral promontory. 
The various ligaments passing between the last lumbar vertebra and the sacrum and ilium 
retain the weight of the trunk in position upon the anterior end of the sacrum, and resist its 
tendency to shp forwards and downwards towards the pelvic cavity. The entire weight of the 
trunk and pelvis is transmitted to the heads of the thigh bones in the most advantageous 
position, both for effectiveness and the strengthening of the inverted innominate arch, for it will 
be evident that the heads of the femora thrust inwards upon the convex side of the arch, very 
much at the place where the arches are weakest, viz. at the springing of the arch from its piers. 
The forces which tend to cause movement of the pelvic bones during parturition act from within 
the pelvis, and have for their object the increase of the various pelvic diameters, in order that 
the foetal head may more readily be transmitted. For this purpose the wedge-like dorsal surface 
of the sacrum is driven backwards, and a certain amount of extra space may thereby be obtained. 
An important factor, however, in the increase of the pelvic capacity at this period is found in 
the relaxation of its various ligaments. 
THE ARTICULATIONS OF THE LOWER EXTREMITY. 
THE HIP-JOINT. 
The human body provides no more perfect example of an enarthrodial diarthrosis 
than the hip-joint (articulatio coxe). Combined with all that variety of movement 
which characterises a multi-axial joint, it nevertheless presents great stability, which 
has been obtained by simple arrangements, for restricting the range of its natural 
movements. This stability 
is of paramount importance 
for the maintenance of the 
erect attitude, and the me- 
chanical adaptations whereby 
this result is obtained are + 
such that the erect attitude 
may be preserved without 
any great degree of sustained \ 
muscular effort. 
Articular Surfaces.—The 
head of the femur is globular 
in shape, and considerably 
exceeds a hemisphere. It is 
clothed by hyaline articular 
cartilage on those parts which 
come into direct contact with 
the acetabulum. There is 
frequently more or less of 
extension of the articular 
cartilage from the head to ‘Transverse acetabular ligament 
the adjoining anterior part Retinacula 
of the neck, an extension 
which is accounted for by 
the close and constant apposi- 
tion of this portion of the 
neck with the hinder aspect ae 
of the ilio-femoral ligament. Bottom of the acetabulum removed, and capsule of the joint thrown 
debi ; outwards towards the trochanters. 
The limit of the articular 
cartilage covering the head is indicated by a sinuous border. Further, there is an 
absence of articular cartilage from the pit or depression on the head of the femur. 
~[schial spine 
Cotyloid ligament 
- Ligamentum teres 
Capsular ligament (reflected) 
Fic. 221.—DIssEcTION OF THE HIp-JOINT. 
