#Surgical Anatomy.#--The strength of the hip-joint depends primarily on its osseous elements--the rounded head of the femur filling the deep socket of the acetabulum, to the bottom of which it is attached through the medium of the ligamentum teres. The edge of the acetabulum is specially strong above and behind, while at its lower margin there is a gap, bridged over by the labrum glenoidale (cotyloid ligament). In relation to fractures of the upper end of the femur, it is to be borne in mind that as the antero-posterior diameter of the neck is less than that of the shaft, and as a considerable portion of the great trochanter lies behind the junction of the neck with the shaft, the greater part of any strain put upon the upper end of the femur is borne by the neck of the bone and not by the trochanter. The head and neck of the femur are nourished chiefly by the thick, vascular periosteum, and through certain strong fibrous bands reflected from the attachment of the capsule--the retinacular or cervical ligaments of Stanley. The integrity of these ligaments plays an important part in determining union in fractures of the neck of the femur, both by keeping the fragments in position and by maintaining the blood-supply to the short fragment. Whether it be true or not that an alteration in the angle of the femoral neck takes place with advancing years, it is generally recognised that this change is of no importance in relation to fractures in this region. The articular capsule of the hip is of exceptional strength. It is attached above to the entire circumference of the acetabulum, and below to the neck of the femur in such a way that while the whole of the anterior and inferior aspects of the neck lies within its attachment, only the inner half of the posterior and superior aspects is intra-capsular. The capsule is augmented by several accessory bands, the most important of which is the _ilio-femoral or [inverted Y]-shaped ligament_ of Bigelow, which passes from the anterior inferior iliac spine to the anterior inter-trochanteric line, its fasciculi being specially thick towards the upper and lower ends of this ridge. The medial limb of this ligament limits extension of the thigh, while the lateral limits eversion and adduction. The weakest part of the capsular ligament lies opposite the lower and back part of the joint. The hip-joint is surrounded by muscles which contribute to its strength, the most important from the surgical point of view being the obturator internus, which plays an important part in certain dislocations, and the ilio-psoas, which influences the attitude of the limb in various lesions in this region. Except in thin subjects, the constituent elements of the hip-joint cannot be palpated through the skin. A line drawn vertically downwards from the middle of Poupart's ligament passes over the centre of the joint, which in adults lies on the same level as the tip of the great trochanter. In children it is somewhat higher.