THE BONY PELVIS. 



493 



the arch toward the ends, equilibrium becomes more unstable until, when it passes 

 beyond them, it is lost and the body begins to fall. In maintaining a normal erect 

 posture hyperextension of the hip-joint is prevented by the anterior or iliofemoral 

 ligament; hyperextension of the knee is prevented by the lateral, posterior, and 

 crucial ligaments (Fig. 494, A}. The main muscular efforts required are those of the 

 muscles of the back of the neck to hold the head level, owing to the head being 

 heavier anterior to the condyles, and the muscles of the back of the leg to prevent 

 the dorsal flexion at the ankle, due to the centre of gravity falling in front of the ankle- 

 joint. When a person falls asleep in the erect posture the head drops forward and 

 when a soldier is shot his calf muscles give way and he falls forward on his face. 



Deformities of the spine affecting its curves have already been alluded to (page 

 478). When the spine is the part affected it is usually the case that the secondary 

 curve fully compensates for the increased primary one, hence there is no necessity 



The tendino- and 



iliotrochanteric 



bands 



Iliotibial band 



A. 



B. C. 



FIG. 495. Lateral equilibrium. 



Fig. A. The body being erect, a vertical line a-b through the centre of gravity c falls midway between the 

 ankles or base of support d-e and the body is in stable equilibrium. 



Fig. B. The trunk being inclined to the right, the centre of gravity c is shifted to the right and a vertical 

 line a-b through it falls still within the line of support d-e and the upright position can still be maintained. 



Fig. C. If the relative length of the two legs is altered, as by placing a block beneath one of them, the pelvis 

 and upper portion of the body inclines to the opposite side, until a vertical line a-b through the centre of gravity 

 C falls beyond the extremity of the base of support d-e and the body is in a position of unstable equilibrium. 



Fig. D. The body in a position of rest. The weight is borne mainly on the left leg; the right side of the 

 pelvis falls until the iliotrochanteric and iliotibial bands are tense, when the position can be maintained without 

 muscular effort. 



for any change in the position of the joints below, and we find people with marked 

 deformities of the spine who are normal from the waist down and who stand and 

 walk perfectly well. Occasionally a case presents itself in which the secondary curve 

 has not entirely compensated the primary one and then the body is bent at the hips 

 until the centre of gravity is brought over the base of support (Fig. 494, B}. If the 

 deformity throws the centre of gravity too far back, by bending the hips it will be 

 brought forward, but if for any reason, such as ankylosis, flexion is impossible, 

 then it cannot be corrected at the hip-joint, and therefore in such cases equilib- 

 rium is unstable and the body falls (Fig. 494, C). If from deformity the centre of 

 gravity is thrown so far forward as to fall beyond the base of support then a cane 

 or crutches is required (Fig. 494, D}. 



When the hip-joint is involved it is never affected by hyperextension (the ilio- 

 femoral ligament prevents that), but always by flexion. This throws the centre of 

 gravity forward; to bring it back a secondary curve is produced in the lumbar 

 region, and we have a condition of lordosis established; if this is insufficient then 



