ii MECHANICS OF LOCOMOTOE APPAEATUS 111 



tractions are not necessary for maintaining equilibrium in lying 

 down, that is therefore the position of rest and sleep. We may 

 distinguish between the sternal, sterno-costal, lateral and dorsal 

 postures. This last is almost confined to man, as in no other 

 vertebrate is the back sufficiently flat to support the weight of the 

 body conveniently. 



In the sitting posture, if the trunk is leaning against the back 

 of the seat, all the muscles are in repose, except the elevators of 

 the head which keep it in the vertical position. In fact, in 

 sleeping while seated the head drops forward towards the chest, 

 which shows that the centre of gravity for the head is placed in 

 front of the occipito-atlantoid articulation. 



When seated on a stool with no support for the back, the base 

 of support is represented by the line that connects the outer 

 margins of the sciatic tuberosities and of the feet which rest on 

 the ground. In order to maintain the centre of gravity of the 

 head and trunk within this base, it is necessary to obtain the 

 antero-posterior balance by the alternate activity of the dorsal, 

 the lumbar, and the psoas-iliac muscles. 



In the erect posture, with the two feet set square, the centre 

 of gravity of the body is brought much higher from the base of 

 support, and this base is much smaller; it would therefore be 

 natural to assume that a much greater muscular force would be 

 necessary to preserve equilibrium. Meyer, on the contrary, 

 demonstrated that in the most comfortable erect posture, the 

 muscular activity necessary to preserve equilibrium is small, as this 

 is due principally to the tension of the ligaments, especially the 

 ileo-femoral ligaments. 



As its articulations are mainly synchondroses, the vertebral 

 column may be regarded as an elastic bar, capable of supporting 

 the entire weight of the head, trunk, and upper limbs. It has 

 various curvatures ; it is convex forwards in the cervical and 

 lumbar regions, and concave in the thoracic and sacral regions 

 (Fig. 72). It is wholly immobile in the sacral region owing to the 

 fusion of the vertebrae, but little movable and flexible in the 

 lumbar region, much more mobile and flexible in the dorsal 

 part, and in the cervical region it is remarkably flexible in all 

 directions. The neck muscles fix the head, and therefore make 

 the cervical spine relatively rigid. 



The line of gravity of the trunk and head in the easy (or 

 military) position shown in Fig. 69 falls behind the line of 

 junction of the ileo-femoral articulations. The trunk would 

 primarily fall back, but for the resistance, as Meyer showed, of the 

 strong ligament which runs from the anterior inferior iliac spine 

 to the anterior intertrochanter line of the femur ; the balancing 

 of the trunk on the heads of the femur is chiefly due to the 

 elastic tension of this ileo-femoral ligament, but this is aided by 



