PHYSIOLOGY OF MOVEMENT. 733 



being slightly flexed on the femora, the axis of the joint lies a little 

 behind the line of gravity, and the inclination thus produced to fall 

 forward is prevented by the glutei muscles, which are likewise concerned 

 in regaining the erect posture after bending the trunk forward. The 

 motions between the pelvis and vertebrae are practical^ so slight as to be 

 disregarded, and the vertebral column, with the exception of the motions 

 existing between the head and the upper cervical vertebrae, may be re- 

 garded as a rigid column. Between the occiput and atlas lateral and 

 rotatory motions are "possible to a considerable extent, so that balancing 

 the head is rendered possible only by co-ordinated muscular contractions, 

 since no ligaments are present which can fix the occipito-atlantoid artic- 

 ulation. 



Sitting is that position of equilibrium where the body is supported 

 on the tubera ischii. The line of gravity may pass either in front of the 

 tubera ischii, in which the body must be supported by some fixed object, 

 or the line of gravity may fall behind the tubera in the backward pos- 

 ture, in which case falling backward may be prevented by leaning upon 

 a support or by the counter-weight of the extended legs. In sitting 

 erect the line of gravity falls between the tubera themselves, and but 

 slight muscular action, such as is required in the balancing of the head, 

 is sufficient to maintain equilibrium. 



In quadrupeds the four limbs act like four columns, as in a chair or 

 table, in supporting the centre of gravity of the body, so that the base 

 of support is a parallelogram whose corners are represented by the point 

 of contact of the feet with the ground, and which is about four times as 

 long as it is broad. In consequence of the greater base of support, equi- 

 librium is more readily preserved in quadrupeds than in bipeds. The 

 centre of gravity in the large quadrupeds, such as the horse, ox, etc., lies 

 at the intersection of a line passing vertically behind the xyphoid carti- 

 lage and one passing horizontally through the end of the second third 

 of the sterno-vertebral diameter. In the small quadrupeds, such as the 

 dog, the centre of gravity is located somewhat more anteriorly. From 

 the fact that the centre of gravity lies below the vertebrae in quadrupeds, 

 in the erect position the tendency of the weight at the centre of gravity 

 is to curve the vertebral column inward. This is prevented by both mus- 

 cular action and ligamentous support. The fore extremities and scapula 

 are only attached to the trunk through muscular and ligamentous con- 

 nections which are continually on a stretch and so serve to render the 

 shoulder-blade immovable, while by means of the greater serrati muscles 

 the trunk is supported as in a sling, so that it cannot be pushed forward 

 against the shoulder-blade. In the posterior extremities the relationships 

 differ in that the single bones are not, as in the fore extremities, verti- 

 cally over each other. Here, also, the erect oosition is maintained through 



