48 ANIMAL PHYSIOLOGY. 



cup; the elbow and the ankle furnish, examples of hinge- 

 joints, permitting angular movement in only one path, that 

 of flexion and extension; and in the articulation of the first 

 and second vertebrae, we have an instance of a pivot-joint, 

 the odontoid process of the axis or second vertebra, being the 

 pivot round which the atlas or first vertebra revolves. 



26. Mechanics of the Skeleton. By the contraction of 

 muscles passing over joints and attached to bones, the parts 

 of the skeleton are thrown into different positions, so that 

 we are enabled to support ourselves in different attitudes and 

 to move about. 



For the support of the body, it is first of all.necessary 

 that the centre of gravity be within the basis of support, 

 whether that consist of one foot or of both; and to accom- 

 plish this the body is instinctively balanced by compensatory 

 deviations of its different parts from the vertical position. 

 The weight of the body in standing falls on the arch of the 

 instep, the piers of which are the heel and the balls of the 

 toes. When the feet are together and the knees straight, 

 both the tibia and the femur are thrown forwards further at 

 their upper ends than their lower, and over-extension of the 

 knee is prevented by the construction of that joint; while 

 the weight transmitted from the vertebral column is 

 received by the haunch bones at a point further back 

 than where these bones articulate by the hip-joints with 

 the thigh bones. The leg is prevented from falling for- 

 wards over the foot at the ankle joint by the action of the 

 muscles of the calf, the soleus and gastrocnemius ; and this 

 is probably the only instance, in the standing posture, of a 

 considerable weight being permanently supported by muscular 

 power. At the hip-joint the weight, being behind, makes 

 tense the ligaments in front; and the muscles passing over 

 the back of that joint, while they may be felt to be rigid 

 during their activity in recovering the body from stooping, 

 are flaccid when the erect position is attained. The vertebral 

 column is balanced by being curved in different directions : 

 in the loins it is thrown forwards; where it supports the 

 chest it is curved well backwards; it turns forwards again in 

 the upper part of the chest and at the root of the neck; and 

 on the top of the column the head maintains its position by 



