MOVEMENTS OF THE HEAD. 185 



postures, but those that we are about to examine 

 will serve to afford examples of the application of 

 our principles of movements, and in particular they 

 will demonstrate how movements may result from, 

 and be modified by, external agencies. In a strong 

 and healthy man the head is held erect, and sym- 

 metrical, unless some central condition or external 

 agent changes the posture. In a strong man the 

 centre of the forehead is in the mid-plane of the 

 body, the antero-posterior and the transverse axes 

 are horizontal, with both ears on the same level : 

 this is a normal posture of quiescence. 



Rotation is a movement always involving asym- 

 metry of action ; flexion and extension are symme- 

 trical movements. 



A slight stimulus, as the sight of an object or 

 the hearing of a sound, may cause rotation. A 

 slight condition of weakness of the nerve-centres is 

 expressed by flexion. 



In the brain condition whose mental action is 

 called " shame," and in the state of mental abstrac- 

 tion, the lessened kinetic function of the brain leads 

 to head-drooping. Rotation, from the pathological 

 conditions, hemiplegia and hemispasm, has been 

 described in chap. vi. p. 105. 



We all know that rotation of the head may result 

 from the sight of an object, or the hearing of a sound. 

 We will inquire first as to the effects of the sight of 

 objects, or the direct rays of a luminous object, in 

 causing rotation of the head. When the sight of 

 an object is followed by the forehead being turned 

 towards it, it is said to have attracted the attention 



