THE BRAIN 241 



diseases. The centripetal nerves which acquaint these 

 centres with the position of the body are : 



(a) The sensory nerves of the entire body which end in 

 the muscles, tendons, and joints and which notify the centre 

 of the relative position of individual members to each other 

 and of the extent of the tension of the muscles. 



(b) The optic nerve, which, by means of visual perception, 

 acquaints the centre with the position of the body with refer- 

 ence to the objects of the external world. 



(c) Certain fibres of the auditory nerve which end in the 

 semicircular canals of the internal ear. These semicircular 

 canals are sense organs for ascertaining the position and 

 movements of the head. 



In the execution of compensatory movements all the 

 skeletal muscles take part. 



As to the position of the centres, it is supposed that the 

 coordinated movements of the lower extremities which chiefly 

 function in locomotion and standing are governed by the 

 cerebellum. The centres in the corpora quadrigemina are 

 supposed to regulate chiefly the movements of the arms and 

 hands. 



Nothing is definitely known in detail concerning the posi- 

 tion and limits of the centres. This is not surprising when 

 it is borne in mind that in these centres the greater part of 

 all the sensory and motor nerves are connected. 



If, because of pathological changes and disturbances, interrup- 

 tions in the connections between the afferent and efferent nerves 

 for these centres or in the centres themselves take place, a disturb- 

 ance in the coordinated movements of the body results. Hence 

 in locomotor ataxia, in which the sensory nerves of the lower limbs 

 are paralyzed, uncoordinated movements are made during walking. 

 A person suffering with locomotor ataxia cannot stand erect when, 

 by closing his eyes, he deprives himself of the only remaining 

 means of orientating himself. 



It is also possible that each centre or its tract on one side only may 

 be paralyzed or abnormally stimulated by disease. The result is 

 that the strength of the stimulation which is unconsciously imparted 

 to the muscles is not equal on both sides. This results in abnormal 

 positions and movements of the body, called forced position and 

 forced movements because they are called forth involuntarily, indeed 



