NEURO-MUSCULAR MECHANISM 149 



part in the labyrinthine mechanism. As the head is moved 

 the pressure of the otolith will change from one part of the 

 macula to another, and may thus give different stimuli for 

 different positions of the head. 



The importance of this labyrinthine mechanism in muscular 

 co-ordination is shown by the effect of its destruction. This 

 leads to inco-ordinate movements of the eyes, head, and limbs, 

 and to loss of tone in the muscles. When injury to the 

 labyrinth is sudden, as when a " knock-out " blow is received 

 on the chin, driving the condyles of the lower jaw against 

 the petrous part of the temporal, the general loss of muscular 

 tone may be so complete that the individual collapses. The 

 muscles chiefly under the tonic influence of this mechanism 

 are those of the neck and trunk, and the extensor and abductor 

 muscles of the limbs of the same side. 



Just as the eye dominates the movements of the muscles of 

 the body through the cerebrum (see p. 142), so the labyrinthine 

 mechanism dominates the tonus of the muscles as a whole through 

 the cerebellum (see p. 177). 



5. THE CONNECTIONS BETWEEN THE RECEIVING AND 

 REACTING MECHANISMS 



1. THE SPINAL NERVES 



The connections between the peripheral receiving mechan- 

 isms and the central nervous system, the activity of which 

 leads to the appropriate reaction of muscles, have been in part 

 studied in connection with the various special senses. 



The main connecting channels between the peripheral receiv- 

 ing organs and the central nervous system on the one hand, 

 and the central nervous system and the reacting structures on 

 the other, may now be considered more generally. 



These connections are seen in their most typical arrange- 

 ment in the spinal nerves, a pair of which comes off, one on each 

 side, from each level of the spinal cord, and passes outwards 

 between the vertebrae. 



General Arrangement 



These nerves may be classified as ingoing and outgoing, 

 and they may be divided into those connected with the 



