NERVE 121 



3. Physiology. 



A. Lihyrintho- Spinal and Cerebellar Arcs. — This 

 mechanism acts in two ways : — 



1 St. As a great tonic dominator of the muscular system. 



2nd. As a great proprioceptive reflex adjuster of move- 

 ments. 



B. Labyrintho- Cerebral Arc. — This acts by modifying 

 consciousness. 



A. Labyrintho-Spinal-Cerebellar. — 1. Tonic Action.— One 

 effect of removal of the labyrinth on one side is to cause 

 a loss of tone in the muscles of the same side. If 

 the labyrinths on both sides are removed, a general loss 

 of tone occurs. The result of this is that a very small 

 force is capable of preventing the muscles from adjust- 

 ing the position of the head. In a pigeon deprived of 

 its labyrinths, if a small weight is attached to the head and 

 the head is bent over the back, it remains in this abnormal 

 position. It is the cerebellar arc which is involved in the 

 maintenance of tonus, and after removal of the deep nuclei 

 of the cerebellum and the paracerebellar nuclei the tonus is 

 lost. The cerebellar arc acts upon the spinal arcs (a,) 

 through its connections with the red nucleus (p. 127), under 

 the tectum ; (6) probably directly through descending fibres 

 of the vestibulo-spinal tract. If the spinal arcs are interfered 

 with by section of a series of posterior roots, the influence of 

 the labyrintho-cerebellar arc is lost. 



The importance of this tonic action is illustrated by the 

 pugilist's "knock-out" blow on the chin, which drives the 

 condyles of the lower jaw against the petrous part of the 

 temporal bone which contains the labyrinths, and throws 

 them out of action. Instantly there is a complete loss of 

 muscular tone, and the man falls in a heap on the ground. 



The cerebral arc is not involved. In fact, an increased 

 tonus appears after removal of the cerebrum. 



2. Proprioceptive Reflex Adjustment of Movements. — (1) 

 This action of the labyrinth is shown by the effects of injury 

 and removal. When one of the canals is opened, an 

 operation which can be performed in the pigeon with 



