THE NERVOUS SYSTEM 483 



tured the middle lobe, or vermis, with a scalpel. The animal at 

 once shook his head, but there were no convulsions. After a 

 second and third puncture the gait became staggering and the 

 limbs splayed to preserve the equilibrium. When the animal 

 walked, the body was balanced alternately to the right and left, 

 as if at each step he expected to fall. He then showed a marked 

 tendency to lean forward in the attitude of draught, as if deter- 

 mined to negotiate a stiff hill ; it was only with difficulty that 

 seven or eight assistants held him back. 



The cerebellum has been regarded as the centre of muscle 

 sense — i.e., as the centre by which the position of the body and its 

 movements are recognised in the absence of visual and tactual 

 impressions. The necessary impulses are brought to it from 

 the footpads in soft-footed animals, also from the depths of the 

 body — i.e., the muscles, tendons, ligaments, joints (pp. 458,470) 

 — and, as we shall presently see, from the internal ear. Loss 

 of sensation in the soles of the feet in man leads to disturbances 

 of equilibrium ; in animals this does not appear to be present. 

 A cat may have its feet desensitised by division of all sensory 

 nerves with no evident interference with locomotion, and in the 

 solid-footed horse robbing the part of sensation gives similar 

 results. 



The question of maintaining the body equilibrium, a function 

 markedly lost as the result of injury or disease of the cerebellum, 

 (cerebellar ataxia) , is explained by the connection which exists 

 between the cerebellum and the internal ear. This connection 

 is the most important path by which afferent impressions 

 associated with the mechanism of equilibration are con- 

 veyed. The nerve from the semicircular canals enters the 

 pons and connects with the nucleus of Deiters, which in turn 

 connects with the cerebellum. Some fibres may even pass 

 direct to the cerebellum. Through the nucleus of Deiters the 

 vestibular nerve is brought into indirect communication with a 

 nucleus which gives rise to the nerves supplying the muscles of 

 the eyeball, and by means of the vestibulo-spinal tract it estab- 

 lishes connection with the motor cells in the spinal cord. The 

 influence of the semicircular canals on equilibration will receive 

 consideration with the organs of special sense, but it is due to 

 afferent impressions received by the cerebellum from the semi- 

 circular canals that the cerebellum is enabled to judge of the 

 position of the body in space. Other afferent impressions 

 arising in the muscles and joints convey to the cerebellum 

 impulses which maintain the muscles in a condition of reflex 

 tonus, enable the attitude of the body to be maintained during 

 rest, and co-ordinated movement to be executed during loco- 

 motion. It will be remembered that this explanation of the 

 action of the cerebellum is due to the researches of Sherrington. 



