570 TEXT-BOOK OF PHYSIOLOGY. 



transmission of afferent impulses from the previously mentioned peripheral 

 sense-organs to the cerebellum. Tactile, muscle, visual, and labyrinthine 

 impressions and sensations not only cooperate in the development and or- 

 ganization of the motor adjustments necessary to the maintenance of the 

 equilibrium and locomotive coordination, but even after their organization 

 they are necessary to the excitation of cerebellar activity. The manner in 

 which they lead to the development of this capability on the part of the cere- 

 bellum is conjectural. Their ever-present influence is shown by the effects 

 which follow their removal, as the following facts indicate. 



The prevention of the development of tactile impulses by freezing or 

 anesthetizing the soles of the feet, and the blocking of normally developed 

 impulses through destruction of afferent pathways in diseases of the spinal 

 cord lead at once to make impairment in the coordinating power. The 

 removal of the skin from the hind legs of the frog, previously deprived of its 

 cerebrum, destroys its coordinating power, which it would otherwise possess 

 in a high degree. 



The blocking, in consequence of destructive lesions of the spinal cord, 

 of the impulses, which come from the muscles, tendons, etc., and which inform 

 us of the activity and the degree of activity of our muscles, the location of 

 limbs, the amount of effort necessary to produce a given movement, etc., 

 also gives rise to much incoordination. A blocking of both tactile and 

 muscle impulses frequently exists in degeneration or sclerosis of the posterior 

 columns of the spinal cord. The coordinating power is so much impaired 

 in this disease that the patient is unable to maintain, without strained effort, 

 the erect position and especially if the directive power of the eyes be removed 

 by closure of the lids. Walking becomes extremely difficult; the gait is 

 irregular and jerky, and equilibrium is maintained only by keeping the 

 eyes fixed on the ground in front and by artificially increasing the basis of 

 support by the use of canes. 



An interference with the development of the customary visual impres- 

 sions which in a measure maintain the sense of relation of the individual 

 to surrounding objects also gives rise to equilibratory disturbances. A 

 rapid change in the relation of the individual to surrounding objects or the 

 reverse; a change in the direction of one optic axis from the use of a prism 

 or from paralysis of an eye muscle; the destruction of an eye; these and 

 similar conditions frequently give rise to such marked disturbances of the 

 equilibratory power that displacement is difficult to prevent. 



An interference with the development of the so-called labyrinthine im- 

 pressions by destruction of the semicircular canals gives rise to the most 

 remarkable disturbances in this respect. Section of one horizontal canal 1 

 in the pigeon is followed by oscillations of the head in a horizontal plane 

 around a vertical axis. Bilateral section so increases these oscillations that 

 the pigeon is unable to maintain equilibrium and forced to fall and turn con- 

 tinuously around the vertical axis. Bilateral section of the posterior vertical 

 canals gives rise to oscillations around a horizontal axis which frequently be- 

 come so exaggerated as to eventuate in the turning of backward somersaults, 



1 The physiologic anatomy of the semicircular canals is described in the chapter devoted 

 to the ear, to which the reader is referred. 



