938 THE CENTRAL NERVOUS SYSTEM 



This is composed of two phases, a slow movement towards the injiircd 

 side and a quick jerk back towards the uninjured side. The nystag- 

 mus is synchronous in both eyes, and is constantly present for" some 

 days after the operation. There is reason to believe that the slow 

 deviation is due to excitation arising in the labyrinth and conveyed 

 through the vestibular nuclei to the oculomotor nuclei by way of the 

 posterior longitudinal bundle. The slow movement can still be ob- 

 tained after removal of the cerebral cortex. The quick return jerk is 

 different in its origin, since it is abolished by the removal of the cere- 

 bral hemisphere on the side to which the eyes are moved in the slow 

 deviation i.e., the side of the cerebrum from which the efferent im- 

 pulses concerned in pulling the eyes back to the primary position of 

 equilibrium arise. 



In speaking of the postural reflexes it was stated that evidence 

 had been obtained of the widespread influence of the labyrinth on 

 the tonus of the skeletal muscles. The effects on the postural tonus 

 of the limb muscles were separated into two components, one due 

 to the labyrinth directly, and the other to impulses passing along 

 the afferent nerves of the neck muscles whose postural tonus is 

 itself affected from the labyrinth. These phenomena have been 

 demonstrated especially by Magnus and his pupils, in mammals 

 like the rabbit, cat, and dog, and to a certain extent in man. They 

 studied the effects of excitation of the labyrinth as well as the 

 effects of its extirpation. To excite the labyrinth they employed 

 such ' adequate ' stimuli as are generated when the position of the 

 head is altered with reference to the vertical. Without going into the 

 details of this elaborate work, it may be said that it has placed in 

 a clear light the manner in which the posture of the head and of 

 the neck reacts upon the postural contractions of the limbs. The 

 strange attitudes so often seen after injury or disease of the laby- 

 rinth are to a great extent the consequences of the abnormal 

 postures assumed by the neck.' 



In the frog, in which no direct influence of destruction of the labyrinth 

 on the tonus of the extremities has been established, the effects of the 

 altered neck posture on the postural tonus of the limbs have also been 

 clearly demonstrated. On the injured side the extremities, especially 

 the anterior limb, are flexed and adducted, the extremities of the other 

 side extended and abducted; the head and vertebral column are ro- 

 tated to the injured side (Ewald). All that is necessary to abolish the 

 difference in the posture of the limbs on the two sides is to divide the 

 posterior roots of the spinal nerves supplying the neck muscles 

 (Fig. 377). 



The effects of destructive lesions of the labyrinth have their 

 counterpart in the phenomena caused by stimulation ; excitation of a 

 posterior canal, for example, in the pigeon causes movements of the 

 head from side to side. 



Lee's results in fishes are, on the whole, of similar tenor. Mechan- 

 ical stimulation of the ampullae in the dogfish, by pressing on them 

 with a blunt needle, calls forth characteristic movements of the 



