10 A. C. IVY 



normal (32° to 35°C.) and it was very depressed, being comatose 

 forty-eight hours previous to death. The quick component was 

 absent during the last forty-eight hours. Autopsy revealed the 

 thalamus to have undergone complete malacia. At the time 

 observations were being made on this dog I was not aware of the 

 effect of body temperature upon the quick component of nystag- 

 mus, and hence did not study the effect of raising the body tem- 

 perature as was done in the case of the rabbits reported above. 



A clean ablation of the occipital cortex in the dog does not alter 

 vestibular nystagmus as judged by the results from such a pro- 

 cedure in three dogs. 



DISCUSSION 



Tozer and Sherrington ('10) have demonstrated histologically 

 and physiologically the presence of sensory tendon nerves in the 

 extrinsic eye muscles which pass back to the midbrain via the 

 Illrd, IVth, and Vlth nerves. Wilson and Pike ('15) suggest 

 that afferent impulses from these tendon nerves "set up efferent 

 impulses in the oculomotor cells of the cerebrum, which result in 

 a quick, jerky contraction of the internal rectus on the side of the 

 slow deviation and of the external rectus of the opposite side, with 

 relaxation of the antagonistic muscles," which effects a restora- 

 tion of the eyes to the primary position. In other words, these 

 latter investigators are of the opinion that the quick component 

 is dependent upon the presence of the neopallium or upon the 

 integrity of a cerebral reflex arc. The presence of true vestib- 

 ular nystagmus in the frog, pigeon, and turtle questions this 

 idea from the viewpoint of comparative anatomy. The persist- 

 ence of true vestibular nystagmus following the removal of the 

 cerebral hemispheres in these forms) which is a very simple mat- 

 ter and causes no great physiological disturbance — questions this 

 idea physiologically. The observation that no disturbance of 

 nystagmus follows decerebration in these forms, while in the 

 higher forms (rabbit, cat, dog) there is a change in the number 

 and (limit ion of the nystagmic movements, shows that some con- 

 stituenl is absent in the cerebrum of the frog, turtle, and pigeon 

 which is present in the cerebrum of the rabbit, cat, and dog. 



