CEREJBRAL FUNCTION IN THE RAT 73 



common elements, in apparently dissimilar mental conditions. 

 We also find that the cerebral structures do not differ widely 

 in fundamentals. Many of the cells have the same general 

 mass and appearance, but in one place may differ in number 

 from those in another location. Their staining qualities are 

 similar, the neurofibrils are much the same, the most marked 

 differences are the direction and the length of the axones and 

 the number of collaterals. Consequently, it should be obvious 

 that great care should be taken that points of minor difference 

 are not made the basis for major deductions. 



At the same time care should be exercised that apparent simi- 

 larities are not accepted as equalities and that a certain condi- 

 tion should not too hastily be assumed to point exclusively to 

 one explanation. But this mistake is one of the most common 

 in dealing with cerebral functions. Von Monakow has empha- 

 sized the distinctions which must be made in certain cases, and 

 has pointed out some of the difficulties of interpretation. In 

 one direction these difficulties are to be appreciated by the care- 

 ful consideration of the temporary and the residual defects fol- 

 lowing a cerebral accident. But there are many more. For 

 example, after the destruction of certain parts of the cerebrum 

 of an animal, if a beam of light is thrown into the eye, or if an 

 object crosses the visual field, and the eyes turn in the proper 

 direction one cannot be certain whether the animal sees or the 

 reaction is a reflex without vision. 



For cases such as that just mentioned we have a means of 

 more exact determination. This is the method of training which 

 can be used both prior and subsequent to any experimental 

 destructions. We may set such conditions to an operated ani- 

 mal which exhibits a reaction to light stimuli that the animal 

 will be able to form a habit provided the cerebral sensory, asso- 

 ciational (if they exist), and motor elements be intact. If a 

 necessary part of the sensory-motor mechanism be out of order 

 such a habit can not be acquired. By varying the end condi- 

 tions, either the sensory or the motor or both, it is possible to 

 deduce the effects of destructions if they be more than tem- 

 porary, and only in this way is it possible to determine accu- 



