GENERAL SURVEY 601 



some other part, making the interpretation of results very difficult. Moreover 

 this method has to contend with the difficulty of limiting the lesion produced 

 exactly to the region intended. 



In some parts of the brain, as for example the cerebral cortex, it is relatively 

 easy to get clear results with ordinary electrical stimulation, but in others the 

 results are too easily obscured by by-currents and in the deeper parts it is prac- 

 tically impossible to employ the method without a serious operation. 



Comparative physiology has shown very conclusively that the importance of 

 different parts of the brain is very different in different vertebrates. Hence one 

 cannot apply to man the results obtained upon animals without some qualifica- 

 tion ; hence also it is highly important that analogous results should be had upon 

 man himself. The great variety of mental diseases furnish us the necessary 

 material for this purpose, and in many respects the information obtained from 

 them supplements the information we obtain from animal experimentation. 



The weight of the evidence accruing from such material must, however, be 

 estimated with caution and only by observance of certain definite principles. 

 Thus a tumor may be located some place in the brain and all sorts of disturb- 

 ances may appear in both the bodily and the mental functions of the patient. 

 But one is not justified in concluding from this alone that all these disturbances 

 result directly from destruction of the part where the tumor is located, for it 

 may be that the tumor raises the intracranial pressure and has by this means 

 disturbed functions far removed from the seat of the lesion. Again, a sudden 

 hemorrhage in the brain occurs; the patient shows various severe symptoms and 

 dies within a few hours. Now this is not equivalent to saying that the different 

 disturbances observed were produced alone by paralysis of the part destroyed in 

 the hemorrhage; they certainly were the result, in part, at least, of shock, and 

 would doubtless have disappeared to a certain extent had the patient lived longer. 

 Only from cases where there is no rise of intracranial pressure and where the 

 patient lives some time after the inception of the lesion can conclusions of any 

 physiological importance be drawn. 



These preliminary remarks with regard to the principles which must be borne 

 in mind in the study of brain functions must suffice for the present. As we 

 proceed with the subject we shall have opportunity of discussing these funda- 

 mental propositions more in detail. 



B. DIVISIONS OF THE BRAIN 



His has divided the brain on the basis of its embryological development 

 as given in the table l on opposite page. 



The parts of the brain derived from the first primary brain vesicle (hind- 

 brain) inclusive of the diencephalon ('tweenbrain), were formerly described 

 collectively as the "brain-stem" in contradistinction to the endbrain (telen- 

 cephalon). In presenting the subject of the brain functions it seems advisable 

 for several reasons to continue as formerly the use of this division and to 

 apply the name cerebrum only to the parts developed from the endbrain. 



1 This classification has been very slightly modified in accordance with more modern 

 usage in English. ED. 



