CHAPTER XIV 



The Beain of Mammals axd the Olfactohy Apparatus. 



In the first chapters the relations of the forebrain of mammals could 

 only Ije touched upon in a very general way. At present, when you are 

 better acquainted with the structure of the human brain, it will pay you to 

 glance at other mammalian brains. j\Iueh of that which is known regarding 

 the fiber-systems, etc., has been gained, as you know, not from the human 

 brain, but from a study of that of animals. A great many things that appear 

 hardly intelligible in human brains are met with much better developed in 

 lower vertebrates. 



If it is desired clearly to understand the enormous differences in degree 

 of perfection of the mantle of the forebrain which are found in the various 

 mammals, they must be considered in regard to a fact which up to the 

 present time has not been sufficiently recognized. The mantle is not a unit 

 functionally. It is composed raiher of a large number of different parts — 

 called centers — and numerous physiological experiments have shown thai 

 movements which must be acquired, and probably most of the intellectual 

 combinations, are only made possible by the existence of such centers. 



The introductory chapters have shown that the real motor and sensory 

 centers are situated low down, from the spinal cord to the midbrain, and 

 that these, even if the forebrain is wanting, are in themselves sufficient 

 for necessary activity. These centers, present in abundance and found at 

 an early period, are connected with one another to form series. 



Experimental physiology shows, however, that many of the lower centers are 

 connected with higher centers, located above in the cortex, in such a manner that 

 irritation of the latter produces movement. Of ^^ hat nature and importance the in- 

 fluence of the higher centers on the lower may be is still a matter of special dis- 

 cussion. For this reason an endeavor has been made to study as exactly as possible 

 the phenomena that appear after the removal of portions of the cortex. Doubtless 

 the importance of the role played by the cerebral cortex is different in different verte- 

 brates. While the removal of the entire cerebrum in loAver vertebrates does not de- 

 stroy the ability to perform coarser movements with efficiency, in mammals, after de- 

 struction of circumscribed portions of the motor zone, paralyses appear, which are 

 very transitory. In man disease even of relatively small portions of the cortex often 

 leads to permanent paralyses. Manifestly all motor, and many senso-psi/chieal, func- 

 tions may 6e performed iy parts of the central nerrotis system situated lower down. 

 The higher we ascend in the rertcbrate series, however, the more is the carter con- 

 cerned in the activity of the hrain, and the more is consciousness met witJi as a con- 



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