252 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



between the olivary bodies. According to Spitzka, 1 these bundles are absent 

 in the case of the elephant and porpoise, :i condition which is correlated with 

 the slight differentiation of the limbs, which are not modified either for fine 

 movements or for tactile purposes. It has been pointed out, too, that removal 

 of a hemisphere causes in the dog and most rodents a degeneration of other 

 parts of the cord (dorsal columns) than those occupied by the pyramidal tracts 

 in man. 2 The fibres passing from the cortex to the efferent cell-groups in the 

 cord do not, therefore, hold exactly the same position in various mammals. 



Size of Pyramidal Tracts. — It has been clearly shown that if the cross- 

 sections of the r>'\\\> of the dog, monkey, and man be drawn of the same size, 

 the pyramidal fibres being indicated, then the area of this bundle is propor- 

 tionately greatest in man and least in the dog, the monkey being inter- 

 mediate in this respect. The relations thus indicated are evident — namely, 

 that the number of fibres controlling the cell-groups in man is the largest, 

 and also is much larger than that in the lower animals. 



The relative areas of the pyramidal tract at corresponding levels, the 

 ana of the entire cord being taken as 100 per cent., are given by v. Len- 

 hossek 3 for the following animals: 



Mouse 1.14 per cent. 



Guinea-pig 3.0 " 



Rabbit 5.3 " 



(at 7.76 " 



Man 11.87 « 



This relation is to be carefully noted, for with it is correlated the degree 

 of the disturbances in the reactions of the entire nervous system following 

 removal of parts of the cerebrum, the effect being slight when the cerebrum 

 is connected with the cord by a small number of fibres, and serious when the 

 connection is by many fibres, as in the case of man and the highest mammals. 



E. Localization in the Cerebral Cortex of the Cell-groups 



RECEIVING THE AFFERENT IMPULSES. 



Sensory Regions. — If an attempt is made to unify the construction of 

 the entire cortex by bringing the motor and sensory areas under a common law, 

 it must be based on the fact that the system of axones bringing impulses to 

 the motor region forms part of the pathway for conducting the afferent im- 

 pulse.- t'roin the skin and muscles back to some organ controlled by the 

 efferent nerves. To Munk 4 is due the credit of having from the first looked 

 upon the responsive cortex as marked off into areas within which certain 

 groups of these fibres terminate, so that apart from the sensory anas named 

 from the special senses, he calls the area which controls the skeletal muscles 

 the "Fuhlsphare" or body-sense area, on the assumption that in it end the 



: Journal of Gomparatiw Medicine and Surgery, 1886, vol. vii. 



a von Lenhossek : Anatomi*cher Anzeiyer, 1889. 



3 THefeiner Bait des Nervensystemi im l/ichte neuester Forschungen, Basel, 1893. 



* Ueber die Functionen der Qro&hirnrinde, 1881. 



