CENTRAL NERVOUS SYSTEM. 695 



cortical areas with the cell-groups controlling the muscles of the eye are 

 independent of each other. 



This instance of the direct control of the same axial cell-groups from dif- 

 ferent areas of the cortex is analogous to the control of efferent cell-groups in 

 the spinal cord, either by impulses coming down from the cerebrum or by 

 those entering the cord directly through the dorsal roots, and the instance 

 here cited is typical of a general arrangement. 



Cortical Control Crossed. Where the stimulation of the cerebral cortex 

 causes a response on one side only, that response is on the side opposite to the 

 stimulated hemisphere. It sometimes happens, however, that two groups of 

 symmetrically placed muscles both respond to the stimulus applied to one 

 hemisphere only, but these cases : the conjugate movements of the eyes ; 

 movements of the jaw muscles or those of the larynx, always depend on the 

 response of muscles which are naturally contracted together. 



This reaction depends on the arrangement of the fibres in the cord, since 

 in lower mammals (dog and rabbit, for example) it is not seriously disturbed 

 by the removal of one hemisphere. 



Course of Impulses Leaving the Cortex. In the higher mammals, as 

 well as in man, it is by way of the pyramidal fibres that impulses travel from 

 the cortex to the cell-groups of the axis. The pyramidal tracts by definition 

 form in part of their course the bundles of fibres lying on the ventral aspect 

 of the bulb, caudad to the pons, veutrad to the trapezium, and between the 

 olivary bodies. According to Spitzka, 1 these are absent in the case of the 

 elephant and porpoise. It has been pointed out, too, that removal of a hemi- 

 sphere 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 cords 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 intermediate 

 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 is much 

 larger than that in the lower animals. 



The relative areas of the pyramidal tract, the area of the entire cord being 

 taken as 100 per cent, at corresponding levels, are given by v. Lenhossek 3 for 

 the following animals : 



Mouse 1.14 per cent. 



Guinea-pig 3.0 " 



Babbit 5.3 " 



Cat 7.76 " 



Man 11.87 " 



1 Journal of Comparative Medicine and Surgery, 1886, vol. vii. 



2 von Lenhossek : Anatomischer Anzeiger, 1889. 



3 Diefeiner au des Nervensystems im Lichte neuester Forschungen, Basel, 1893. 



