862 



THE CENTRAL NERVOUS SYSTEM 



distinct though relatively ill-developed cerebral hemispheres, it 

 would seem by no means unlikely in the case of fishes and animals 

 below them, which are practically devoid of a cerebral cortex 

 altogether. 



The functions of the spinal cord may be classified thus: 



1. The conduction of impulses set up elsewhere either in the 



brain or at the periphery. 



2. The modification of impulses set up elsewhere (reflex action). 



3. The origination of impulses (?). 



Ce.re.br ai 



2)e cassation 

 of Pyramids 



oft.rect 

 Pyramidal tract 



J\ferue-Cetts^-* 

 of Ant Horn ^-^-" 

 Termirxil Arbonsvtioriy-^^ 

 of a Pyramids. I Fi 

 aroundL cell of 

 ' ' #oro 



Anterior 



Soot 



Fibres 



Fig- 355- Some Possible Paths of Efferent Impulses in the Central Nervous System 

 (Schematic). Details are omitted from the scheme. For instance, each pyra- 

 midal fibre is represented as arborizing around one anterior cornual cell only, 

 and no collaterals are shown. The hypothetical intercalated neurons between 

 the pyramidal fibres and the anterior horn cells (p. 848) are not shown. 



I. Conduction of Nervous Impulses by the Cord. The old con- 

 troversy as to whether the white fibres of the spinal cord are directly 

 excitable has long been settled in the affirmative. 



The inquiry was complicated by the presence of the spinal roots, 

 which, since the experiments of Charles Bell, have been known to be 

 capable of excitation by artificial stimuli. But at length the difficulty 

 was overcome in this way: The posterior (dorsal) portion of several 



