FUNCTIONS OF CENTRAL NERVOUS SYSTEiM 887 



We have purposely omitted to enumerate other paths by which 

 the various tracts of grey matter in the brain are brought into com- 

 munication with each other, and our knowledge of such connections 

 is no doubt far from complete. When we add that not only are the 

 cerebral hemispheres united by many ties to the subordinate portions 

 of the cerebro-spinal axis and to each other, but that cortical areas 

 of one and the same hemisphere are in communication by short 

 connecting loops of ' association ' fibres (Fig. 360), it will be seen that 

 the linkage of the various parts of the central nervous system is 

 extremely complex; that an excitation, blocked out from one path, 

 may have the choice of many alternative routes; and that the ap- 

 parent simplicity and isolation of the pyramidal tracts must not be 

 allowed too far to govern our views of the possibilities open to a 

 nervous impulse once it has been set going in the labyrinth of the 

 nervous network. Nor is it only by the main channel of the axis- 

 cylinder that nervous impulses can be conducted, they can also pass 

 along the collaterals. And the actual route taken by a given impulse 

 is determined not only by anatomical relations, but also by molec- 

 ular conditions, particularly in the terminal fibrils of the axons, 

 collaterals and dendrites, and in the substance, if such a substance 

 there be, which intervenes between the end arborizations of a neuron 

 and the dendrites or cell-bodies of the neurons with which they lie in 

 contact. So that a road open at one moment may be closed at 

 another. (See p. 902.) We may suppose that the greater the number 

 of connections between the cells of the central nervous system, the 

 greater is the complexity of the processes which may be carried on 

 within it. And, indeed, comparison of the brains of different animals 

 shows that it is not so much by excess in the number of nerve-cells 

 as by the increased complexity of linkage, that a highly-developed 

 brain differs from a brain of lower type; the higher the brain, the 

 more richly branched are the dendrites and the terminations of the 

 axons and their collaterals, and, therefore, the greater is the numbei 

 of possible paths between one nerve-cell and another. 



SECTION VIII. FUNCTIONS OF THE CENTRAL NERVOUS SYSTEM 

 (i) THE SPINAL CORD (INCLUDING THE MEDULLA OBLONGATA). 



Much of our knowledge of the functions of the central nervous 

 system and of its divisions has been gained by the removal or 

 destruction of more or less extensive tracts of nervous substance, or 

 the cutting off of connection between one part and another. But it 

 is well to warn the reader at the very outset that in no other part of 

 physiology is such caution required in making deductions as to the 

 working of the intact mechanism from the phenomena which mani- 

 fest themselves after such lesions. 



