5?8 THE NERVOUS SYSTEM. 



part affects the size of its vessels and the supply of blood, 

 this change cannot be considered as the only source of the 

 change in its mode of secretion or nutrition. 



It is, of course, very difficult to determine the relative 

 share exercised by the true sympathetic and the ordinary 

 cerebro-spinal fibres in the contraction of blood-vessels, 

 and in the general processes of nutrition and secretion, 

 since both kinds of fibres appear to be distributed to most 

 parts, and there seems to be no possibility of isolating 

 them. The difficulty of determining this point is much 

 greater in the higher than in the lower Yertebrata ; for 

 it would appear that, in the same proportion as the 

 centres of the cerebro-spinal system are developed, so is 

 its connection with the processes of organic life more inti- 

 mate. In frogs, for instance, all the organic functions 

 may be earned on for several days after the removal of 

 the brain and spinal cord, if only the medulla oblongata 

 has been spared for the maintenance of respiration ; but 

 in Mammalia, and, most of all, in man, even a slight 

 injury of either brain or spinal cord, may disturb all the 

 organic functions. The regularity of the movements of 

 the stomach and intestines, the heart and urinary bladder, 

 independently of the spinal cord or brain, is manifested 

 by numerous experiments in reptiles and Amphibia ; but 

 in Mammalia, the separation of these organs from the 

 influence of the spinal cord or brain, is sufficient to render 

 their actions feeble and irregular, or, after a short time, 

 to stop them altogether. 



Probably, therefore, the safest view of the question at 

 present is, still to regard all the processes of organic life, 

 in man, as liable to the combined influences of the cere- 

 bro-spinal and the sympathetic systems ; to consider that 

 those influences may be so combined as that the sympa- 

 thetic nerves and ganglia may be in man, as in the lower 

 animals, the parts through which the ordinary and constant 

 influence of nervous force is exercised on the organic 



