THE CENTRAL NERVOUS SYSTEM 693 



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. 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 pro- 

 cesses which may be carried on within it. And, indeed, 

 comparison of the brains of different animals shows us 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 number of possible paths 

 between one nerve-cell and another. 



II. Functions of the Central Nervous System. 



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. 



In the first place, every operation of any magnitude on the brain 

 or cord is immediately followed by a depression of the functional 

 power of the nervous tissue, a depression which may extend far from 

 the actual seat of injury and manifest itself by various phenomena, 

 which are grouped together under the name of ' shock.' Thus, when 

 the spinal cord of a dog is divided, e.g., in the dorsal region, all power, 

 all vitality, one might almost say, seems to be for ever gone from the 

 portion of the body below the level of the section. The legs hang 

 limp and useless. Pinching or tickling them calls forth no reflex 

 movements. The vaso-motor tone is destroyed, and the vessels 

 gorged with blood. The urine accumulates, overfills the paralyzed 

 bladder, and continually dribbles away from it. The sphincter of 

 the anus has lost its tone, and the faeces escape involuntarily. And 

 if we were to continue our observations only for a short time, a few 

 hours or days, we should be apt to appraise at a very low value the 

 functions of that part of the cord which still remains in connection 

 with the paralyzed extremities. But these symptoms are essentially 



