EXPERIMENTAL PSYCHOLOGY 



for example, in a muscle or in the skin. Such a fibril 

 may have about it a protective covering, which is 

 known as the sheath of Schwann ; but the fibril itself is 

 the essential nerve tract; and in many cases, as Re- 

 mak presently discovered, the sheath is dispensed with, 

 particularly in case of the nerves of the so-called sym- 

 pathetic system. 



This sympathetic system of ganglia and nerves, by- 

 the-bye, had long been a puzzle to the physiologists. 

 Its ganglia, the seeming centre of the system, usually 

 minute in size and never very large, are found every- 

 where through the organism, but in particular are 

 gathered into a long double chain which lies within the 

 body cavity, outside the spinal column, and represents 

 the sole nervous system of the non-vertebrated or- 

 ganisms. Fibrils from these ganglia were seen to join 

 the cranial and spinal nerve fibrils and to accompany 

 them everywhere, but what special function they sub- 

 served was long a mere matter of conjecture and led 

 to many absurd speculations. Fact was not substi- 

 tuted for conjecture until about the year 1851, when 

 the great Frenchman Claude Bernard conclusively 

 proved that at least one chief function of the sympa- 

 thetic fibrils is to cause contraction of the walls of the 

 arterioles of the system, thus regulating the blood- 

 supply of any given part. Ten years earlier Henle 

 had demonstrated the existence of annular bands of 

 muscle fibres in the arterioles, hitherto a much-mooted 

 question, and several tentative explanations of the 

 action of these fibres had been made, particularly by 

 the brothers Weber, by Stilling, who, as early as 1840, 

 had ventured to speak of " vaso-motor" nerves, and by 



259 



