THE STORY OF NINETEENTH-CENTURY SCIENCE 



the brain. Further studies went to show that such phe- 

 nomena of reflex action on the part of centres lying out- 

 side the range of consciousness, both in the spinal cord 

 and in the brain itself, are extremely common ; that, in 

 short, they enter constantly into the activities of every 

 living organism, and have a most important share in the 

 sum total of vital movements. Hence, Hall's discovery 

 must always stand as one of the great mile-stones of the 

 advance of neurological science. 



All these considerations as to nerve currents and 

 nerve tracts becoming stock knowledge of science, it 

 was natural that interest should become stimulated as 

 to the exact character of these nerve tracts in them- 

 selves ; and all the more natural in that the perfected 

 microscope was just now claiming all fields for its own. 

 A troop of observers soon entered upon the study of the 

 nerves ; and the leader here, as in so many other lines 

 of microscopical research, was no other than Theodor 

 Schwann. Through his efforts, and with the invaluable 

 aid of such other workers as Remak, Purkinje, Henle, 

 Miiller, and the rest, all the mystery as to the general 

 characteristics of nerve tracts was cleared away. It 

 came to be known that in its essentials a nerve tract is 

 a tenuous fibre or thread of protoplasm, stretching be- 

 tween two terminal points in the organism one of such 

 termini being usually a cell of the brain or spinal cord ; 

 the other, a distribution point at or near the periphery 

 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 Remak presently dis- 

 covered, the sheath is dispensed with, particularly in 

 case of the nerves of the so-called sympathetic system. 



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