NERVE ELEMENTS AND THEIR FUNCTIONS. 85 



of the organism to stimuli. Our object therefore is to trace out 

 these pathways of impulses, to arrange them into systems of 

 nerve elements with reference to their appropriate stimuli and 

 responses, and to give an orderly account of these systems. 



THE NEURONE THEORY. 



The cell theory of Schleiden and Schwann (1838-39) was a state- 

 ment of the general conclusions of anatomists and embryologists 

 up to that time regarding the mode of construction of the animal 

 and plant organism. In brief, it was to the effect that every 

 higher animal or plant is made up of many individual organisms 

 ■known as cells. The cell was regarded as the unit of structure. 

 Each cell carried on its own processes and had its own life history, 

 and at the same time joined with others to produce the structure 

 and actions of an organism of higher order. The cells of each 

 tissue were Hke one another and the differences between tissues 

 depended upon the characters of the cells constituting each. As 

 the knowledge of various animal and plant tissue was increased, this 

 theory was extended and amplified and remained for fifty years 

 the best expression of our knowledge. In 1891 the cell theory was 

 stated in a special form as it applied to the nervous system. This 

 statement of the cell theory of the nervous system has since been 

 known as the neurone theory of Waldeyer, who formulated it. The 

 theory may be stated as follows. The nervous system consists 

 •of cells each of which (i) arises from a single embryonic cell, 

 processes of which grow out to form the neurite and dendrites; (2) 

 remains as an independent cell in adult life, making connections 

 with other cells only by contact of its processes with the processes 

 ■or cell-bodies of other cells. The constituent elements of the 

 nervous system were called by Waldeyer neurones. As the result 

 ■of later discussion of this theory two further points have been 

 added to it; (3) the structural and functional polarity of the neurone; 

 and (4) that all parts of the neurone constitute a trophic unit for 

 whose continued metabolic activity the presence of the nucleus 

 is necessary. When the neurone is cut in two in any way only 

 that part which retains the nucleus is capable of long continued 

 functional existence. 



