92 NERVOUS SYSTEM OF VERTEBRATES. 



invertebrates, neurones do not show a functional differentiation 

 of their processes, typical neurones in vertebrates present a struc- 

 tural and functional polarity. Each neurone consists of dendrites, 

 cell-body and neurite; and the impulse passes through the parts in 

 the order named, or passes from dendrites to neurite without 

 passing through the cell-body. Certain peripheral plexuses in 

 which the dendrites of adjacent neurones are fused into an intricate 

 network require further study with reference to this statement. 



3. When the neurite of one neurone has a functional connec- 

 tion with some part of another neurone, the plasm of the two 

 neurones at the point of connection may be fused. Whether 

 such a continuity of structure between the neurones of a chain 

 or pathway is universal, or how frequent it is, must be determined 

 by further study. 



4. In the protoplasm of neurones the apparent fibrillar structure 

 is a constant and normal feature. The real nature of the neuro- 

 fibrillae in the living cell, the mode of their formation, and the 

 structural changes which they may undergo during the different 

 phases of functional activity of the neurone are not yet understood. 

 When a fusion between two neurones takes place the neuro- 

 fibrillae take part in it. 



5. The whole neurone is necessar)^ for continued functional 

 existence. Any part of the neurone which is cut away from the 

 nucleated portion degenerates. The mode of repair of such 

 injury is by growing out of the cut fiber. 



6. The form and position of the neurones and especially the 

 disposition and connections of their processes determine the path- 

 ways of impulses and hence the work done by the nervous system. 



The practical value of the neurone theory is found in the last 

 statement. The linking together of neurones in pathways is 

 determined in part by inheritance of the general plan of arrange- 

 ment and in part by the experience of the individual. The nervous 

 system considered as a complex of neurones variously linked 

 together into functional systems, is at once the mechanism by 

 which certain work is done and a record of the experience of the 

 individual and the race. A knowledge of how neurones are linked 

 together in functional systems is necessary for the pathologist 



