THE NERVOUS SYSTEM. 



FIG. 834. 



THE nervous system the complex apparatus by which the organism is brought 

 into relation with its surroundings and by which its various parts are united into one 

 coordinated whole consists essentially of structural units, the neurones, held together 

 by a special sustentacular tissue, the neuroglia, assisted by ingrowths of connective 

 tissue from the investing membrane, the pia mater. 



The neurone, the morphological unit of the nervous system, includes a 

 nucleated protoplasmic accumulation, the cell-body, and the processes. The former, 

 usually spoken of as the nerve-cell, presides over the nutrition of the neurone and is 

 the seat of the subtle changes giving rise to nervous impulse. The processes arise 

 as outgrowths from the cell-body and provide the paths along which impulses are 

 conveyed. They are very variable in length, some extending only a fraction of a 

 millimeter beyond the cell-body, while others continue for many centimeters to 

 distant parts of the body. The longer processes, which usually acquire protecting 

 sheaths, are known as the nerve-fibres, and these, associated in bundles, constitute 

 the nerve-trunks that pass to the muscles and various other organs. 



Reduced to its simplest terms, the nervous system consists of the two parts rep- 

 resented in the accompanying diagram (Fig. 834). The one, the sensory neurone, 



{A} takes up the stimulus received upon the 

 integument or other sensory surface and, by means 

 of its process (nerve-fibre), conveys such impulse 

 from the periphery towards the central aggregations 

 of nerve-cells that commonly lie in the vicinity of the 

 body-axis. Functionally, such a path constitutes a 

 centripetal or afferent fibre (#). The impressions 

 thus carried are transferred to the second element, 

 the motor neurone (.#), which in response sends 

 out the impulse originating within the cell-body 

 (nerve-cell) along the process known as the centri- 

 fugal or efferent fibre (e*), to the muscle-cell 

 and causes contraction. The simple relations of 

 the foregoing apparatus are, in fact, superceded 

 by much greater complexity in consequence of the 

 introduction of additional neurones by which the 

 afferent impressions are distributed to nerve-cells 

 situated not only in the immediate vicinity of the 

 first neurone, but at different and often distant levels. 



Although very exceptionally the relation between the neurones may perhaps be 

 that of actual continuity in consequence of a secondary union of their processes 

 (Held), the view concerning the constitution of the nervous system most worthy of 

 confidence, notwithstanding the bitter attacks by certain histologists, regards the 

 neurones as separate and distinct units. While chained together to form the various 

 paths of conduction, they are probably seldom, if ever, actually united to one another 

 but only intimately related, since their processes, although in close contact, are not 

 directly continuous, contiguity but not continuity being the ordinary relation. 



During the evolution of the nervous system from the simpler type, the cell- 

 bodies of the neurones forsake their primary superficial position and recede from the 

 periphery. In vertebrates this recession is expressed in the axial accumulation of 

 cell-bodies either within the wall or in the immediate vicinity of the neural tube 

 (brain and spinal cord), from or to which the processes pass. The nervous system 

 is often divided, therefore, into a cenfra/and a peripheral portion. The former, also 

 known as the cerebro- spinal axis, includes the brain and spinal cord and contains 

 the chief axial collections of nerve-cells ; the peripheral portion, on the contrary, 

 996 



Diagram showing fundamental units 

 of nervous system. A, sensory neurone, 

 conducting afferent impulses by its pro- 

 cess (a) from periphery (S) ; B, motor 

 neurone sending efferent impulses by its 

 process (e) to muscle. 



