Dendrites 



THE NEURONES. 6 5 



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. This recession is expressed in the axial accumu- 

 lation -of the cell-bodies either within the wall, or in the immediate vicinity 

 of the neural tube (brain and spinal cord), to or from which the processes 

 pass. The nervous system is often divided, therefore, into a central 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 nervous system, on the contrary, con- 

 tains the nerve-cells of the sensory ganglia and is 

 composed principally of the nerve-fibres that pass 

 to and from the end-organs. Intimately associ- 

 ated with and in fact a part of the peripheral 

 nervous system, but at the same time possessing 

 a certain degree of independence, stands the 

 sympathetic system, which provides for the inner- 

 vation of the involuntary muscle and glandular 

 tissue throughout the body and the muscle of 

 the heart. 



The Neurones. As before stated, the neu- 

 rones consist of the cell-body (nerve-cell) and 

 the processes. The latter, as seen in the case 

 of a typical motor neurone extending from the 

 spinal cord to a muscle, are of two kinds : (#) 

 the branched protoplasmic extensions, the den- 

 drites, which are usually multiple and form 

 elaborate arborescent ramifications that establish 

 relations with other neurones, and (6) the single 

 unbranched axone (neuraxis, neurite) that ordi- 

 narily is prolonged to form the core or axis- 

 cylinder of a nerve-fibre, and, hence, is also 

 often termed the nerve- or axis-cylinder process. 

 Although '/ chained ' ' together as the links that 

 form the various paths along which impulses 

 are conveyed, the neurones are seldom, if ever 

 primarily, actually united to one another, but only 

 intimately related. Their processes, although in 

 close contact, are not directly continuous, contigu- 

 ity and not continuity being the ordinary relation. 

 The dendrites are usually uneven in contour 



and robust as they leave the cell-body (nerve-cell), but rapidly become 

 thinner in consequence of their repeated division, until they are reduced to 

 delicate threads that constitute the terminal arborizations, the telodendria t 

 formed by the end-branches. The dendrites collect and carry impulses toward 

 the cell -body; the axones carry impulses away from the cell-body. The 

 axone, slender and smooth and of uniform thickness, is much less con- 

 spicuous than the dendrites. It may be short and extend only to nearby 

 cells, or it may be of great length and connect distant parts that lie either 

 wholly within the cerebro-spinal axis (as from the brain-cortex to the lower 

 part of the spinal cord) or extend beyond (as from the lower end of the 

 spinal cord to the plantar muscles of the foot). On reaching its destination, 

 the axone terminates in an end-arborization or telodendrion, in a manner 



Telodendrion 



FIG. 84. Diagram of typical 

 neurone. 



