814 



THE NERVE SYSTEM 



narrower as they divide into many branches in a dichotoinous or arborescent manner to end free, 

 according to most observers, or to be joined with the dendrites of other neurones by means of 

 minute fibrilla? (as claimed by Apathy) or by concrescence (Held). The contour of the dendrite, 

 while occasionally irregular in some specimens, with varicosities along its course, is, as a rule, 

 beset with numerous lateral buds called gemmules. Various hypotheses have been advanced 

 in explanation of these appearances, it being held by some investigators that they are related to 

 conditions of activity as contrasted to those of repose, while others believe them to be artefacts 

 produced by the fixing and staining methods at present employed. However, it is no longer 

 disputed that the function of the dendrites is receptive and conductive (or cellulipetal) for nerve 

 impulses, although they probably serve the nutritional requirements of the cell body as well. 



This functional distinction gives the clue to the correct interpretation of the central and 

 peripheral prolongations of the cerebrospinal ganglionic neurones. The cells of these ganglia 

 are at first bipolar in form, but gradually undergo transformation into apparently unipolar cells 

 by the migration of the cell body to one side and the consequent approximation of the two 

 processes to form a common pedicle in a T-shaped manner so typical of the spinal ganglion cell of 

 the adult. The central branch invariably remains cellulifugal, the peripheral branch invariably 

 remains cellulipetal, and as such ft equivalent to the dendrites of all other neurones. It is 

 merely a modified dendrite in that it courses a longer distance without branching until it reaches 

 the periphery and is usually myelinic. Such a peripheral prolongation of the ganglion cell is 

 also termed a centripetal nerve fibre or myelinic (medullated) peripheral axone of a peripheral 

 centripetal neurone. 



B 



FIG. 587. .1. Myelinic axones in fresh state, showing a few nodes, B. Portion of a myelinic axone treated 

 with boiling ether and alcohol to remove the myelin and leaving the neurokeratin network, a. Axone. 



The Axone. The axone is usually much longer than any of the dendrites, thin, pale, smooth, 

 emerging from the nerve cell as a direct continuation of the neurofibrillar ground substance 

 of the cell body, and devoid, so far as at present known, of chromatophile granules. Its 

 calibre varies for the different cells, corresponding in general to the length of its course, 

 but it is practically of uniform diameter throughout its extent. Axones may be extremely 

 short or fully a meter in length. Most cells give rise to only one axone (monaxonic neurones), 

 but in certain localities diaxonic (two axones) and polyaxonic (several axones) neurones are 

 found. In a Golgi preparation axones stand out like pieces of black thread, taking a more 

 direct course than do the irregular dendrites, and rarely branching before reaching the ultimate 

 termination, although giving off collaterals along their course. The central axones of spinal 

 ganglion (sensor) neurones are the principal exception to this rule in that they bifurcate in a 

 Y-shaped manner after their entrance into the central nerve system. In the case of another 

 group of neurones, Golgi's Cell, Type II, the axone is observed to break up into numerous 



