THE NERVOUS SYSTEM. 799 



always the same appearance, and that this appearance furnishes a norm for comparison 



with cells showing pathological changes and 

 which have been subjected to the same technique. 



Pigment. In addition to the elements already 

 described, more or less pigment in the shape of 

 fine brownish-yellow granules is often present in 

 nerve cells. It is not found in cells of the new- 

 born. It increases with age, and in old age often 

 fills up a large part of the cell body. Its signifi- 

 cance is not known. 



The protoplasmic processes or dendrites are at 

 least the larger trunks of the same structure as 

 the cell body (Plate XIII., 1). That these larger 

 trunks have functions similar to those of the cell 

 body seems probable from the fact that the axone 

 not infrequently takes origin from a large dendri- 



F,G. 524.-NEURONE (NERVE CELL OR ^ *"* Jf 6 * 1 f ^^ fr m .^ ^ f 



GANGLION-CELL PROCESSES). FROM THE Me cel l- The chromatic substance is present m 

 GRANULAR LAYER OF THE CEREBELLAR the dendrites, as elongated rods w r ith triangular 

 CORTEX OF a GUINEA-PIG. masses at the points of bifurcation. The achro- 



The short axone passes off to the left and matic elements are apparently of the same charac- 



terminates near its cell of origin. ter as in the body of the cell. The protoplasmic 



processes divide dichotomously, becoming rapidly 



smaller, and end at a comparatively short distance from the cell body. 1 Stained by the 

 method of Golgi, the dendrites are seen to be covered with minute projections or 

 "gemmules," often ending in a small bulb. These processes are cellulipetal in charac- 

 ter, carrying impulses always toward the cell. 



The axis-cylinder process or axone so-called from its often becoming the axis cylin- 

 der of a nerve fibre is usually single. It generally arises directly from the body of the 

 cell, but may arise, as already mentioned, from one of the larger protoplasmic trunks. 

 It is differentiated from the dendrites in Mssl preparations by always taking origin 

 from an area in the cell body free from chromatic substance, and by being itself entirely 

 achromatic (Plate XIII., 1); in Golgi specimens, it is recognized by its straight course, 

 uniform diameter, and smooth outline (Fig. 523). It sends off few branches, and these 

 at right angles (collaterals). This axone may extend a great distance from the cell 

 body ; for example, the axones of certain motor cells of the spinal cord extend to the 

 muscles of the hands or feet. Both the axone proper and its collaterals end in terminal 

 arborizations. Their conductivity is cellulifugal that is, they always carry impulses 

 from the cell. In certain cells the axis-cylinder processes branch rapidly and end in 

 the gray matter in the vicinity of their cells of origin (Fig. 524). An axone may pass 

 from its cell of origin to its termination uncovered by any sheath. Such axones are 

 found in certain portions of the gray matter. An axone may be enveloped only by a 

 thin membrane, the sheath of Schwann, as, e.g., the fibres of Remak found mainly in 

 the sympathetic system. An axone may be surrounded by a myelin sheath alone, as 

 in the white matter of the brain and cord, or by both a myelin sheath and a sheath of 

 Schwaun, as in the fibres of the peripheral nerves, with the exception of the olfactory 

 and optic. 



According to the strict neurone conception of nervous-system structure, each neu- 

 rone is considered a complete morphological, and, to a lesser degree, physiological, 

 entity. It has no anatomical connection with any other neurone. Association between 

 neurones takes place by contact or contiguity, and never by continuity of their proto- 

 plasm. The passage of an impulse is always from the terminal arborizations of axones 

 or collaterals to the dendrites or cell body of the other neurone. 



The validity of the neurone theory has been recently called in question by such 

 prominent neurologists as Held, Apathy, Bethe, and Nissl, 2 on the ground that in many 

 cases axones pass directly into the protoplasm of another neurone and that the neuro- 



1 Exception, peripheral arm of spinal ganglion cell. 



'-' For a critical review of the opinions of these writers, consult the last edition of 

 Van Gehuchten's "Anatomic du Systeme Nerveux." 



