414 GENERAL STRUCTURE AND FUNCTIONS 



and spinal cord, on the contrary, they present three, four, five, or even 

 eight prolongations, running in different directions and giving to the 

 cell a peculiar radiated appearance. The prolongations after a certain 

 distance become branched, the branches thus formed again dividing 

 and subdividing, growing at the same time smaller in size, until they 

 terminate in a more or less abundant tuft or ramification of exceedingly 

 slender filaments. According to Gerlach, the terminal fibres of this 

 ramification constitute a plexus of fine nervous threads, penetrating 

 the interstitial substance of the gray matter. It is not, however, known 

 with certainty whether these fibres terminate by free extremities, or 

 whether they form a network of communication between different nerve 

 cells. 



Connection between the Nerve Cells and Nerve Fibres. In all cases 

 the nerve fibres are connected, at their central origin, with masses of 

 gray substance, into which they penetrate and in which they are inti- 

 mately mingled with the nerve cells. In some instances, a direct con- 

 tinuity can be seen between the nerve fibres and certain prolongations 

 of the nerve cells ; in others, such a direct anatomical connection is only 

 rendered more or less probable by the similarity in direction between 

 the nerve fibres and the processes of the nerve cells, and by their resem- 

 blance in physical constitution. 



In the ganglia of the sympathetic system and in those of the roots of 

 the spinal and cranial nerves, the nerve cells of a rounded form give 

 off, as a rule, in man and the mammalians, only a single, pale, undivided 

 process, which at first presents the appearance of an axis cylinder of 

 small diameter, but which subsequently increases in size and becomes 

 provided with a medullary layer, assuming at the same time the distinct 

 double contour, characteristic of a fully formed nerve fibre. These cells, 

 sending out a single nerve process in one direction, are called " unipolar" 

 nerve cells. In the ganglia of the spinal nerve roots, and in the Gasse- 

 rian ganglion of fishes, nerve cells are found which send off two such 

 processes in opposite directions ; the medullary layer of the nerve fibre 

 ceasing on each side just before its union with the body of the cell. 

 Such cells, with two opposite nerve processes, are called "bipolar" nerve 

 cells. These connections have been recognized by all observers, and 

 there is no doubt as to their existence. 



In the gray substance of the brain and spinal cord, the nerve cells, as 

 above described, are " multipolar," or send out a number of prolonga- 

 tions, in different directions, which divide and ramify without making 

 any certain anatomical connection with other parts. Beside these 

 branching prolongations, however, according to the observations of 

 Deiters, confirmed by Schultze, Gerlach, and Kolliker, the multipolar 

 nerve cell also sends out a single prolongation which is different from 

 the others, in that it does not branch but continues on its course for a 

 considerable distance, presenting the usual physical aspect of a naked 

 axis cylinder. This simple unbranched process is called the "axis 

 cylinder process," to distinguish it from the remaining ramified pro- 



