PROPERTIES OF THE NERVE CELL. 131 



The Varieties of Neurons. — The neurons differ greatly in 

 size, shape, and internal structure, and it is impossible to classify 

 them \\ith entire success from either a physiological or an anatomical 

 standpoint. Neglecting the unusual forms whose occurrence is 

 limited and whose structure is perhaps incompletely knowTi, there 

 are three distinct types whose form and structure throw some 

 light on their functional significance: 



I. The bipolar cells. This cell is found in the dorsal root gan- 

 glia of the spinal nerves and in the ganglia attached to the sensory 

 fibers of the cranial nerves, the ganglion semilunare (Gasserian) 

 for the fifth cranial, the g. geniculi for the seventh, the g. vestibu- 

 lare and g. spirale for the eighth, the g. superius and g. pe- 

 trosum for the ninth, the g. jugulare and g. nodosum for the 

 tenth. 



The typical cell of this group is found in the dorsal root ganglia. 

 In the adult the two processes arise as one, so that the cell seems to 

 be unipolar, but at some distance from the cell this process divides 

 in T, one branch passing into the spinal cord via the posterior 

 root, the other entering the spinal nerve as a sensory nerve fiber 

 to be distributed to some sensory surface. Both processes become 

 medullated and form typical nerve fibers. That these apparently 

 unipolar cells are really bipolar is sho\\Ti not only by this division 

 into two distinct fibers, but also by a study of their developm^^nt 

 in the embryo. In early embryonic life the two processes arise 

 from different poles of the cell, and later become fused into an ap- 

 parently simple process (Fig. 60). The striking characteristics of 

 this cell, therefore, are that it gives rise to two nerve fibers, and that 

 it possesses no dendritic processes. On the physiological side these 

 cells might be designated as sensory cells, since they appear to be 

 associated always with sensory nerve fibers. 



The nerve cells found in the sensory ganglia exhibit, as a matter of fact, 

 a number of different types, some of which possess short dendritic processes. 

 These histological variations cannot as yet be given a physiological signifi- 

 cance, but their occurrence certainly seems to indicate a possibility that 

 the sensory ganglia may have a much more varied physiological activity 

 than has been attributed to them heretofore. Afferent fibers, especially those 

 from the viscera, may terminate in these ganglia by making synaptic connec- 

 tions with the nerve cells. For a description of these ganglia and a classifica- 

 tion of their cells under eight different types consult Cajal in "Ergebnisse 

 der Anat. u. Entwickelungsgeschichte," vol. xvi, 1906, and Dogiel, "Bau 

 der Spinalganglien, etc.," Jena, 1908. 



So far as the sensory fibers of the spinal and cranial nerves 

 are concerned, it is worth noting also that all of them arise from 

 cells lying outside the main axis of the central nervous system. 

 It has been a question whether the sensory impulses brought 



