THE SPINAL NERVES 63 



origin from the ganglia on the dorsal roots we will do well to interrupt for a 

 moment our functional analyses of the spinal nerves and consider the struc- 

 ture of these ganglia. 



The spinal ganglia are rather simple structures so far as their fundamental 

 plan is concerned, but in recent years, chiefly through the studies of Cajal 

 (1906) and Dogiel (1908), we have learned to recognize in them many complex 

 histologic details, the significance of which is not yet understood. It has long 

 been known that the typical cells of the mammalian spinal ganglion are uni- 

 polar. The cell body is irregularly spheric. The axon, 1 which is attached to 

 the perikaryon by an implantation cone, is coiled on itself in the neighborhood 

 of the cell, forming what is known as a glomerulus (Fig. 39, /). It then runs 

 into one of the central fiber bundles of the ganglion and divides in the form 

 of a T or Y into two branches, of which one is directed toward the spinal cord 

 in the dorsal root. The other and somewhat larger branch is directed distally 

 in the spinal nerve. The cells vary greatly in size and the diameter of the axon 

 varies with that of the cell from which it springs. An axon arising from a 

 large cell usually forms a very pronounced glomerulus and soon becomes en- 

 sheathed with myelin, and this myelin sheath is continued along both branches 

 into which it divides. The branching occurs at a node of Ranvier. 



As was originally pointed out by Cajal (1906) and Dogiel (1908) and 

 recently emphasized by Ranson (1911) the small cells of these ganglia give rise 

 to fine unmyelinated fibers. These coil but little near the cell, or the glomerulus 

 may be entirely lacking (Fig. 39, a). They divide dichotomously, just as do 

 the myelinated fibers, into finer central and coarser peripheral branches. At 

 the point of bifurcation there is a triangular expansion in place of the constric- 

 tion so characteristic of a dividing myelinated fiber. It has been shown by 

 Hatai (1902) and Warrington and Griffith (1904) that the small cells are con- 

 siderably more numerous than the large cells, though because of their small 

 size they constitute a less conspicuous element. 



A few cells retain the bipolar form characteristic of all the spinal ganglion 

 cells at an early stage of development (Figs. 21, 40, d). 



The spinal ganglion cells are each surrounded by a capsule or membranous 

 sheath with nuclei on its inner surface (Fig. 39, d, /) which is continuous with 

 the neurilemma sheath of the associated nerve-fiber. The cells forming the 

 capsule are of ectodermal origin, being derived like the spinal ganglion cells 

 themselves from the neural crest. 



1 See fine print, page 45. 



