184 



THE HUMAN BODY. 



the medulla oblongata and through it with the rest of the 



brain, so that nervous disturbances can pass by anatomically 



continuous paths from one to the other. 



The Structure of a Spinal Ganglion. When one of these 



ganglia is cut lengthwise, and the section examined micro- 

 scopically, it is seen that 

 connective tissue forms an 

 envelope for it, and sends 

 ramifying bundles through 

 it. The fibres of the poste- 

 rior root become separated 

 into bundles when they 

 enter a ganglion and unite 

 into a single bunch when 

 they leave it to join the 

 mixed spinal nerve trunk. 

 Between the bundles of 

 nerve-fibres within the gan- 

 glion are groups of nerve- 

 cells, and probably each fibre 

 on its way through the gan- 

 glion is connected with a 



FIG. 86. Diagram of a spinal ganglion -M rnr,- 



cell: uc, its flbrillated process, which ac- Cell. IhlS Connection OC- 



quiring primitive sheath, ps, and medullary e Orr ,p W V, a t rPPnliir 

 sheath. m, becomes a fibre which at the c 



node of Ranvier nr joins a posterior root w v TVm polla ^TTio- 8fi\ 



fibre, part of its axis cylinder, c, running Wa J' ne C6llS I'H?' *V 



centrally in this, and part, d, distaiiy. are pear-shaped, granular, 

 contain a large nucleus and nucleolus, and average -$ mm. 

 (^l-Q inch) in long diameter. Near its narrow end the cell 

 substance is fibrillated, and a bundle of these fine fibres, ac, 

 passes from it, something like the stalk from a pear. This 

 stalk is an axis cylinder, and has on it small nuclei. A little 

 way from the cell the axis cylinder acquires a primitive sheath, 

 ps, and a little farther on a medullary sheath, ms, so that it 

 becomes a fully formed white nerve-fibre. At a node of Ran- 

 vier (usually that one nearest the cell), nr, this divides, its 

 branches diverging from it at right angles: one branch runs 

 to the posterior root and enters the spinal cord; the other 

 continues through the ganglion as a fibre of the mixed 

 nerve-trunk. The axis cylinders of these branches, c and d, 

 in some cases at least, contain fibrillae not derived from the 

 pear-shaped cell in addition to those which are. Each cell as 

 it lies in the ganglion is gncased in a delicate envelope of 



