THE NERVOUS SYSTEM 



527 



returns to the ventral spinal root as a grey ramus, or post-ganglionic 

 nerve (Fig. 156), and, with the motor nerve, is distributed to the 

 parts concerned. The vasomotor nerves are mainly constrictor ; 

 the fibres for the sweat glands are secretory ; those for the hair 

 are motor. The grey ramus does not necessarily return to the 

 same ventral root of the spinal nerve which the white ramus left, 

 but may join the spinal efferent nerves farther backwards. The 

 vasomotor, sweat, and pilomotor fibres for the head and neck 



DORSAL ROOT 



SPINAL GANGLION 



MIXED NERVE 



POSTGANGLIONIC FIBRE 



PREGANGLIONIC^ 

 FIBRE 



SYMPATHETIC 

 GANGLION 



Fig. 156. — Diagram showing the Arrangement of the Dorsal and Ventral 

 Roots of the Cord and the Connection of the White and Grey Rami 

 with the Ventral Root. 



The sensory fibres are shown passing through the spinal ganglion and entering the 

 cord through the dorsal root. The motor fibres leave the cord by the ventral 

 root. Shortly after leaving, the white ramus is given off, shown in the 

 diagram as the pre -ganglionic fibre. This passes to a lateral (vertebral) 

 ganglion of the sympathetic system, makes a cell connection as shown in the 

 diagram, and issues as a grey ramus, or post-ganglionic fibre, which rejoins the 

 ventral root. 



do not return as post-ganglionic fibres to the spinal nerve, but 

 are distributed through the cervical sympathetic, and from the 

 superior cervical ganglion are issued to the head as post-ganglionic 

 fibres. The fibres — vasomotor, sweat, and pilomotor — for the 

 fore-limbs also have their cell-station in the stellate ganglion, 

 and from this post-ganglionic fibres are issued. All these details 

 are fully dealt with at pp. 84, 306, and 309. The matter is only 

 referred to here in order to illustrate the principle underlying the 

 distribution of the autonomic fibres, which we are now in a 

 better position to understand. 



