NEUKO-MUSCULAPt MECHANISM 151 



the involuntary structures in the somatopleur, contains 1st. 

 Incoming Fibres. These take origin either in definite periph- 

 eral structures, such as Pacinian corpuscles, or in some less 

 defined endings, and as medullated fibres pass through the 

 various ganglia, and have their cell stations in the ganglion 

 on the ventral root. The nerve fibres coming from Pacinian 

 bodies and from muscles, like the similar fibres in the somatic 

 branch, are large and are connected with large cells in the 

 spinal ganglion, and they become myelinated at the same time. 

 A set of smaller fibres similar to those coming from the skin in 

 the somatic branch are also found in the visceral branch, and 

 they seem to be connected with smaller cells in the ganglion. 



V.N. 



FIG. 75. Structure of a Typical Spinal Nerve. P.R., dorsal root with 

 ganglion; A.R., ventral root; S.I., ganglion of sympathetic chain; 

 W.R., its white ramus ; Q.R., its grey ramus ; V.N., visceral nerve 

 with collateral ganglion ; S.N~,, somatic nerve. 



2nd. The Outgoing Fibres are characterised by their small size, 

 take origin chiefly in a lateral column of cells, which is well 

 developed in the dorsal region of the cord, and pass out as 

 medullated fibres by the ventral root. From this they pass 

 by the white root to a sympathetic ganglion, whence they may 

 proceed in one of two different ways (fig. 75). 



(a) They may form synapses with cells, and fibres from 

 these cells may pass 



1. Outwards with the splanchnic nerves ; or, 



2. Back into the spinal nerve by the grey root, and so down 

 the somatic nerve to blood vessels, muscles of the hairs, sweat 

 glands, etc. The ganglia from which fibres pass back into spinal 

 nerve are known as lateral ganglia. 



