SYMPATHETIC SYSTEM. 163 
there are also visceral fibres of both kinds, supplying the viscera 
(alimentary canal, excretory and reproductive organs) and the circula- 
tory system. The dorsal and ventral rami contain mostly somatic 
fibres with a few of the visceral type, while the visceral rami are com- 
posed of visceral fibres alone. The farther subdivision of these nerves 
will be considered later. 
To the statement that the dorsal roots are purely sensory the exception must be 
made that in the lower vertebrates some of the visceral motor fibres, arising in 
the neighborhood of the lateral cornu, pass out from the cord through the dorsal 
root. In the mammals they are said to leave by the ventral roots like all other 
motor fibres. 
In the regions of the appendages the spinal nerves usually form 
networks or plexuses, branches of a varying number of ventral rami 
interlacing in a complicated manner before entering the appendage. 
Plexuses are poorly developed in the fishes, but here many spinal nerves 
are united before entering a limb by means of a longitudinal ‘col- 
lector’ nerve, there being no exchange of fibres such as occurs in a 
plexus. In the amphibia there are two plexuses, a cervico-brachial 
near the fore limb, and a lumbo-sacral for the hind limb. In the 
higher groups there may be four plexuses: cervical, brachial, lum- 
bar and sacral, the positions of which are indicated by their names. 
The Sympathetic System. 
The function of the sympathetic system is the control of the viscera, 
various glands, the smooth muscles, and through the latter, of the size 
of the blood-vessels and the supply of blood to the various parts. 
The system is connected with the spinal nerves by the visceral rami 
(rami communicantes) already mentioned. As has just been said, 
these visceral rami contain both motor and sensory fibres. As these 
rami extend downward in their development, they carry with them 
ganglion cells derived from the ganglia of the dorsal roots of the 
spinal nerves, and these give rise to the sympathetic ganglia. Of 
these there are three groups. Nearest to the spinal nerves on either 
side are a series of the sympathetic trunk (chain ganglia), usually 
connected with each other by a longitudinal sympathetic trunk. 
Nerves run from these chain ganglia to the prevertebral ganglia, 
some of which, like the cardiac, pelvic, hypogastric and solar 
(plexuses) are of considerable size. From these nerves go to the 
