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 



