124 CHAPTER 14. 



Tlic second, or Sympathetic, oi* Ganglionic system consists of a double 

 chain of ganglia, one on each side, phiced along the spinal column from 

 the cranium to the coccyx, and connected to each other by nervous cords. 

 Each ganglion is connected not only with the one immediately before and 

 behind it, but also with the cerebro-spinal system by one or two filaments. 

 From these ganglia the nervous filaments are given off, which pass to the 

 thoracic, abdominal, and pelvic viscera. This system is concerned prin- 

 cipally with the processes of organic life. 



The separation of the nervous system into two divisions is certainly 

 convenient for the purposes of explanation ; but the doctrine implied in 

 it, namely, that there are two distinct nervous systems is objected to by 

 many authors. The pneumogastric nerves, for instance, although belong- 

 ing to the cerebro-spinal system, which presides over the processes of 

 animal life, are yet associated with some important functions of organic 

 life. 



The Cerebro-spinal nerves have a white colour, whilst the Sympathetic 

 are yellowish grey. In them the outer layer of white substance is 

 wanting. 



A ganglion is an independent nerve-centre, consisting of vesicular 

 nerve maiter traversed by tubular and gelatinous nerve fibres, enclosed 

 in a fine membrane of areolar tissue. 



242. The Nerves. 



Each nerve consists of a bundle of nerve fibres enclosed in a sheath. 

 It is connected at one end, called its origin, either to the cerebro-spinal 

 system or to one of the ganglia of the sympathetic system. The other 

 end is distributed to various parts of the body. 



The nerve fibres of both the cerebro-spinal and sympathetic systems 

 convey two kinds of impression, namely sensation and motion. The 

 sensitive nerves convey impressions from their further or peripheral ex- 

 tremities to the nervous centres, and from thence to the brain. The 

 motor nerves, on the other hand, transmit impressions from the neiwous 

 centres to the parts to which they are distributed, producing muscular 

 action and influencing various functions of the body. There are no 

 characteristic distinctions between sensitive and motor nerves, except 

 that the tubes of the sensitive are smaller than those of the motor. 



Nerves of special sense differ from nerves of common sense only in 

 their functions. Irritation of them does not produce pain, but gives rise 

 to various phenomena peculiar to each. 



Nearly all nerves except those of special sense consist of both sensitive 

 and motor filaments. They are distinct only at their origin. Afterwards 

 they mingle together. Reflex action is produced by communication of 

 impressions from sensitive to motor fibres. All reflex actions are in- 

 voluntary. 



A motor fibre can only convey motor impulses, a sensitive fibre only 

 sensation, whilst nerves of special sense can only convey impressions 

 which produce peculiar sensations. In certain parts, however, of their 

 course nerves form plexuses and anastomose with each other and intei;- 



