830 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



nected with them ; but each system transmitting its own fibres into the trunks 

 of the other, and even ganglia of the Sympathetic being dispersed among the 

 Cerebro-spinal tubules. 



847. It is in virtue of the connections of the Sympathetic with the Cerebro- 

 spinal system, that the parts which are solely supplied with nerves from the 

 former are capable of transmitting sensory impressions to the Sensorium, and 

 of receiving motor impulses from the Encephalic centres. It is true that, under 

 ordinary circumstances, these parts are insensible, that is, impressions made upon 

 them do not travel onwards through the Spinal Cord to the Encephalon ; but 

 their sensibility is acutely manifested in morbid states, in which the impressions 

 seem to be propagated further than usual, in virtue of their greater potency. 

 That it is the office of the ganglia on the roots of the Spinal nerves to " cut off 

 sensation/' that is, to prevent the further transmission of sensory impressions, 

 is an old doctrine ; and there seems much reason to believe that this may be 

 effected by the free communication between one fibre and another, which is es- 

 tablished through the vesicular substance of a ganglion, so that the whole force 

 of ordinary impressions on the nerve-fibres is lost in diffusion among the rest of 

 their contents. The same principle seems to apply to the motor fibres ; for there 

 are cases which show that when fibres obviously belonging to Cerebro-spinal 

 nerves pass through Sympathetic ganglia, they do not so rapidly or so surely 

 transmit motor impulses, as when they have no; such relation to ganglia. 1 

 Although it is not easy to obtain definite evidence of the influence of the Sym- 

 pathetic system on Muscular Contraction, since this influence is extinguished 

 within a short time after death, yet it has been established by the elaborate re- 

 searches of Prof. Valentin 3 that contractions of the various muscular parts sup- 

 plied by the three great visceral plexuses may be excited by irritation applied 

 to their nerves and ganglia. But he has further shown that the same effects 

 may be produced by irritating the Prevertebral ganglia, the cords of commu- 

 nication with the Spinal nerves which have been sometimes termed the roots of 

 the Sympathetic, and the roots of the Spinal nerves themselves. It results 

 from his inquiries, that, although any particular division of the Sympathetic 

 nerve must be regarded as extremely complex in its relations, deriving its motor 

 fibres from many different sources, the ultimate distribution of these fibres is 

 sufficiently simple, so that each organ is definitely supplied from a certain part 

 of the cerebro-spinal axis. But the fibres proceeding from the roots of the 

 cerebro-spinal nerves do not pass into the nearest organs, being transmitted 

 through three or more of the prevertebral ganglia of the Sympathetic before 

 reaching their ultimate destination ; thus the motor fibres of the cardiac plexus 

 are principally derived from the cervical portion of the Spinal Cord, those of 

 the solar plexus from the thoracic region, and those of the hypogastric plexus 

 from the dorsal region. No experimental evidence has yet been afforded, that 

 the proper fibres of the Sympathetic System have any power of exciting mus- 

 cular contraction, or that its ganglia can serve as centres of reflex action to the 

 organs which they supply ; on the contrary, it is quite certain that the ganglia 

 in the posterior roots of the Spinal nerves have no such endowment. And as 

 all the facts which have been supposed to indicate the existence of such a power 

 may be otherwise explained in accordance with our fundamental doctrine ( 

 432-3, 497-8), it seems fair to conclude that the motor power of the Sympathetic 

 system which is chiefly exercised on the muscular substance of the heart and 

 the walls of the bloodvessels, on the muscular coat of the alimentary canal and 

 of the larger gland-ducts that open into it, and on the muscular walls of the 



1 See Messrs. Kirkes and Paget's " Handbook of Physiology," p. 361, Am. Ed. 



2 "De Functionibus Nervorum Cerebralium et Nervi Sympatkici," Bernae, 1839; lib. ii. 

 cap. 2. 



