STRUCTURE OF NERVOUS TISSUE. 



335 



formed of trunks, which are distributed to different parts of the body, and espe- 

 cially to the muscles and to the sensory surfaces; and of ganglia, or masses with 

 which the central terminations of those trunks come into connection, It is 

 easily established by experiment, that the trunks themselves have no power of 

 originating changes; and that they only serve to conduct or convey the influence 

 of operations, which take place at their central or their peripheral extremities. 

 For if a trunk be divided in any part of its course, all the organs to which the 

 portion thus cut off from the ganglion is distributed, are completely paralyzed ; 

 that is, no impression made upon them is felt as a sensation, or produces any 

 respondent movement ; and no motion can be excited in them by any act of the 

 mind. Or, if the substance of the ganglion be destroyed, all the parts which 

 are exclusively supplied by nervous trunks proceeding from it, are in like man- 

 ner paralyzed. But if, when a trunk is divided, the portion still connected with 

 the ganglion be pinched or otherwise irritated, sensations are felt, which are 

 referred to the points supplied by the separated portion of the trunk ; which 

 shows that the part remaining in connection with the ganglion is still capable of 

 conveying impressions, and that the ganglion itself receives these impressions, 

 and makes them felt as sensations. On the other hand, if the separated portion 

 of the trunk be irritated, motions are excited in the muscles which it supplies ; 

 showing that it is still capable of conveying the motor influence, though cut off 

 from the usual source of that influence. 



337. In the ordinary Nerve-trunks, we find only one form of Nervous tissue; 

 that which may be designated as the fibrous or tubular. In the Ganglia, we 

 find, in addition to this, a substance made up of peculiar cells or vesicles ; which 

 may be distinguished as the vesicular nervous matter. In fact, the character of 

 a Granglionic centre (which is frequently not otherwise clearly distinguished as 

 such) is derived from the presence of this vesicular substance (Fig. 105). 



Fig. 105. 



Dorsal Ganglion of Sympathetic nerve of Mouse: a, b, cords of connection with adjacent sympathetic ganglia; 

 c, c, c, c, branches to the viscera and spinal nerves ; d, ganglionic globules or cells ; e, nervous fibres traversing 

 the ganglion. 



338. The ultimate Nerve- Fibre, in its most complete form such as is pre- 

 sented to us in the ordinary cranio-spinal nerves is distinctly tubular; being 

 composed of an external cylindrical membranous sheath, within which the pecu- 

 liar nervous matter is contained. This membranous tube, like the Myolemma 

 of muscular fibre, is extremely delicate and transparent; and is nearly or quite 

 homogeneous. It is not penetrated by bloodvessels; nor is it ever seen to branch 

 or anastomose with others; so that there is reason to regard it as forming one 



