NERVOUS SYSTEM; ITS GENERAL STRUCTURE. 



205 



to an adjacent one. The occasional appearance of a termination to a nervous 

 fibril is caused by its dipping down between the muscular fibres, to pas^ to- 

 wards another stratum. The nerves are almost exclusively of the motor 

 kind; but a few sensory are blended with them. We see this most clearly 

 in cases in which the motor and sensory trunks supplying the muscles are 

 distinct; as in the muscles of the orbit. The non-striated muscles are very 

 sparingly supplied with nerves ; and these are derived (for the most part, if 

 not entirely), from the Sympathetic system, rather than from the Cerebro- 

 Spinal. 



241. We have, lastly, to consider the structure, composition, actions, and 

 mode of growth and regeneration of the Nervous Tissue ; the one which is 

 most distinctive of the Animal fabric, and which serves as the instrument of 

 the operations that are most peculiar to it. Wherever a distinct Nervous Sys- 

 tem can be made out (which has not yet been found possible in the lowest of 

 those beings, that, from their general structure and habits of life, are unques- 

 tionably to be ranked in the Animal Kingdom), it consists of two very different 

 forms of structure ; the presence of both of which, therefore, is essential to 

 our idea of it as a whole. We observe, in the first place, that it is formed of 

 trunks, which are distributed to different parts of the body, and especially to 

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

 which the central terminations of those trunks come into connexion. 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 in- 

 fluence of operations which take place at their central or peripheral extremi- 

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

 the portion thus cast off from the ganglion is distributed, are completely para- 

 lyzed ; that is, no impression made upon them is felt as a sensation ; 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 manner paralyzed. But if, 



Fig. 109. 



Dorsal ganglion of Sympathetic nerve of Mouse ; a, 6, cords of connection with adjacent sympathetic 

 ganglia ; c, c, c, c, branches lo the viscera and spinal nerves ; d, ganglionic globules or cells j e, nervous 

 fibres traversing the ganglion. 



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 sup- 

 plied by the separated portion of the trunk ; which shows that the part re- 

 maining in connexion with the ganglion is still capable of conveying impres- 

 18 



