ELEMENTARY STRUCTURE OF NERVOUS SYSTEM. 



91 



the auditory nerves, are equally small in every part of their course ; and thus 

 resemble, in size as well as in structure, those portions of the other nervous 

 fibres which are continued into the nervous centres. A different structure 

 has been described by Ehrenberg, as composing the bulk of the medullary 

 substance of the brain, under the name of varicose tubes ; and he states that 

 these are also found largely in the spinal cord, and less abundantly in the 

 nerves of special sense ; but that they are seldom to be met with in the other 

 trunks. These tubes were so named, from their not being cylindrical, but 

 presenting dilatations at intervals, so as to resemble a string of beads ; and the 

 appearance of these dilatations has given rise to the opinion, that the brain is 

 composed of globules. It is now, however, satisfactorily shown that they are 

 the result of the pressure and other manipulations, to which the objects are 

 subjected in preparation for the microscope ; and that, if the nervous fibres of 

 the brain and other parts are examined in a recent state, they are cylindrical, 

 like those of the nervous trunks in general. Still there must be some differ- 

 ence in their structure, since they exhibit this tendency to become varicose, 

 which, is elsewhere wanting ; this difference appears chiefly to consist in the 

 greater thinness and delicacy of the walls of the tube itself, rendering it more 

 liable to be distended at certain points, by the accumulation of the contained 

 substance in little masses, when coagulating. Besides these tubular fibres, 

 which constitute the white portions of the nervous matter, there are other fila- 

 ments of a yellowish-gray colour, and of about half the usual diameter, with- 

 out distinct cavities, which exist especially in the sympathetic nerves, but 

 which may also be detected in others. These fibres may be termed organic; 

 those existing in the sympathetic system of nerves may be traced to its gan- 

 glipnic centres ; whilst those which are found in the cerebro-spinal nerves, are 

 connected with the ganglia upon their posterior roots.* 



Fig. 7. 



Structure of nerve-tubes, magnified 350 Diam. A, cylindrical tubuli from nerve. B, varicose tubuli 

 from brain, c, nerve-tubes, of which one exhibits the remains of nuclei in its walls. (After Wagner.) 



111. The other elementary form of Nervous structure is termed the dneri- 

 tious or gray matter. It seems to consist principally of a plexus of blood-ves- 

 sels, into which the fibres of the former may be traced ; and amongst these lie 

 a number of nucleated globules, which do not seem to have any definite ar- 

 rangement. This substance is usually disposed in the interior of the larger 



* The distinctness of the sympathetic fibres from the cerebro-spinal, which has been 

 denied by some Microscopists, appears to have been fully established by the recent 

 laborious inquiries of Volkmann and Bidder. 



