NERVOUS SYSTEM. (NERVE.) 



591 



no means free from them, and they will be 

 employed in the course of this article. 



Such are the only subdivisions of the nervous 

 system which anatomy appears to warrant. 

 Others have been proposed ; but as they are 

 founded upon physiological opinions which are 

 as yet hypothetical, it is unnecessary to discuss 

 them at present. 



Having thus given a brief and general account 

 of the nervous system and of nervous matter, 

 we proceed to consider the anatomy and phy- 

 siology of this system under the following 

 divisions: I. THE GENERAL ANATOMY OF 



NERVES. II. THE COMPARATIVE ANATOMY 

 OF THE NERVOUS SYSTEM OR ITS DISPOSI- 

 TION THROUGHOUT THE ANIMAL KINGDOM. 

 III. THE ANATOMY OF THE NERVOUS CEN- 

 TRES, THE GANGLIA, BRAIN, AND SPINAL 



CORD. IV. AND LASTLY, THE OFFICE OF 



THE NERVOUS SYSTEM AND THE FUNCTIONS 

 OF ITS VARIOUS PARTS. 



NERVE. (vsvgov, nervus; Germ, nerve; 

 Fr. nerf.) The nerves perform the inttrnuncial 

 office in the nervous system by maintaining 

 communications between the various organs 

 and tissues and the nervous centres. They are 

 bundles of threads of various size, surrounded 

 by sheaths of membrane, with more or less of 

 areolar tissue interposed. 



The nerves of the cerebro-spinal system and 

 of the great sympathetic exhibit such different 

 characters as regards their anatomy, that they 

 may be examined separately. 



Cerebro-spinal nerves. In examining a cere- 

 bro-spinal nerve, we find it invested by a sheath 

 of membrane, which has adherent to its inner 

 surface thin layers of areolar tissue which pass, 

 like so many partitions, between the threads or 

 fibres of which the nerve is composed. This 

 sheath is commonly called the neurilemma; it 

 is analogous to the sheath which surrounds mus- 

 cles. Its office is chiefly mechanical, namely, 

 that of binding the constituent fibrillae and fas- 

 cicles of the nerve together, so as to protect 

 them and to support the delicate plexus of 

 capillary bloodvessels from which they derive 

 their nutriment. 



The neurilemma is composed of fibres of the 

 white fibrous kind. It exhibits to the naked 

 eye the appearance of a fibrous membrane, 

 white and almost silvery ; and its microscopic 

 characters are those of the white fibrous ele- 

 ment, although not presenting much appearance 

 of wavy fibres. The fibres are, for the most 

 part, parallel to the axis of the nerve; but 

 there are some which cross the nerve at right 

 angles, or appear to pass spirally round it. The 

 septa between the secondary bundles of the 

 nerves seem to consist of a less perfect fibrous 

 tissue, containing the remains of numerous 

 cytoblasts. A yellow fibrous tissue of the 

 finest kind exists here in very small quantity. 



The bloodvessels are distributed upon the 

 external investing sheath and upon the septa. 

 In some of the large nerves, the sciatic for 

 example, these may be often seen minutely 

 injected with blood. They are disposed some- 

 what similarly to those of muscles, running 



parallel to the fibres and fascicles of the nerve. 

 The capillaries are among the smallest in the 

 body : they form oblong meshes of considerable 

 length, completed at long intervals by vessels 

 which cross the fibres of the nerve more or less 

 in the transverse direction. Ilenle assigns to 

 them, when empty, a diameter not exceeding 

 g g g s th of an inch. These bloodvessels are 

 generally derived from neighbouring arterial 

 branches; sometimes a special vessel accom- 

 panies a nervous trunk, and even perforates it, 

 passing along its central axis, as is well known 

 to descriptive anatomists in the sciatic and the 

 optic nerves. 



After the external part of the neurilemma 

 has been dissected off, the nerve may be torn by 

 needles and divided into secondary bundles and 

 fibres. The ultimate fibre can then be readily 

 distinguished by the aid of the microscope, from 

 its being incapable of further subdivision by 

 mechanical means ; and an accurate knowledge 

 of its structure is of the utmost importance to 

 the formation of correct opinions respecting the 

 actions of the nervous system. 



The ultimate (or, as it is also called, the pri- 

 mitive) nervous fibre, is a tube composed of a 

 fine transparent homogeneous membrane, in a 

 great degree resembling the sarcolemma of 

 muscle. It is elastic, like that membrane, per- 

 fectly homogeneous, and, according to Schvvann, 

 in young nerves has the nuclei of cells connected 

 with it. It may be called the tabular membrane 

 of nerve (a, Jig- 329). The contents of this tube 

 consist in a soft, semifluid, whitish, pulpy sub- 

 stance, which is readily pressed out of its cut 

 extremity. In the nerve that is quite fresh, 

 having been taken from an animal just dead, 

 this pulpy matter is quite transparent and appa- 

 rently homogeneous. The tube membrane pre- 

 sents the appearance of a delicate line, resem- 



Fig. 329. 



Nerve tubes altered by re-agents. 



a, tube altered by water. The light external line 

 is the tubular membrane ; the dark inner double- 

 edged one, broken here and there, is the white 

 substance of Schwann. b, shows the change pro- 

 duced by the action of ether on the nerve-tube of 

 the common eel ; several oil globules have coalesced 

 in the interior, and others have accumulated around 

 the exterior of the tube. 



