NERVOUS SYSTEM. (NERVE.) 



593 



from each other, and are extremely variable in 

 shape and size. Two conditions appear to 

 favour the production of this varicose state of 

 the nerve-tube, namely, a feeble power of re- 

 sistance in the tubular membrane, and, se- 

 condly, perhaps, a semi-fluid state of the con- 

 tained nervous pulp; and hence we find that 

 some nerve-tubes are much more prone to 

 become varicose than others. In the nerves 

 of pure sense the tubes are very delicate in 

 structure and very apt to exhibit this change of 

 form, and in the brain and spinal cord they 

 exhibit the same tendency. Ehrenberg sup- 

 posed formerly that these varicosities were 

 natural and existed during life, and that they 

 afforded a valuable morphological character of 

 the nerves of pure sense and the cerebro-spinal 

 centres. But many circumstances favour the 

 opinion that the varicosities are accidental ; 

 thus, the very irregularities above noticed in 

 their shape, size, form, and number on a single 

 tube are not likely to occur in the natural 

 state. Moreover, in a piece of the brain or 

 spinal cord not much pressed nor torn, the 

 cylinders may be distinctly seen : even in 

 the manipulated specimens the varicose tubes 

 form only a small portion of the whole. And 

 in those nerves whose fibres are not prone 

 to become varicose, such as muscular nerves, 

 they may be made so by firm pressure and 

 violence in manipulation. In the nerve-tubes 

 of young animals, in whom the tissues are more 

 tender and contain more abundant water, these 

 changes are also very apt to take place. 



A cerebro-spinal nerve, then, consists of a 

 congeries of fascicles or bundles of the nerve- 

 fibres or nerve-tubes (and we shall use these 

 terms synonymously) above described, enve- 

 loped and bound together by fibrous mem- 

 brane, the nerve-sheath. The nerve-tubes lie 

 side by side, parallel, and sometimes have 

 a wavy course within the general sheath (Jig. 

 331). The relation of the nerve-tubes to each 



Fig. 331. 



Diagram to illustrate the wavy course of the nerve 

 tubes within the neurilemma. 



other is simply that of juxta-position. All 

 observers, from Fontana down to those of 

 the present day, agree in denying the ex- 

 istence of any inosculation or anastomosis 

 between the fibres in vertebrate animals; 

 and it seems almost certain that this complete 

 isolation of the nerve-tubes is not limited 

 to those of the nerves, properly so called, but 

 may be observed in the nervous centres also. 

 When a piece of nerve is examined on a dark 

 ground, as an opaque object, with an object 

 glass of a quarter of an inch focus, the disposi- 

 tion and relation of its component tubules are 

 more beautifully seen than by any other mode 

 of examination. The primitive fibres present 

 the appearance of a series of transparent tubes, 

 containing an exquisitely delicate, soft, pearly- 

 white material. 



VOL. III. 



In point of size the nerve tubes present 

 considerable variety even in the same trunk, 

 while they maintain an identity of structure. 

 The smallest tubes have very delicate walls, 

 and are more easily rendered varicose than the 

 larger ones. The following table gives a state- 

 ment of the results of the admeasurement of 

 the cerebro-spinal tubules in Man and other 

 Vertebrata. 



Man, and other Mammalia, from ^ to ^ s 

 of an inch. 



Birds, ^ m to 5^5 of an inch. 



Reptiles, Frog, B ' g5 to ^ of an inch. 



Fish, Eel, Ts ' 33 of an inch. 



Codfish, optic nerve, gL, of an inch. 



It has already been remarked that no such 

 thing as subdivision or branching of the pri- 

 mitive tubules takes place in the cerebro-spinal 

 nerves of the vertebrate series. Whatever be 

 the connection which each primitive tubule 

 forms with the nervous centre, or with the 

 textures to which it is distributed at its pe- 

 riphery, it passes from one point to the other 

 without any change, save perhaps in size, and 

 without any communication with neighbour- 

 ing tubules, beyond simple juxta-position, or 

 investment by a common sheath. This fact 

 was recognized by Fontana, whose description 

 of the structure of nerve, although drawn up 

 from observations made at a great disadvan- 

 tage through the imperfection of his instru- 

 ments, corresponds in all essential particulars 

 with modern observations.* And as there is 

 the same absence of subdivision in the con- 

 tinuations of these nerve-tubes in the nervous 

 centres, we may fairly infer that each point on 

 the periphery which is in contact with a nerve- 

 tube, is, as it were, represented by that same 

 nerve-tube in the centre. 



The structure of the cerebro-spinal nerve 

 admits of an obvious comparison with that 

 of the striped muscle. Both are composed 

 of bundles of fibres, united by a sheath, 

 which also passes between the bundles, 

 and is a nidus for the support of the nu- 

 trient vessels. Both admit of being subdi- 

 vided into primitive fibres, which are very 

 analogous in structure. The primitive fibre of 

 muscle (primitive fasciculus of some authors) 

 consists of the true muscular tissue, or sarcous 

 elements contained in a transparent sheath of 

 homogeneous elastic membrane called sarco- 

 lemma by Mr. Bowman. The peculiar mor- 

 phological characters of the primitive fibre 

 depend upon the arrangement of the sarcous 

 particles within this transparent tube ; and to 

 this arrangement is due any further subdivision 

 of which the primitive muscular fibre may be 

 susceptible. So is it with the primitive nerve- 

 fibre : its tubular membrane is strictly analo- 

 gous in structure and other characters with the 

 sarcolemma. It contains the elements of the 

 true nervous tissue or neurine, and this ad- 

 mits of a certain subdivision which may be ren- 

 dered more apparent under the influence of re- 

 agents, and which is variously interpreted by 

 different observers, and has been compared to 



* Fontana sur le venin de la vipnre. 



2 Q 



