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 (fig. 331). The relation of the nerve-tubes to each 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.
In point of size the nerve Lubec 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 Tdo to thi, of an inch.
Birds, 1.0,S6 to Nam of an inch.
Reptiles, Frog, Thz tO iliNy of an inch. Fish, Eel, Tdo of an inch.
Codfish, optic nerve, Ad 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 vertebmte 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 fascieulus 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. Sn 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 v-ariously interpreted by different observers, and has been compared to the separation which talces place in the fat cells between the solid and the fluid elements of fat. As the combination of the primitive muscular fibres, in a common sheath, forms the muscle, so the union of the primitive nervous fibres, in a similar way, forms the nerve. And as the primitive fibre of muscle passes undivided from one point of the muscle to another, so the nerve tube exhibits no subdivision in its course.
Branching of nerves.—The main trunk of a nerve breaks up into its component bundles, as it passes from centre to periphery, yielding up branches to the various parts it is destined to connect with the nervous centre. These branches generally come off at acute angles, and soon plunge into the muscles and other parts to which they tend, dividing and subdividing as they pro ceed. Such is the most common mode of sub division, but there are many exceptions : some times a branch separates from the parent trunk at an acute angle, and then turns to run in an op posite direction, forming an arch, from the con vexity of which several branches are given off'. Such a nerve is said to be recurrent; the in ferior laryngeal nerve takes this course. The anastomotic arches between the emerging spinal nerves, round the vertebral laminw, are also exceptions to the separation at acute angles.