Courvoisier describes both fibres as acquiring a medullary sheath, the straight one first. He has found the above described structure in the ganglia of fish, birds, and mammals; but whilst in the frog the cell has never, or scarcely ever, more than one straight and very rarely more than one spiral fibre, he finds that in other vertebrates a cell may give off such twin fibres from two or more parts of its circumference. In the spinal ganglia of the skate, torpedo, and dog-fish, there is a different arrangement. In these, as first pointed out by R. Wagner, two fibres are connected with each ganglion cell, at opposite sides or opposite poles,-one directed centrally toward the root of the nerve, and the other outwardly towards its branches. CEREBRO-SPINAL NERVES. These are formed of the nerve-fibres already described, collected together and bound up in sheaths of connective tissue. A larger or smaller number of fibres inclosed in a tubular sheath form a slender round cord of no determinate size, usually named a funiculus; if a nerve be very small it may Fig. LXXXIV. consist of but one such cord, but in larger nerves several funiculi are united together into one or more bundles, which, being wrapped up in a common membranous covering constitute the nerve (fig. LXXXIV.). Accordingly, in dissecting a nerve, we first come to an outward covering, formed of connective tissue, often so strong and dense that it might well be called fibrous. From this common sheath we trace lamina passing inwards between the larger and smaller bundles of funiculi, and finally between the funiculi themselves, connecting them together as well as conducting and supporting the fine blood-vessels which are distributed to the nerve. But, besides the interposed areolar tissue which connects these smallest cords, each funiculus has a special sheath of its own, as will be further noticed presently. Fig. LXXXIV.-PORTION OF THE TRUNK OF A NERVe, A, the nerve; B, a single funiculus drawn out from the rest (from Sir C. Bell). The common sheath and its subdivisions consist of connective tissue, presenting the usual white and yellow constituent fibres of that texture, the latter being present in considerable proportion. The special sheaths of the funiculi, on the other hand, appear to be formed essentially of a fine transparent membrane, which may without difficulty be stripped off in form of a tube from the little bundle of nerve-fibres of which the funiculus consists. When examined with a high power of the microscope, this membrane presents the aspect of a thin transparent film, which in some parts appears to be quite simple and homogeneous, but is more generally marked with extremely fine reticulated fibres. Corpuscles resembling elongated cellnuclei may also he seen upon it when acetic acid is applied. The tissue investing a nerve and inclosing its proper fibres, as now described, is named the neurilemma, and the term is for the most part applied indiscriminately to the whole of the enveloping structure, though some anatomists use it to denote only the sheaths of the funiculi and smaller fasciculi, whilst they name the general external covering of the nerve its "cellular sheath" (vagina cellulosa). Some recent writers, believing that the primitive sheath or membranous tube of the nerve-fibre corresponds to the sarcolemma of muscle, have proposed to designate it as the neurilemma, and to use the term perineurium for the coarser sheathing of the nerves and nervous cords, to which the terin CEREBRO-SPINAL NERVES. cxlv neurilemma has been heretofore applied. The use of the term perineurium is unobjectionable and may sometimes be convenient, but the proposed new and restricted application of the term neurilemma will, I think, lead to ambiguity, and is of doubtful propriety. The funiculi of a nerve are not all of one size, but all are sufficiently large to be readily seen with the naked eye, and easily dissected out from each other. In a nerve so dissected into its component funiculi, it is seen that these do not run along the nerve as parallel insulated cords, but join together obliquely at short distances as they proceed in their course, the cords resulting from such union dividing in their further progress to form junctions again with collateral cords; so that in fact the funiculi composing a single nervous trunk have an arrangement with respect to each other similar to that which we shall presently find to hold in a plexus formed by the branches of different nerves. It must be distinctly understood, however, that in these communications the proper nerve-fibres do not join to gether or coalesce. They pass off from one nervous cord to enter another, with whose fibres they become intermixed, and part of them thus intermixed may again pass off to a third funiculus, or go through a series of funiculi and undergo still further intermixture; but throughout all these successive associations (until near the termination of the nerve) the fibres remain, as far as known, individually distinct, like the threads in a rope. The fibres of the cerebro-spinal nerves are chiefly, in some cases perhaps exclusively, of the white or medullated kind, but in most instances there are also grey fibres in greater or less number. Moreover, it has often appeared to me as if there were filaments of extreme tenuity, like the white filaments of connective tissue, but of doubtful nature, mixed up with wellcharacterised nerve-fibres within the sheaths of the funiculi. Lying alongside each other, the fibres of a funiculus form a little skein or bundle, which runs in a waving or serpentine manner within its sheath; and the alternate lights and shadows caused by the successive bendings being seen through the sheath, give rise to the appearance of alternate light and dark cross stripes on the funiculi, or even on larger cords consisting of several funiculi. On stretching the nerve, the fibres are straightened and the striped appearance is lost. 6000 Vessels. The blood vessels of a nerve supported by the sheath divide into very fine capillaries, said by Henle to measure in the empty state not more thanth of an inch in diameter. These, which are numerous, run parallel with the fibres, many of them within the funicular sheaths, but are connected at intervals by short transverse branches, so as in fact to form a network with long narrow meshes. Branching and conjunction of Nerves.-Nerves in their progress very commonly divide into branches, and the branches of different nerves not unfrequently join with each other. As regards the arrangement of the fibres in these cases, it is to be observed, that, in the branching of a nerve, collections of its fibres successively leave the trunk and form branches; and that, when different nerves or their branches intercommunicate, fibres pass from one nerve to become associated with those of the other in their further progress; but in neither case (unless towards their peripheral terminations) is there any such thing as a division or splitting of an elementary nervefibre into two, or an actual junction or coalescence of two such fibres together. A communication between two nerves is sometimes effected by one or two connecting branches. In such comparatively simple modes of connec tion, which are not unusual, both nerves commonly give and receive fibres; so that, after the junction, each contains a mixture of fibres derived from two originally distinct sources. More rarely the fibres pass only from one of the nerves to the other, and the contribution is not reciprocal. In the former case the communicating branch or branches will of course contain fibres of both nerves, in the latter of one only. In other cases the branches of a nerve, or branches derived from two or from several different nerves, are connected in a more complicated manner, and form what is termed a plexus. In plexuses-of which the one named "brachial" or "axillary," formed by the great nerves of the arm, and the "lumbar" and "sacral," formed by those of the lower limb and pelvis, are appropriate examples-the nerves or their branches join and divide again and again, interchanging and intermixing their fibres so thoroughly, that, by the time a branch leaves the plexus, it may contain fibres from all the nerves entering the plexus. Still, as in the more simple communications already spoken of, the fibres, so far as is known, remain individually distinct throughout. Some farther circumstances remain to be noticed as to the course of the fibres in nerves and nervous plexuses. Gerber has described and figured nerve-fibres, which, after running a certain way in a nerve, apparently join in form of loops with neighbouring fibres of the same bundle, and proceed no further. Such loops might of course be represented as formed by fibres which bend back and return to the nervous centre; and so Gerber considers them. He regards them as looped terminations of sentient fibres appropriated to the nerve itself as the nervi nervorum, in short, on which depends the sensibility of the nerve to impressions, painful or otherwise, applied to it elsewhere than at its extremities. The whole matter is, however, involved in doubt for, admitting the existence of the loops referred to, which yet requires confirmation, it is not impossible that they may be produced by fibres which run back only a certain way, and then, entering another bundle, proceed onwards to the termination of the nerve. Again, it has been supposed, that, in some instances, of nervous conjunctions, certain collections of fibres, after passing from one nerve to another, take a retrograde course in that second nerve, and, in place of being distributed peripherally with its branches, turn back to its root and rejoin the cerebro-spinal centre. An apparent example of such nervous arches without peripheral distribution is afforded by the optic nerves, in which various anatomists admit the existence of arched fibres that seem to pass across the commissure between these nerves from one optic tract to the other, and to return again to the brain. These, however, are perhaps to be compared with the commissural fibres of the brain itself, of which there is a great system connecting the symmetrical halves of that organ. But instances of a similar kind occurring in other nerves have been pointed out by Volkmann; as in the connection between the second and third cervical nerves of the cat, also in that of the fourth cranial nerve with the first branch of the fifth in other quadrupeds, and in the communications of the cervical nerves with the spinal accessory and the descendens noni. But certain fibres of the optic nerves take a course deviating still more from that followed generally, for they appear to be continued across the commissure from the eyeball and optic nerve of one side to the opposite nerve and eye, without being connected with the brain at all, and thus to form arches with peripheral terminations, but no central connection. In looking, however, for an explanation of this arrangement, it must be borne in mind that the retina contains nerve-cells, like those of the nervous centres, and perhaps the fibres referred to may be intended merely to bring the collections of nerve-cells of the two sides into relation independently of the brain. Julius Arnold has found an arrangement of fibres at the junctions of the nerve-plexus of the iris similar to that in the optic commissure.* The disposition of the fibres at the points of division and junction of the branches * Virchow's Archiv. 1863. of nerves still requires further investigation. For some interesting observations on the subject the reader is referred to a paper by Dr. Beale.* Origins or Roots of the Nerves.-The cerebro-spinal nerves, as already said, are connected by one extremity to the brain or to the spinal cord, and this central extremity of a nerve is, in the language of anatomy, named its origin or root. In some cases the root is single, that is, the funiculi or fibres by which the nerve arises are all attached at one spot or along one line or tract; in other nerves, on the contrary, they form two or more separate collections, which arise apart from each other and are connected with different parts of the nervous centre, and such nerves are accordingly said to have two or more origins or roots. In the latter case, moreover, the different roots of a nerve may differ not only in their anatomical characters and connections, but also in function, as is well exemplified in the spinal nerves, each of which arises by two roots, an anterior and a posterior the former containing the motory fibres of the nerve, the latter the sensory. The fibres of a nerve, or at least a considerable share of them, may be traced to some depth in the substance of the brain or spinal cord, and hence the term "apparent or superficial origin" has been employed to denote the place where the root of a nerve is attached to the surface, in order to distinguish it from the "real or deep origin" which is beneath the surface and concealed from view. To trace the different nerves back to their real origin, and to determine the points where, and the modes in which their fibres are connected with the nervous centre, is a matter of great difficulty and uncertainty; and, accordingly, the statements of anatomists respecting the origin of particular nerves are in many cases conflicting and unsatisfactory. Confining ourselves here to what applies to the nerves generally, it may be stated, that their roots, or part of their roots, can usually be followed for some way beneath the surface, in form of white tracts or bands distinguishable from the surrounding substance; and very generally these tracts of origin may be traced towards deposits of grey nervous matter situated in the neighbourhood; such, for instance, as the central grey matter of the spinal cord, the grey centres of the pneumo-gastric and glosso-pharyngeal nerves, the corpora geniculata and other larger grey masses connected with the origin of the optic nerve. It would further seem probable that certain fibres of the nerve roots take their origin in these local deposits of grey matter, whilst others become continuous with the white fibres of the spinal cord or encephalon, which are themselves connected with the larger and more general collections of grey matter situated in the interior or on the surface of the cerebrospinal centre. There is still much uncertainty as to the precise mode in which the nervefibres originating or terminating in the grey matter are related to its elements, and for the most part, indeed, individual fibres on being traced into the grey matter, become so hidden in the mass as to elude further scrutiny. Nevertheless, as a continuity between the nerve-fibres and nerve-cells in the grey matter has now been traced in individual examples by many different observers, and as such connections may be held to be general in the ganglions, it is not unfair to infer that, but for the obstacles to successful investigation, the cells in the grey matter of the cerebro-spinal centre would by this time also have been shown to be generally connected with the nerve-fibres. * On the Branching of Nerve Trunks, &c., Archives of Medicine, vol. iv. p. 127. Three modes of connection of cells with fibres are described. 1. From a cell, which may have several branched outrunners, one stout unbranched process is continued into a nerve-fibre, at first naked, and probably representing only the axis cylinder, then acquiring a medullary sheath and dark borders, and finally a membranous tube or primitive sheath. 2. From one or more finely divided branches of a cell, or of more than one cell, equally fine fibrils are prolonged, which coalesce into a pale fibre, having the characters of an axis cylinder, which then, as in the former case, may in its progress become a dark-bordered medullated fibre. 3. The extreme ramifications of a cell or cells become connected, as in the last case, with fibrils, which join into a nerve-fibre; but the connection takes place by the intervention of small bipolar cells, which are by one pole continuous with the branches of the larger cell or cells, and by the other with fine fibrils which join into a pale fibre, or into an axis cylinder of a dark bordered fibre. Gerlach, and after him Waldeyer and others, have described this last mode of connection, as seen by them in the cerebellum. In the cortical grey matter of the cerebellum there are well known large cells generally with one undivided process directed centrally, and two or three finely divided branches towards the surface (fig. LXXIX.). Scattered in the neighbourhood of these large cells, and also collected in a layer named the stratum ferrugineum, or rust-coloured layer, are numerous small cells, often called granules (fig. LXXX. c); and it is alleged by the above named authorities that fine ramifications of the large cells join neighbouring small cells or pass inwards to join those of the stratum ferrugineum, and that the small or intermediate cells are, on the other hand, connected with filaments which coalesce into nerve fibres as above described. This statement derives support from the important observations of Mr. Lockhart Clarke, on the structure of the olfactory bulb. Along with this indirect connection through small intervening cells, Gerlach supposes that a process or processes of the large cells pass directly into nerve-fibres; and should such direct connection take place by the prolongation of an unbranched cell-process into a nervefibre, the arrangement would be analogous to that in the ganglia; the simple origin. representing that of the straight fibre from the ganglion-cell, whilst the ramified origin, with the intervening small cells, might be compared to that of the superficial or spiral fibre, with its interposed nuclei. The fibres of origin of a nerve, whether deeply implanted or not, on quitting the surface of the brain or spinal cord to form the apparent origin or free part of the root, are in most cases collected into funiculi, which are each invested with a sheath of neurilemma. This investment is generally regarded as a prolongation of the pia mater, and in fact its continuity with that membrane may be seen very plainly at the roots of several of the nerves, especially those of the cervical and dorsal nerves within the vertebral canal, for in that situation the neurilemma, like the pia mater itself, is much stronger than in the cranium. The funiculi, approaching each other if originally scattered, advance towards the foramen of the skull or spine which gives issue to the nerve, and pass through the dura mater, either in one bundle and by a single aperture, or in two or more fasciculi, for which there are two or more openings in the membrane. The nerve roots in their course run beneath the arachnoid membrane, and do not perforate it on issuing from the cranio-vertebral cavity; for the loose or visceral layer of the arachnoid is prolonged on the nerve and loosely surrounds it as far as the aperture of egress in the dura mater, where, quitting the nerve, it is reflected upon the inner surface of the latter membrane, and becomes continuous with the parietal or adherent layer of the arachnoid. The nerve, on escaping from the skull or spine, acquires its external, stout, fibrous sheath, which connects all its funiculi into a firm cord, and then, too, the nerve appears much thicker than before its exit. The dura mater accompanies the nerves through the bony foramina, and becomes continuous with their external sheath and (at the cranial foramina) with the pericranium ; |