When a portion of a sympathetic nerve is examined under the microscope, it is found to contain an unusually large quantity of white fibrous tissue, the fibres of which are arranged longitudinally. Crossing these are some fine circular fibres (of yellow elastic tissue) which are placed at some distance apart from each other. When the nerve is tom up by needles, numerous small oval cells may he seen among the fibres, their long axes being parallel to the fibres; these cells become much more visible when the fibrous tissue has been acted upon by acetic acid. They are scattered among the other elements of the nerve, and are probably persis tent nuclei of the sarne kind as those which exist in muscle and other tissues. Numerous nerve-tubes are also seen entering into the for rnation of these nerves. These tubes appear to correspond in structure exactly with those of the cerebro-spinal system; they present the same clear outline, and contain a semifluid pulpy matter, which is acted upon in a similar way by reagents as that in the nerve-tubes of the cerebro-spinal system. They resemble, however, the nerve-tubes of the brain or spinal cord more than those of nerves, for they are much smaller and more delicate than the latter, and more prone to form varicosities. They lie side by side of each other as in other nerves, and do not inosculate. The number of these nerve-tubes seems to vary in different parts of the sympathetic, apparently without regard to the size of the nerve, so that a small nerve may contain several nerve-tubes, while a large one contains but a few. In the abdominal ramifi cations the nerve-tubes are very numerous, and also in the cardiac nerves, while the sympa thetic trunk in the neck contains but a few, which are situated quite in the centre of the nerve. So far all observers appear to agree in their statements respecting the elementary composi tion of this nerve, and so far its intimate structure justifies the opinion that in its func tions it must be intimately connected with the cerebro-spinal nerves. A coarser anatomy had already taught us that this nerve has extensive communications with the cerebro-spinal system, with all the encephalic nerves, excepting those. of pure sense, and with all the spinal nerves by their anterior and posterior roots. It is now evident from microscopic observation that the object of these communications must be to enable cerebro-spinal nerve-tubes to pass into, the sympathetic system ; and, in short, that! these communications may be regarded as so many origins of the sympathetic from the brain and spinal cord.
It remains to inquire whether there is any good foundation for the doctrine that the sym pathetic nerve contains distinct and peculiar fibres, (gre y fibres of some authors,) which are independent of the brain and spinal cord, and which by anastomosing with cerebral or spinal nerves may confer upon them, to a certain extent, the peculiar endowment which is supposed to characterise the nerves of the former kind.
Itetzius and Muller appear to have been the first to put forward distinctly the opinion that certain cerebro-spinal nerves received parti cular fibres from the sympathetic, as the latter received filaments from the former. And Aliiller* suggested that both the ganglionic and the cerebro-spinal nerves should be looked upon as compound in structure ; " that the ganglionic nerves contain motor, sensitive, and organic fibres, of which the latter kind alone have the power of regulating the vegetative processes, and have a special relation to the ganglia ; that the cerebro-spinal nerves are likewise composed of motor, sensitive, and organic fibres, of which those of each kind have their specific destination, and run their course together without uniting with the others; that the ganglionic nerve consequently differs only in having numerous ganglia, and in con taining a large number of grey fibres, which give it a proportionally greyer colour; while, in the cerebro-spinal system, the grey fibres are less numerous, and are seen as grey fasciculi lying in the larger mass of white fibres." Neither Retzius nor Muller has given a clear description of these organic fibres as seen by them. Muller quotes and adopts Remak's
account of the microscopic examination of these fibres. " They are," according to the latter anatomist, " much more minute than the cere bro-spinal fibres ; they are perfectly homo geneous, that is to say, not composed, as far as can be distinguished with the microscope, of a tube and contained portion; and are so pale and transparent that in a strong light they are not visible ; lastly, a completely characteristic appearance is produced by the small roundish or oval bodies which here and there beset their surface." They are almost gelatinous in their nature ; they have on their surface the appear ance of fine longitudinal lines, and are easily resolved into very fine fibres.* Schwann seems to confirm this description, and to regard the organic fibre' as a less per fectly developed state of the nerve-tube of the cerebro-spinal system.
Henle in his description .of the grey or soft nerves gives the following account of these fibres (fig.334).They are flatfibres,very clear, of horno geneous appearance, in diameter from 0.002 to 0.003 of a line (8.56th to qolaith of an inch), with numerous nuclei of cells, round and oval, most of them laid flat, and arranged at nearly equal distances, many presenting regular nucleoli, and pointed at their opposite poles. Their longest diameter is generally parallel to the longitudinal axis of the nerve. Sometimes one of these fibres resolves itself into more delicate fibrillT, resembling the primitive fibre of cellu lar tissue. Acetic acid dissolves them and leaves the nuclei untouched. Henle a'dmits that the greyish colour of the nerves depends on the quantity of these fibres ; the greater the number of nerve-tubes, the more thi bundle resembles an ordinary cerebro-spinal 4rve. In the roots of the sympathetic the number of the grey fibres is in large proportion, there being four to six of them for one nerve-tube, so that each nerve-tube appears surrounded by the nucleated fibres.* Valentin, who admits the existence of fibres of a similar kind to those described by Henle, maintains that they are continuations of the sheaths of the globules which exist in the gan glia, and which are prolonged from them into the nervous trunks, and they serve as an enve lope or protecting sheath to the cerebro-spinal nerve-tubes. IIenle, who had formerly regarded these fibres as nerves distributed to the con tractile cellular tissue and to vessels, (" the slight developement of the nerves of these tissues seeming to correspond to the imperfec tion of their contractile power,") now expresses great doubts as to their nature and office, and proposes to call them gelatinous nervous fibres; " a name," he says, " which has no other end but to designate their presence in certain nerves, in the same way as we continue to call the fibres of cellular tissue, which are met with in tendons, tendinous fibres." Midler conjectures that they may serve the purpose of establishing a communication be tween the ganglia; in short, that they are so many commissures between these centres.t Purkinje and Rosenthal describe the organic nerve-fibre as the same as the central axis of the cerebro-spinal nerve-tube deprived of its investing membrane, and from comparing the sympathetic fibres with the cerebro-spinal fibre in the young embryo, they state their opinion that the latter, in an early stage of develope ment, is identical with the former, but they do not appear to recognise, as Remak did, any continuity between these organic fibres and the ganglionic globules./ Volkmann and Bidder have lately put for ward an examination of this question; and these observers maintain the existence of a series of fibres peculiar to the sympathetic and distinct frotn those of the brain and spinal cord. Their work, however, contains many statements so much at variance vvith those of preceding writers, and with what I have myself seen, that I am led to entertain a strong suspicion that there must have been some fallacy affecting their observations throughout.