The cord is made up of grey substance, forming the ganglionic centres and consisting of ganglion cells in the midst of a fine network, and of white substance consisting of medullated nerve fibres having essentially the structure of those in the peripheral nerves.
The grey substance of the cord is arranged in the well-known form of an anterior and a posterior cornu on either side, with a commissure across the middle line. The ganglionic centres have their seat chiefly in the anterior cornua, and form tolerably definite groups of cells whose arrangement may be followed by reference to Fig. 303, which shows sections of the lumbar and cervical enlargements. There are the lateral groups divided into antero-lateral and postero-lateral (al and pl), the anterior group (a), and the internal group (i). Towards the centre of the horn is the central group (c). The median group (m) is much larger in the cervical than in other parts of the cord, and so causes the horn to be extended laterally (see Fig. 303). Lastly, there is a group of cells, generally called Clarke's vesicular column, situated near the internal border of the posterior horns close to the posterior commissure. The group is present only in the lower part of the cervical enlargement, in the dorsal region, and in the upper part of the lumbar enlargement (see vc, Fig. 304).
The white substance, consisting of nerve fibres, forms connections in the first place with the various orders of centres in the cord itself, and in the second place with the higher centres above the cord, in the brain. We may thus distinguish two sets of nerve fibres, one forming connections within the cord and medulla and the other forming communications between the cord and the cerebellum and cerebrum. These two sets of fibres will be divisible again into afferent and efferent.
Fig. 303. - Sections of spinal cord from middle of lumbar and cervical enlargements, showing groups of ganglion cells; aI and pl, antero-lateral and posterolateral, a, anterior, i, internal, c, central, m, median, groups. (Ross).
It is a fact of very peculiar interest, that the two sets of fibres distinguished above seem to be developed not only separately but at different periods, and so the aid of embryology has been sought to enable us to distinguish between them. The fibres which connect the different parts of the cord and medulla with each other may be regarded as the Primary or Fundamental fibres, and it is found that they are the first formed, while those forming higher connections are of subsequent development, and may be named Secondary or Accessory fibres. As nerve fibres are first developed without the medullary sheath, and recently formed tracts are therefore much paler than those which have acquired it, we have in this a means of distinguishing the fundamental from the secondary. What follows will be understood by reference to Fig. 305, which represents a transverse section of the cervical cord in the foetus of nine months.
Fig. 304. - Section of dorsal region of cord. Letters same as in previous figuie, with the addition that the antero-lateral and posterolateral groups are separated by a medio-lateral area (ml), and Clarke's column (vc) is shown. (Ross).
The first developed fibres immediately surround the grey matter, and are called the Anterior and Posterior root-zones, the latter being also sometimes called the columns of Burdach. As they form communications between one part of the grey matter and another, the fibres are comparatively short and their number is generally in proportion to the amount of grey matter, or at least of ganglion cells in the horns. These fibres therefore do not diminish from above downwards as do the others.
Fig. 305. - Cervical enlargement of cord in a foetus of nine months. A and P, anterior and posterior cornua; G, columns of Goli; T, columns of Ttirck; ar and pr, anterior and posterior root-zones; pt, pyramidal tract; dc, direct cerebellar tract. (Ross).
Of the fibres forming communications between the brain *and cord, the best known are those which convey the motor impulses from the brain to the cord. These form the Pyramidal tract. We shall afterwards trace them from the cortex of the brain downwards, but at present we take them up at the medulla oblongata. Here they form the anterior pyramids and most of the fibres decussate so that in the cord they occupy the opposite side to that which they have in the brain. Some of them, however, do not decussate, but remain in the anterior parts of the cord, forming a small band on either side of the anterior lorgitudinal fissure, the Column of Turck (T in figure). The great mass of the fibres, having decussated, pass to the lateral column of the cord, where they occupy a definite position in its posterior parts (pt in figure). The fibres in both these positions diminish in number from above downwards, as they pass into the grey substance of the cord at successive levels in order to come into relation with the centres in the anterior comua.
The secondary centripetal fibres, or those which form sensory connections between the cord and brain, are represented by a tract in the posterior columns lying next the posterior longitudinal fissure, and occupying a position somewhat similar to that of the columns of Ttirck anteriorly. These Are called the Columns of Goll (G in figure).
Besides this there is a tract which forms communications between the cerebellum and the cord, but which is not of late development. This is the so-called Direct cerebellar tract (dc in figure), which lies in the lateral column outside the pyramidal tract, and as if flattened against the surface. The function of this tract is not known, but it is composed of centripetal fibres and diminishes from above downwards even more quickly than the pyramidal tract, so that by its disappearance the latter may come to the surface.
According to Gowers there is a centripetal tract anterior to the direct cerebellar and pyramidal tracts, and lying for the most part close to the surface of the cord, where it extends almost to the anterior median fissure. This is called the Anterolateral ascending tract or Column of Gowers. It is thus similar in position to the cerebellar tract.
In the Medulla oblongata the grey and white substances of the cord may be traced upwards, but they undergo considerable dislocation. From the examination of Fig. 306 it will be seen that, as the central canal passes backwards and finally opens out in the fourth ventricle, the grey matter departing from its arrangement into cornua but still aggregated in the neighbourhood of the central canal and ventricle, forms various masses in the posterior part of the medulla.
These masses have special importance as being the nuclei of origin of certain nerves, and will be afterwards more particularly referred to, in connection with bulbar paralysis. The white substance gradually comes to occupy the middle and anterior parts of the medulla, and the olivary body is intercalated in its midst. The pyramidal tract is easily recognized here, forming the anterior pyramids (p), which decussate at the lower part of the medulla (A). The motor fibres, having assumed a position in front, remain anterior to the sensory in the rest of their course in the brain.
Fig. 306. - Medulla oblongata at various levels, A, at decussation of pyramids (p), the general shape of the cornua still retained. B, higher up, the grey matter passing backwards, and pyramids becoming more isolated. C, in fourth ventricle, the nuclei in the floor of which are shown. (Quain).
Post-mortem changes and injuries. Artefacts. The soft structures of the cord and brain are very liable to injury during the process of removal, and to distortions and other changes in the course of hardening. Great care should be exercised in removing the parts, and also in placing them into the hardening media in such a way as that they shall not be twisted or distorted. In spite of all precautions there are liable to be artificial changes in the tissue, especially when the post-mortem is made some time after death. These various changes and injuries constitute the so-called Artefacts. It is well to have the post-mortem made as soon as possible, and in the meantime the body may be placed with the face downwards and in a cold atmosphere. Recently a method of injecting formaline into the cranial cavity through the orbit has come into use. This has decided advantages both in rendering the brain and cord more easily handled without risk of injury and in preventing post-mortem changes.
For a very full exposition of the structure and development of the cord see Ross's Nervous system, vol. i. An important contribution is that by Flechsig, Die Leitungsbahnen im Gehirn und Rlickenmark des Menschen, 1876. For diseases of spinal cord, Ross and Gowers are very complete; also Erb's very excellent work in Ziemssen's Encyclopaedia; Charcot, Lectures on dis. of nervous syst., New Syd. Soc. transl., 2nd series, 1877 and 1881. Bramwell, Dis. of spinal cord, 3rd ed., 1895; Hammond, A treatise on dis. of nervous system, 6th ed., 1876; Leyden, Klinik der Ruckenmarkskrankheiten, 1875. Paul Blocq and Albert Londe, Anat. path, de la moelle epiniere, Paris, 1891; Pierre Marie, Lect. on dis. of spinal cord, New Syd. Soc. transl., 1895; Van Gieson, Artefacts of the nervous system, reprint from New York Medical Journal, 1892.