The Cerebro- Spinal Axis consists of two portions, the brain or encephalon, which is contained within the cranium, and the spinal cord, continuous with the brain, which is enclosed in the spinal canal. The cerebro-spinal centre consists of two lateral symmetrical halves, which correspond in their structure in every respect; they are partially separated by longitudinal fissures, and connected together by broad transverse bands of nervous substance, called commissures.

  The cerebro-spinal axis consists of two substances, which differ from each other in density and colour; they are called the grey cineritious or cortical substance, and the white or medullary.

  The grey or cortical substance is disposed in the form of a thin layer upon the outer surface of the convolutions of the cerebrum and lamina; of the cerebellum; it is not confined, however, to the external surface, for it exists in the interior of the spinal cord throughout its entire length, and from this part may be traced up through the medulla oblongata, pons Varolii, and crura cerebri, to the central parts of the hemispheres, the optic thalami, and corpora striata. It also forms at the base of the brain, the lamina cinerea, the tuber cinereum, and the grey 'matter in the anterior and posterior perforated spaces. The grey matter may be traced from the anterior perforated space into the olfactory nerve as far as the bulb, and from the posterior space as forming part of the infundibulum and pituitary body. The grey matter in this situation is continued upon the sides of the thalami, forms the soft commissure, surrounds the anterior pillars of the fornix, enters below into the substance of the corpus albicans; and, above, forms part of the lateral walls of the septum lucidum. It is also found in the centre of each of the corpora quadrigemina, in the pineal gland, and corpora geniculata. It forms also the corpus dentatum in the centre of each lateral lobe of the cerebellum.

  The white or medullary portion of the cerebro-spinal axis consists of fibres, which are arranged chiefly in a longitudinal direction, or interlace at various angles with transverse fibres; they may be arranged into three classes, ascending, transverse, and longitudinal. The ascending fibres pass up from the medulla oblongata, increase in number as they ascend through the pons, the optic thalami, and striated bodies, and then diverge to every part of the surface of the hemispheres. They were called by Gall the diverging fibres. The transverse or com-missural fibres commence at the surface of the hemispheres, and proceed inwards towards the centre, connecting the two hemispheres together; these were named by Gall the converging fibres. The longitudinal fibres, also commissural, connect together different parts of the same hemisphere, being confined to the same side of the middle line.

  Chemical Composition. The following analysis by Lassaigne represents the relative proportion of the different constituents composing the grey and white matter of the brain.

  It appears from this analysis, that the cerebral substance consists of albumen, dissolved in water, combined with fatty matters and salts. The fatty matters, according to Fremy, consist of cerebric acid, which is most abundant, cholesterin, oleophosphorie acid, and olein, margarin, and traces of their acids. The same analyst states, that the fat contained in the brain is confined almost exclusively to the white substance, and that its colour becomes lost when the fatty matters are removed. According to Vauquelin, the cord contains a larger proportion of fat than the brain; and according to L'Heritier, the nerves contain more albumen and more soft fat than the brain.

  The Ganglia may be regarded as separate and independent nervous centres, of smaller size and less complex structure than the brain, connected with each other, with the cerebro-spinal axis, and with the nerves in various situations. They are found on the posterior root of each of the spinal nerves; on the posterior or sensory root of the fifth cranial nerve; on the seventh nerve; on the two sensory divisions of the eighth pair (the glosso-pharyngeal and pneumogastric); in a connected series along each side of the vertebral column, forming the trunk of the sympathetic; on the branches of this nerve in the head, neck, thorax, and abdomen; or at the point of junction of branches of this nerve with the cerebro-spinal nerves. On section, they are seen to consist of a reddish grey substance, traversed by numerous white nerve-fibres: they vary considerably in form and size; the largest are those found in the cavity of the abdomen; the smallest, the microscopic ganglia, which exist in considerable numbers upon the nerves distributed to the different viscera. The ganglia are invested by a smooth and firm closely-adhering membranous envelope, consisting of dense areolar tissue; this sheath is continuous with the neurilemna of the nerves, and sends numerous processes into the interior of the ganglia, which support the blood-vessels supplying its substance.

  The Nerves are round or flattened white cords, communicating on the one hand with the cerebro-spinal centre or the ganglia, and by the other distributed to the various textures of the body, forming the medium of communication between the two. One class of nerve-fibres, the afferent or centripetal, serve to convey im-pressiors to the brain, the great centre of sensation and volition, where they are rendered cognizable to the mind; whilst another class of nerve-fibres, the efferent or centrifugal, convey the stimulus of volition to the organs of motion. The brain and spinal cord are also capable of receiving impressions by means of the afferent nerve-fibres, which results in a motorial stimulus being propagated along the efferent nerves, quite independent of the efforts of volition, and without even consciousness. The movements of this kind are called reflex or excito-motory. The nerves are subdivided into two great classes, the cerebro-spinal, which proceed from the cerebro-spinal axis, and the sympathetic or ganglionic nerves, which proceed from the sympathetic ganglia; the cerebro-spinal are the nerves of animal life, being distributed to the organs of the senses, the skin, and to the active organs of locomotion, the muscles. The sympathetic or ganglionic nerves are distributed chiefly to the viscera and blood-vessels, and are termed the nerves of organic life.

  The cerebro-spinal nerves consist of numerous nerve-fibres, collected together and enclosed in a membranous sheath. A small bundle of primitive fibres enclosed in a tubular sheath is called a funiculus: if the nerve is of small size, it may consist only of a single funiculus, but if large, the funiculi are collected together into larger bundles or fasciculi; and one or more fasciculi bound together in a common membranous investment, termed the sheath, constitutes a nerve. In structure, the common sheath investing the whole nerve, as well as the septa given off from it, which separates the fasciculi, consists of areolar tissue, composed of the white and yellow elastic fibres, the latter existing in greatest abundance. The tubular sheath of the funiculi, or neurllemma, consists of a fine smooth transparent membrane, which may be easily separated, in the form of a tube, from the fibres it encloses; iu structure, it is, for the most part, a simple and homogeneous transparent film, occasionally composed of numerous minute reticular fibres.

  The nerve-fibres, as far as is at present known, do not coalesce, but pursue an uninterrupted course from the centre to the periphery. In dissecting a nerve, however, into its component funiculi, it may be seen that they do not pursue a perfectly insulated course, but occasionally join at a very acute angle with other funiculi proceeding in the same direction; from these again branches are given off, which join again in like manner with other funiculi. It must be remembered, however, that in these communications the nerve-fibres do not coalesce, but merely pass into the sheath of the adjacent nerve, become intermixed with the nerve-fibres, and again pass on to become blended with the nerve-fibres hi some adjoining fasciculus.

  The cerebro-spinal nerves consist almost exclusively of the tubular nerve-fibres, the gelatinous fibres existing in very small proportion.

  The blood-vessels supplying a nerve terminate in a minute capillary plexus, the vessels composing which run, for the most part, parallel with the funiculi; they are connected together by short transverse vessels, forming narrow oblong meshes, similar to the capillary system of muscle.

  Nerves in their course subdivide into branches, and these frequently communicate with branches of a neighbouring nerve. In the subdivision of a nerve, the filaments of which it is composed are continued from the trunk into the branches, and at their junction with the branches of neighbouring nerves, the filaments pass to become intermixed with those of the other nerve in their further progress; in no instance, however, do the separate nerve-fibres either subdivide or inosculate.

  The communications which take place between two or more nerves form what is called a plexus. Sometimes a plexus is formed by the primary branches of the trunks of the nerves, as the cervical, brachial, lumbar, and sacral plexuses, and occasionally by the terminal fasciculi, as in the plexuses formed at the periphery of the body. In the formation of a plexus, the component nerves divide, then join, and again subdivide in such a complex manner that the individual fasciculi become interlaced most intricately; so that each branch leaving a plexus may contain filaments from each of the primary nervous trunks which form it. In the formation also of the smaller plexuses at the periphery of the body, there is a free interchange of the fasciculi and primitive fibrils. In each case, however, the individual filaments remain separate and distinct, neither subdividing nor inosculating.

  Some nerve-fibres have no peripheral termination. Gerber has shewn, that nerve-fibres occasionally form loops by their junction with a neighbouring fibre in the same fasciculus, and return to the cerebro-spinal centre without having any peripheral termination. These he considers to be sentient nerves, appropriated exclusively to the nerve itself, the nervi nervorum, upon which the sensibility of the nerve depends, and quite exclusive of the sensation produced by an impression made at the peripheral end of the nerve. These fibres bear some analogy to those met with in the posterior part of the optic commissure, where a set of fibres pass from one optic tract across the commissure to the opposite tract, having no communication with the optic nerve; also in the communications formed between the cervical nerves and spinal accessory and descendeus noni, the nerve-fibres form an arch connected by each extremity with the cerebro-spinal centre, and have no peripheral termination.

  Again, some nerve-fibres would appear to have no central connection with the cerebro-spinal centre, as those forming the most anterior part of the optic commissure. These inter-retinal fibres, as they are called, commence in the retina, pass along the optic nerve, and across the commissure to the optic nerve and retina of the opposite side.

  The point of connection of a nerve with the brain or spinal cord is called, for convenience of description, its origin or root. If the fasciculi of which the nerve is composed should all arise at or near one point, or along one tract, the root is called single. If, on the contrary, the fasciculi divide into two separate bundles, which are connected at two different points with any part of the cerebro-spinal centre, such nerve is said to have a double origin, or to arise by two roots, each of which may have a separate function, as in the spinal nerves. The point where the separate fasciculi of a nerve are connected to the surface of the cerebro-spinal centre is called the apparent origin of a nerve; the term real or deep origin being given to that part of the centre from which a nerve actually springs.

  The nerve-fibres at their periphery terminate in a varied manner. Occasionally the elementary fibres are disposed in terminal loops or plexuses, which, for a considerable period, was supposed to be their usual mode of termination, but later investigations have shewn that such is not the case. Nerve-fibres most commonly terminate by blunted and slightly-swollen ends, such as is observed in those which enter into the Paccinian bodies, or they may become gradually lost to view in the tissue in which they are distributed, becoming diminished in size, and their tubular sheath and white substance being wanting. Occasionally the elementary nerve-fibres, as in the nerves of special sense, may be brought into connection at their periphery with cells similar to those met with in the grey matter of the brain and ganglia.

  The Sympathetic System consists of numerous parts, which may be arranged as follows, i. A connected series of ganglia placed along both sides of the spinal column, from the cranium above to the coccyx below. 2. Branches of communication passing between the ganglia. 3. Branches of connection between the ganglia and the cranial and spinal nerves. 4. Primary branches of distribution, remarkable for their plexiform communications on the vessels, glands, and neighbouring viscera to which they are distributed, or for passing to other larger ganglia, situated in each of the great cavities of the body, and usually placed on the roots of origin of the larger blood-vessels. 5. Plexuses of nerves proceeding from these secondary ganglia, accompanying the blood-vessels, and receiving branches from the spinal or cerebral nerves. The sympathetic nerves consist of tubular and gelatinous fibres, intermixed with a varying proportion of filamentous areolar tissue, and enclosed in a sheath formed of fibro-areolar tissue. The tubular fibres are, for the most part, smaller than those composing the cerebro-spinal nerves; their double contour is less distinct, and, according to Remak, they present nuclei similar to those found in the gelatinous nerve-fibres. Those branches of the sympathetic which present a well-marked grey colour, are composed more especially of gelatinous nerve-fibres, intermixed with few tubular fibres; whilst those of a white colour contain more of the tubular fibres, and few gelatinous. Occasionally the grey and white cords run together in a single nerve, without any intermixture, as in the branches of communication between the sympathetic ganglia and the spinal nerves, or in the communicating cords between the ganglia.

  The Cerebro-Spinal Centre consists of two parts, the spinal cord and the ence-phalon: the latter may be subdivided into the cerebrum or brain proper, the cerebellum or little brain, the tuber annulare or pons Varolii, and the medulla oblongata.