TILE C'ENTRAL NERVOUS SYSTEM. The brain and spinal cord are covered and protected by three membranes or meninges, as they are frequently termed—viz. the fluor muter, the arachnoid, and the pia mater. The drn•a muter is a strong fibrous membrane, which supplies the cranial bones with blood in early life, and adheres firmly to their inner surface. The arachnoid (so called from its being supposed to be as thin as a spi der's web) is a serous membrane, and, like all serous membranes, is a closed sac, consisting of a parietal and a visceral layer. The parietal layer adheres to tie inner surface of the dur•cr mahr, while the visceral ,layer somewhat luoscly invests the brain and spinal from direct contact with which, however, it is SI Tarated by the inter vention of the pia and some loose areolar tissue. The pia inatcr is an extremely vascular membrane consisting of minute blood-vessels. held together by an extremely tine areolar tissue. It dips down between the convolutions and fis sures of the brain. and is prolonged into the interior, forming the re/i/in iiih•rpositani and the choroid plexuseS of the fourth ventricle. It is by means of this membrane that the blood-vessels are conveyed into the nervous substance.
TUE CEnEIMO-SPINAL NERVOUS SYSTEM. In de scribing this it is more convenient to begin with a description of the spinal cord, the study of which is essential to a proper appreciation of the higher and more complicated centres.
Thc Spinal ('or•d is that portion of the cerebro spinal axis \skid] is contained in the spinal canal. It extends from the upper border of the first cervical vertebra or atlas above, to the middle or lower margin of the first lumbar vertebra below. The cord is continuous superiorly with the me dulla oblongata, while its lower extremity tapers off into a slender cord, the film,' terminale. At two levels, one in the cervical and one in the lumber region, the diameter of the cord is con siderahly larger than elsewhere. These are known respectively as the cerrical and the lum bar enlargements. The poslerirn• medic:cc septum and the anterior median fissure almost divide the cord longitudinally into symmetrical halves, while the spinal nerve roots leaving the cord at quite regular intervals serve to divide it into segments. These segments correspond in number to the spinal nerves. There arc thus eight ccrricol, twelve dorsal, five lumbar, five sacral, and one or two roecygral segments. The internal of the cord can be best appre ciated by studying a transverse section through the cervical enlargement stained by Weigert's method. Such a section shows that two sub stances enter into its compisitimi. one situ ated centrally, the other on the periphery. On account of their appearance in fresh tissues, the former is called gray mat tcr, the latter white mattcr. The gray matter is made up mainly of cells and their• dendrite.;• and of nommedullated axone:; the white matter consists mainly of medullated nerve fibres, its whiteness being due to the myelin. The gray matter presents some
what the form of the letter II. Posteriorly, it is slender, and almost touches the surface— posterior horns—while anteriorly it is broad— anterior horns—and separated from the surface hy a considerable layer of white matter. The expanded tip of the posterior horn is known as the head or entail. and eontains a gelatinous suh• stance, the substantia fielalinosn of Rolando. The narrow portion connecting the head with the central gray matter is railed the neck or ccrri.r. Laterally, the gray matter extends out somewhat into the white matter as the intern/ horn or pr•oeessu.s rcticularis latcralis. The gray horns of the two sides of the cord are connected by a vommlieslre containing the central canal. In foetal life this canal is open and continuous with the ventricles of the brain. In adults it is usually more or less obliterated. It is sur rounded by the substantia yehasw.a and divides the gray eummissure into an un it rowr gray cum insure and a post, taut yru y cum m is.''?? re. Pas,ing out front the anterior horns to the surface of the cord are bundles of fibres tehich constitute the anhrior or motor spinal m ryc routs. Jti,t, to the ue•dialn side of the pos terior horns are ,ut.11 the entering fibres of the posterior or masory ?tern. routs. lit' the pos terior horns the %%bite matter of the cord is di vided into posh rico- cehlIIIIIS Mid aniero-lateral columns. the latter being again rather indefinitely subdivided by the fibres of the anterior roots into lateral and anterior columns. The poste rior columns are also usually subdivided at this level by a connective tissue septum from the pia nutter into an internal portion• the column of Coll, and an external portion, the column of FttutE SYSTEMS OR TRACTS OF TIIE Conn. As al ready noted. the cell bodies of the neurones are grouped in the gray matter of the brain and cord in the ganglia of the cranial and of the spinal nerves and in the end organs of certain of the nerves of special sense. This grouping is for definite physiological purposes. The axone: from many of these neurone groups pass into the white matter as distinct bundles and constiltit,c fibre system, or fibre tracts. In the normal adult cord these various tracts of fibres present the same appearance and cannot he differentiated from (Inc another. Certain of the methods de scribed above. however. especially the niethod of embryology and the method of pathology, have enabled us to determine the following systems: ( 1) Descending tracts: (a) The direct pyramid the direct cerebellar tract ; postero-lateral ascend ing tract or tract of t•lec•hsig; (e) tract of 6101Trr1 or the autero-lateral ascending tract. (3) :Short systems, fundamental columns or ground bundles. composed of short ascending and descending fibres intermingled.