The upon which is haled our present conception of the structure of the nervous system must be considered as he gining with the discovery by (lolgi of the ing method which bears his name. This was bounced by f;olgi although it was not until ten later. when Golgi published a siderable volume of his investigations, that the neurological world began to realize the tance of his To the investigations since that time of Golgi. Van Gelineliten, Forel, lietzins, iiautilu y Cajal. Von and others must be ascribed the over throw of theory of a cliffilse nervous betworl: and the establishment upon a firm basis of the at present accepted neurone theory of ner vous-system structure.
or Ni,clan,octem. I N VESTIG.V111)N. The of the development of our knowledge of the structure of tile nervous is necessarily intimately associated with the history of vanees in of investigation. Among the general methods which have proved of greatest in 1%orking out the structure of the vous the following may be mentioned: 1 Alethoil of .1natomical nly the coarser fibre tracts can be determined by this method, its maim ii-e being in studying the gross anatomy of the (2.) llethod of The extreme value of this method is shown by important investigations of Ily means of this method it is possible to sillily the development of the neurone from its first pearance to its adult emulition and to idiserve bow of neurones beeoine separated off by themselves to form the different parts of the vous system. Of especial importance is the fact that the fibres of different systems of neurones acquire their medullary sheaths at Ilifferent em bryonie ages. In this way it is possible, by study ing the nervous systems of embryos of different ages, to diflereutiate many of the fibre systems. (3) Method of Pathology.—This is based upon the fact that any disease or injury which de stroy, the cell bodies of neurones. or which terrupts the continuity of their axones, causes de generation in the axone:: whose cells are destroyed or whose eontinuity is interrupted. Thus. e.g. in an injury crushing the spinal cord at a tain level, there will be bilincl degenerated above the point of injury all fibres whose cells lie be low the injury, while below the injury there will be found degenerated all fibres whose cells lit above the injury. More recently it has been de termined that not only does the distal portion of a cut nerve or cut fibre trait die, but that in disease of peripheral nerves changes take place in the central slump and in the cells from which the diseased axones originate. It has thus been
possible ill some eases to determine the cells from which diseased nerves originate. (4) Method of Arrested method depends upon the fact that there sometimes occur abnor malities in the nervous system such as absence or malformation of a part, thus causing an ab• sence of development in other parts dependent upon them. (5) Method of Animal method consists essentially in ing animals to certain conditions which cause changes in the nervous system and then studying those changes.
Most of our knowledge of the nervous system has been acquired by the application to the above general methods of certain special staining ods. Of these the four most important are the method of Weigert, the method of Colgi, the method of Nissl, and the method of Ehrlich. For the details of these methods the render is re ferred to text-books on histology and cal DEvrt.nclut:Nr. The differentiation of the vous system begins very early in embryonic life. There is first the formation of a groove or fur rOW ill the outer layer of the blastoderm. This is known as the medullary or neural groove. lty an increase of the cells at the sides of the groove and their union this groove is converted into a (dosed canal. the neural or inedidlary canal. This canal corresponds to the central canal of the spinal cord and to the ventricles of the brain in the adult nervous system, and it is from the epiblastie cells surrounding the medul lary canal that the entire nervous system is developed. At that end of the canal which cor responds to the bead of the embryo, three dila tations appear. These are known respectively as • the fore-brain (anterior cerebral vesi(le—tclen cephalon), the mid-brain (middle cerebral vesicle —mesencephalon), and the bind-brain posterior cerebral These three vesicles soon become five from the development from the fore-brain of the interbrain or dien cephalon, and from the hind-brain of the after brain or myelencephalon. From the anterior cerebral vesicles are developed the cerebral hemi spheres. the corpus striatum. the optic thalami, and posteriorly. as far as the anterior corpora quadrigemina. From the middle cerebral vesicle are developed the corpora quadrigendua and the cerebral peduncles. Frain the posterior cerebral vesicle: are developed the cerebellum. pons, and medulla oblongata. From the remaining undi lated portion of the medullary • canal is formed the spinal cord. During the closure of the canal there become separated off at vary ing intervals groups of cells which form the cerebrospinal ganglia.