Page:The New International Encyclopædia 1st ed. v. 14.djvu/416

NERVOUS system: and brain. 366 NERVOUS SYSTEM AND BRAIN. investigator's death by -Max fSeliullzu in 18C5. Dciters divided the processes given oil' from nerve cells into two kinds, protoplasmic proc- esses, which branched rapidly and the structure of which resembleil that of the cell body, and axis cylinder processes or nervous processes which "were often continuous with the axis cyl- inders of meduUated nerves. Gerlach's publica- tion in 1872 was far-reaching in its intluence. Uy the use of the chloride of gold method, he was able to demonstrate many more and finer proc- esses of nerve cells than had been recognized by the older methods of staining. These delicate processes were believed by Cierlach to form a dif- fuse nervous network within the gray matter. Besides describing this nervous network, Ger- lach built up upon this as a basis a theoiy of nervous system structure which was accepted by most neurologists until the advent of the so- called 'newer investigations.' Gerlach believed that from the anterior horn cells of the spinal cord, i)rocesses passed directly off to form the motor roots of the spinal nerves. According to liis theory the protoplasmic processes of these cells ramified in the gray matter of the cord, ■where they anastomosed with ])rotoi)lasmie proc- esses from otlKT cells, forming a reticulum from which arose the fibres which passed out of the oird as the posterior or sensory roots. Gerlach believed that this same relation of afferent and el^ercnt fibres to the nerve cells and to the rtheviluni obtained throughout the entire nervous system, which thus consisted of a remarkably complex continuum of nervous ])rotoplasm. The so-called 'newer investigations' upon which is based our present conception of the structure of the nervous system must be considered as be- gining with the discovery by Golgi of the stain- ing method which bears his name. This was an- nounced by Golgi in 187.3, although it was not tintil ten years later, when Golgi published a con- siderable volume of his investigations, that the neurological world began to realize the impor- tance of his discovery. To the investigations since that time of Golgi, His. Ki'illiker. Van Gehuchten, Forel, Hctzius, Ramon y Cajal. Von I,enluissek, and others nuist be ascribed the over- throw of Gerlach's theory of a difTuse nervous network and the establishment upon n firm basis of the at present aeceijted neurone theory of ner- vous-.system structure. Method.s ok Nkikolocicm- 1xvestig.tiox. The history of the development of our knowledge of the structure of the nervo>is system is necessarily intimately associiited with the history of ad- vances in methods of investigation. Among the general methods which have proved of greatest value in working out the structiire of the ner- vous system the following may be mentioned: (1) Method of . atomical Dissection. — Only the coarser fibre tracts can be determined by this method, its main use being in studying the gross anatomy of the nervous system. ('2) Metho<l of Embryology. — The extreme value of this method is shown by the important investigations of His. By means of this methoil it i* jiossible to study the development of the neurone from its first ap- pearance to its adult condition and to observe how groups of neurones become separated off by thcni.selves to form the different parts of the ner- vous system. Of especial importance is the fact that tiie fibres of <lifferent systems of neurones acquire their medullary sheaths at different em- bryonic ages. In this way it is possible, by study- ing the nervous systems of embryos of dili'erent ages, to dillerentiate many of the libre systems. (3) Method of Pathology. — This is based upon the fact that any disease or injury which de- stroys the cell bodies of neurones, or which in- terrupts the continuity of their a.ones, causes de- generation in the axones whose cells are destroyed or whose continuity is interrupted. Thus, e.g. in an injury crushing the spinal cord at a cer- tain level, there will be found 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 lie above the injury. More recently it has been de- termined that not only does the distal portion of a cut nerve or cut libre tract die, but that in disease of peripheral nerves changes take place in the central stump and in the cells from which the diseased axones originate. It has thus been possible in some cases to determine the cells from which diseased ner-es originate. (4) Method of Arrested Development. — This 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 develoiiment in other parts dependent upon them. (5) Method of Animal Experimenta- tion. — This method consists essentially in subject- 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 meth- ods. Of these the four most important are the method of Weigert, the method of Golgi, the method of Xissl, and the method of Ehrlich. For the details of these methods the reader is re- ferred to text-books on histologj' and histologi- cal technique. ff.r Neu-por. Med-gr- ■Ant.cerrb.ves. fiil.cereb. vfs. Jhstcerebtvts. Heart. Somite. ASTERIOR niRTIOX OF THE BOHY OF A CHICK. Theliead ilistilH'tly diffcreiitinted. kwii from tliPBnrfnre. n. (., head fold ; .Veli. po/-., iinuro-i>orU(*; //. /).. hfiid idat<': F. g., IdivKUt; r. le.. yolkvelu ; Meil. gr.. iih iliilliirj prnovo; Ant. roreb. re.«., nntorior corebrnl veaicle; .VW. cfrvlt. yrs., middle cerebral vesicle; Post, cereb. rM.,I>08- t4?rior cerebral vesicle. Det:i.op>ient. The differentiation of the ner- vous system begins very early in embryonic life. There is first the formation of a groove or fur- row in the outer layer of the blastoderm. This is known as the medullary or neural groove. By an increase of the cells at the sides of the groove and their union dorsally. this groove is converted into a closed canal, the neural or medullary canal. Tliis canal corresponds to the central canal of the spinal cord and to the ventricles of