Page:The New International Encyclopædia 1st ed. v. 14.djvu/417

* NEEVOUS SYSTEM AND BRAIN. 367 NERVOUS SYSTEM AND BRAIN. the brain in the adult nervous system, and it 13 troiu till' cjiililastic cells surrounding the medul- lary canal that the entire nervous system is developed. At that end of the canal which cor- responds to the head ot" the embrj'o, three dila- tations appear. These are known respectively as • the fore-brain (anterior cerebral vesicle — telen- cephalon), the mid-brain (middle cerebral vesicle — mesencephalon), and the hind-brain (posterior cerebral vesicle — nietencci)halon) . 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 myclencephalon. From the anlorior cerebral vesicles are developed the cerebral hemi- spheres, the corpus striatum, the optic thalami, and posteriorly, as far as the anterior corpora quadri^iemina. From the middle cerebral vesicle are developed the corpora quadrigemina and the cerebral peiluncles. From the posterior cerebral vesicles are de'eloped the cerebellum, pons, and medulla oblongata. From the remaining undi- lated portion ot the medullary • canal is formed the spinal cord. During the closiire of the canal there become separated off at vary- ing intervals groups of cells which form the cerebro-spinal ganglia. Histologically. t w o Kinds of cells may be observed in the walls of the medul- laiy canal, spongio- blasts and neuroblasts. From the former is developed the neuro- glia or connective tis- sue of the nervous sys- tem, from the latter are developed the ner- vous elements or neu- rones. The neuroblast is at first a small round cell. It soon becomes pear-shaped, and from the small end of tle pear a process grows out — • the axis cylinder proc- ess or axonc. Other processes of a different nature appear later. They are known as protoplasmic processes or dendrites. This nerve cell with all its processes is what is known as the 'neurone.' Xekvous Tissue. The structural element of nervous tissue is the neurone. This has already been defined as the nerve cell with all its branches. These neurones in their association with one another to form the organs of the ner- vous system are supported by a peculiar type of connective tissue called neuroglia. As noted above in describing its development, each adult neurone consists of a cell body, and passing off from this two kinds of processes, the axis cylin- der process and the dendritic processes. The cell body, as in most other cells, consists of a mass of protoplasm containing a nucleus. wenvprocess ■ —C0tt*r£PAL bffAHCft —MeduZlary sheath' —Axis cyliji^er —^eurilenurta 1 JTcffMINAt BffAf*CH£S DIAGI[.:I OF A KEURONE. 1. Xcrve process naked ; 2. Euvelojied only by neurilem- ma; :i. Enveloped only by me- dullary Hlieath : 4. Enveloped by both medullary sheath and neurilemma. Our knowledge of the internal structure of the nerve cell has been greallj' enhanced within the last few years by the application of a new method known as the method of Nissl. This method consists essentially in placing fresh tissue in some quick fixative such as absolute alcohol, and staining sections with such a dye as methylene blue. This method demonstrates two very diller- ent types of cell in the nervous tissues. In one of these the nucleus alone stains, the cell body remaining unstained; in the other, both nucleus and cell body are stained. The former are known as caryochrome cells, the latter as somaloehronie cells. The structure of a somatochronie cell as shown by the method of Nissl is as follows: The rather large nucleus situated in the centre of the cell differs in no essential from the nuclei of other cells. It is surrounded by a nuclear mem- brane and traversed by an intranuclear network, both of which stain blue. The nucleoplasm or basement substance of the nucleus remains un- stained. Within eacli nuclexis is a darkly stain- ing nucleolus. The body of the cell shows three distinct elements in its structure, a basement substance, a cytoreticulum or fibrillar element, and chromophilic bodies. The basement sub- stance is unstained by Xissl's method and ap- pears homogeneous. Traversing this basement substance may be seen very delicate fibrils. The.se are described by some investigators as fibrilla; passing through the cell body, by others as a definite reticulum. This reticulum or fibrillar structure extends not only throughout the body of the cell, but also into the dendrites and into the axis cylinder, possiblj" as far as these proc- esses themselves extend. The chromophilic bodies are blue, staining granular masses scattered throughout the cell body. The size and arrange- ment of these bodies differ greatly for difTerent cells, and are quite characteristic for certain types of cells. In addition to the elements al- TWO FORMS OF MULTIPOLAR .NEHVE CELLS. From the ventral horn of the siiinal cord of a new-bom rabbit, showing the richly branclied protoplasmic proc- esses. (Schaper.) ready described, there is present in many nerve eells'more or less yellow or brown pigment. This pigment is absent in the nerve cells of the new- born child. The internal structure of the den- drites is similar to that of the cell body. They usually branch rapidly and end in the gray matter near their cell of origin. The method of Oogli shows them to be covered with little knob like jjrojections known as 'gemmules.' The axonc diflfers from both cell body and dendrites in that it contains no chromophilic bodies. It arises from a portion of the cell called the axone hill, which is also free from chromophilic bodies. In Golgi preparations the axone is distinguished by