Histologically. t w o kinds of cells may be observed in the walls of the medul lary canal. spnngio 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 and from the small end of the pear a process grows out— the axis cylinder proc ess or axone. tither processes of a different nature appear later. They arc known as protoplasmic processes or dendrites. This nerve cell with all its processes is what is known as the Ntaivots 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 frem 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.
Our knowledge of the internal structure of the nerve cell has been greatly 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 sonic quick fixative such as absolute alcohol. and staining sections with such a dye as methylene blue. This method demonstrates two very differ 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 ea ryoehrome cells, the latter as somatuchrome cells. The structure of a ,ffinatochrome cell as shown by the method of Nissl is as follows: The rather large nucleus situated in the centre of the cell differs iu 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 each nucleus is a darkly stain ing nucleolus. The body of the cell shows three distinct elements in its structure. a basement substance, a cytoretieulum or fibrillar element, and chromophilic bodies. The basement sub stance is unstained by NissFs method and ap pears homogeneous. Traversing this basement substance may be seen very delicate fibrils. These are described by sonic investigators as tibrillai 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, possibly as far as these proc esses themselves extend. The chromophilie bodies
are blue. staining granular masses scattered throughout the cell body. The size and arrange ment of these bodies differ greatly for different cells. and are quite characteristic for certain types of cells. In addition to the elements al ready described, there is present in many nerve cells 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 Gogli show; them to be covered with little knoblike projections known as The axone differs from both cell body and dendrites in that it contains no ehronmphilic 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 its fineness, its straight course, approximately uniform diameter,:inch its few branches. These few branches pass off at right angles and are known as eollaterals. In some cells the axona branches rapidly :Ind ends in the gray matter near its cell of origin. Miler axones pass into the white matter and become axis cylinders of nerve fibres. At its origin from the cell body and at its termination. the axone is uncovered by any sheath. Some axones—eontined to the gray matter—are entirely devoid of any covering. tither axones—fibres of Remak, found mainly in the sympathetic system—are covered by a deli cate sheath known as the neurilemma or sheath of Sehwann. The axones of the white matter of the brain and cord are pro tected by a thief: sheath called the medullary sheath. The axones of the peripheral cranial and spinal nerves are covered by a medullary s h ea t and outside of this a neurilemma. Snell axones, with their sheaths, are known as medullated nerve fibres. A wed nerve fibre thus consists of a central core or axis cylinder which is continuous tlr•onglhout the fibre and is the axone of a nerve eel]. This axis cylinder is cov ered by a rather thick sheath of a fatty nature known as the medullary o• myelin sheath. outside of which is the delicate neurilemma. From the• nenrilemnia line septa extend into the myelin called the in eisures of Schmidt. At interva Is t he med ullary sheath is in terrupted and the axis cylinder is un covered or covered only by the neuri lemma. These points are known as tlie nodes of Itanivier, and the portion of the nerve fibre between two nodes as an interimdal segment.