THE NERVOUS TISSUE. 



1113 



THE NERVOUS TISSUE. 



The nervous tissues of the body are comprised in two great systems the 

 cerebro-spinal and. the sympathetic; and each of these systems consist of a central 

 organ, or series of central organs, and of nerves. 



The cerebro-spinal system comprises the brain (including the medulla oblongata), 

 the spinal cord, the cranial nerves, the spinal nerves, and the ganglia connected 

 with both these classes of nerves. The sympathetic system consists of a double 

 chain of ganglia, with the nerves which go to and come from them. It is not 

 directly connected with the brain or spinal cord, though it is so indirectly by 

 means of its numerous communications with the cranial and spinal nerves. 



All these nervous tissues are composed chiefly of two different structures 

 the gray or cineritious and the white or fibrous. It is in the former, as is 

 generally supposed, that nervous impressions and impulses originate, and by the 

 latter that they are conducted. Hence the gray matter forms the essential 

 constituent of all the ganglionic centres, both those in the isolated ganglia and 

 those aggregated in the cerebro-spinal axis; while the white matter is found in all 

 the commissural portions of the nerve-centres and in all the cerebro-spinal nerves. 

 The nerves of the sympathetic system are chiefly composed of a material of a some- 

 what different structure, which is named gray or gelatinous nerve-fibre. This 

 form of nerve-fibre is also found in some of the cerebro-spinal nerves. 



FIG. 645. Neuroglia-cells of brain shown by Golgi's method. (After Andriezen.t (Copied from Schiifer's 

 Essentials of Histology.) A. Cell with branched processes. B. Spider-cell with unbranched processes. 



The gray nervous substance is distinguished by its dark reddish-gray color and 

 soft consistence. It is found in the brain, spinal cord, and various ganglia inter- 

 mingled with the fibrous nervous substance, and also in some of the nerves of 

 special sense, and in gangliform enlargements which are found here and there in 

 the course of certain cerebro-spinal nerves. It is composed of cells, commonly 

 called nerve-cells or ganglion-corpuscles, containing nuclei and riucleoli. The cells 

 together with the blood-vessels in the gray nerve-substance, and the nerve-fibres 

 and vessels in the white nerve-substance, are imbedded in a peculiar ground sub- 

 stance, named by Virchow neuroglia. It consists of fibres and cells. Some of 

 the cells are stellate in shape, and their fine processes become neuroglia-fibres, 

 which extend radially and unbranched (Fig. 645, B) among the nerve-cells and 

 fibres which they aid in supporting. Other cells give off fibres which branch 

 repeatedly (Fig. 645, A). In addition to these fibres there are others which do 



