.‘s to the physiological signifieance of these dilfemnt parts of rho neurone. onr ideas are still based largely upon theoretical grounds. The cell body has been shown to Ia• the birth o• genetic centre of the neuron... From the fact that any portion of the neurone which is cut off from the tell body dies. the yell would seem to be the nutritive or trophic eentre of the neurone. From the absence of nervous activity in portions of the neurone which ha VP been cut off from their cell of origin, the cell body \you'll also appear to be the functional centre of the neurone. The most generally accepted theories of the significance of the component parts of the cell itself may be summed up as follows. function of the nucleus is the same as in other cells. It seems to preside over the constructive activities of the neurone, by which food products brought to the cell by the lymph arc transformed into food elements of the cell, and finally into its nervous elements proper. The basement substance and the chromophilie bodies represent the food ele ments of the cell. The cytoretieulum represents working nervous mechanism of the cell, it alone being concerned in the reception, trans fornmtion, and emission of the nervous impulse. If the processes the axone is centrifugal in func tion, carrying impulses away from the cell body, the dendrites are centripetal in function, bring ing impulses to the cell body. Axones thus act as organs of as regards the nervous impulse. dendrites is organ, of reception. Neu rones are assoc•iatecl with one another by approxi mation or by contact. and not by continuity of their• protoplasm; that is, no two neurones are believed to be directly connected with each other. The axonal terminations of one neurone siinply lie in contiguity. or at most in touch with the dendrites or cell bodies of other neurones. the impulse passing over from one to the other.
Neuroglia, or the connective tissue of the nervous system, differs biith in structure and in origin from the form of connective tiASIle found in other organs. As already stated, it is cid
blostie in origin, developing like the neurone from the cells which line the cuhirt•onie neural eanal. These cells, at first morphologically iden tical, soon differentiate into neuroblasts, or future neurones. and spongioblasts, o• future neuroglia cells.
In adult neuroglia two main types of cells are found, spider cells and mossy cells. The spider cell has a rather small body. from which are given off on all sides straight, unbranehing, spine like processes. They occur mainly in the white matter. 'The mossy cell has also a rather small body, its processes are fewer, are eoarse, rough, and branching: they occur mainly in connection with blood-vessels. As in the ease of the nerve cell, the processes of these cells do not anasto Incise. but interlace, forming a dense felt work.
The newer; is 1.01111)0Sed in all ver tebrated animals of two distinet portions or systems—viz. the cerebro-spinal and sympathetic yanyliosic.
The ecrebro-spina! system ineludes the brain and spinal cord (which form the eerrbr•o..epina! (I.TiN, or el .ntral Sp, t 1'111 ) and the cranial and spinal nerves. It was termed by Tiichat the nervous system of animal life, and comprises all the nervous organs concerned in sensation, voli tion, and mental action.
The sympathetic system consists essentially of a chain of ganglia connected by nervous cords, which extends from the cranium to the pelvis. along each side of t he vertebral column, and from which nerves with large pro ceed to the viscera and blood-vessels in the cavi ties of the chest, abdomen, and pelvis. It was termed by Biehat the nervous system of organie life. since it seems to regulate—almost or quite independently of the will—the due performance of the functions of the organs of respiration, cir culation. and digestion.