NERVOUS TISSUE. The nervous tissue is found in the brain, the spinal cord, the nerve ganglia of the sympathetic system, and the peripheral nerves or nerve trunks distributed throughout the body. The nerve tissue consists of nerve cells and their processes. The body of the cell lies in the nerve centres, that is, the brain, the spinal cord, and the ganglia, and the typical cell has two kinds of processes, one comparatively short and nliuch branched, known as the dendrites or den dritic processes, the other but little branched and in some cases very long, as much as two or three feet. This latter branch is known as the axis cylinder process: it may pass out of the nerve centres to be distributed to the other tissues forming the essential part of what we call nerve fibres. It is important, therefore, to bear in mind that nerve fibres are essentially long processes from nerve cells and form the mechanism by which the nerve centres are connected with and influence the activity of the peripheral tissues. Physiologically, the important function of a nerve fibre is to conduct a change or process known as the nerve impulse. This impulse travels along the fibre at a rate of about one hundred feet per second. The nerve fibres fall into two great groups, those that convey impulses from the peripheral tissues to the nerve centres, and those that convey impulses from the centres toward the periphery. The former group com pose the afferent, or sensory fibres, the latter name being used because in many ea-es the im pulses finally reach the brain and give rise to conscious sensations of various kinds. In many cases, homever, the impulses conveyed to the nerve centres by the afferent fibre- cause no change in consciousness, but manifest themselves by what we call reflex effects, as. for instance, in
the movements of the intestines. the Idood-vessels, or the heart. The group of fibres that carry their impulses outward from the t re to the eral tissues compose the efferent or motor If these fibres end in museh-;, the effect of their impulses is the production of a muscular contrac tion: if they end in a gland, they cause a secre tion, the ' nature of the resulting action depending the kind of tissue with which the nerve eonnected. It should be stated tl there is no known difference in structure between and efferent nerve fibres. Eoh. like a tcleoriph wire, can conduct an impulse in either But just as a telegraph wire with a sending apparatus at one end and a receiver at the other is arranged to conduct messages only in one direc tion, so the afferent and efferent nerve fibres, by the peculiarities of their end connections, are arranged so that normally they can convey ef fective impulses only in one direction. What we call a nerve or a nerve trunk consists of many hundreds or thousands of nerve fibres belonging usually to both the afferent and the efferent group. Though combined in one bundle, each fibre is physiologically independent and may act alone or in combination with others in the same trunk. In a muscle, on the contrary, the hundreds or thousands of muscular fibres of which it is com posed act usually as a unit, all contracting to gether when the muscle is in action.