PERIPHERAL TERNIINATIONS OF SPINAL NERVES,.
The modes of terminations of these processes are extremely varied and complicated. The termina tions are always 'free' in the sense that while possibly sometimes penetrating cells, they are never directly continuous with their protoplasm. The motor nerve fibres end in voluntary and in voluntary muscle. On its way to a muscle a motor fibre, which, as noted above, is the axone of an anterior horn cell. may divide into several branches, a single cell thus innervating more than one muscle fibre. On reaching a voluntary muscle the bundle of nerve fibres breaks up to form a plexus in the connective tissue which sur rounds the bundles of musele fibres. From this plexus nerve fibres pass to the individual muscle fibres. ]laving arrived at the muscle fibre in whieli it terminates, the nerve fibre loses its medullary sheath, and its neurilemina fuses will' the sarcolemma. The naked axone then (accord ing to niost observers) penetrates the sarcolemma and terminates in a more or less elaborate expan sion known as a 'motor end plate.' The sensory or afferent part of the spinal nerves is made by the peripheral processes of the spinal ganglion cells. In the skin, and in those inueous meada-aaes which are covered With sdpiamous epithelium, the nerve fibres lose their medullary sheaths in the snbepithelial tissue, and, penetrating the epithelial layer. split up into minute fibrils which pass in between the veils and terminate there, often in little knob like swellings. In addition to such comparatively simido nerve endings, there are also found in the skin and Duteous membranes, espeeiallY where sensation is most neute, much more elaborate terminations. Among these may be mentioned Merkel's tastzellen, or touch cells, the tactile corpuscles of Meissner, and the Pac•inian bodies. In tendons and in muscle, sensory nerve fibres, after losing their medullary sheaths, divide into minute fibrils which are often studded with irregular expansions. In gland tissue, nerve fibres usually end as fine fibrils, which pass to the epithelial cells.
PurstoLocv OF TILE SPINAL CORD AND SPINAL NERVES. The sensory nerve endings serve as the receptive apparatus by means of which external stimuli may induce a nervous impulse. The motor nerve endings serve as organs of distribu tion through which an impulse may so affect a muscle as to cause a contraction and consequent muscular action. The nerves themselves appar ently serve merely as a. conductive apparatus for transmitting the sensory impulses from the peri phery to the spinal cord and the motor impulses from the cord to the muscles. The spinal cord must be considered as suhse•ving several quite different though related physiological purposes, of which the following are the most important: (1) Its gray matter acts as a reflex centre, i.e. a centre in which an impulse brought to it by the fibres of the posterior nerve root (usually a sensory impulse) can determine an impulse pass ing out in the fibres of an anterior nerve root (usually a motor impulse). in this way a motion is produced by an external stimulus without any involvement of consciousness. A simple reflex is one in which the impulse passes directly from the posterior root fibres to the cells of the an terior horns. A compound reflex is one in which the impulse passes through another neurone sit uated within the cord in passing from the pos terior root fibre to the anterior horn cell. (2) An automatic centre. By an automatic action is meant one which apparently occurs without any determining external stimulus, e.g. the rhyth mical action of the heart or the contraction and expansion of the arteries. (3) A relay station in the transmission of impulses from and to the higher centres. This takes place through the fibre tracts of the cord and has been already re ferred to in connection with the description of those tracts. (4) A conduction path. This also has been noted in connection with the de scription of the fibre tracts of the cord.