442 



NERVOUS SYSTEM 



upper part of the spinal cnrd. The twelfth pair, or 

 hypoglcMul nerve, is the motor nerve of the tongue. 

 Ite origin in near the floor of tin- fourth ventricle, 

 clone to the middle line, and it 

 emerges from the anterior sur- 

 face of the medulla olilonj.Mt.-i 

 in a shallow groove hetween tin- 

 anterior pyramids and the in- 

 ferior olivary body (see BRAIN ). 

 The spinal nerves arise from 

 the spinal cord in pairs, thirty- 

 UK' in number, and are named 

 according to their relation to 

 the vertebra? cervical, dorsal, 

 Itimltar. and sacral. Their 

 mode of origin will he under- 

 stood from tig. 7, which repre- 

 sents diagrammatieally the first 

 part of their course, and on one 

 side their relations with the 

 sympathetic system C 1-8 re- 

 present* the eight paire of cervi- 

 cal nerves; D 1-12, the twejve 

 dorsal pairs ; L 1-5, the live 

 luinliiir pairs ; and S 1-6, the 

 six sacral pairs of nerves. Each 

 spinal nerve arises by two roots, 

 an anterior and a posterior (fig. 

 8, a and y; see also SPINAL 

 CORD). These root* pass out- 

 wards, and unite before they 

 leave the spinal canal. Before 

 their union a small oval swell- 

 ing is found on the posterior 

 root, and is called its ganglion, 

 g. The united nerve leaves the 

 spinal canal by a small aperture 

 l..'i wr.'ii adjacent vertebra. It 

 almost immediately gives off a 

 fine medullated nerve to ito 

 corresponding sympathetic gang- 

 ** ' lion, a branch which can be 



traced into one of the internal 

 organs. It also receives from the ganglion a non- 

 medullated or gray fibre, which is distributed to 

 the muscular coat of the blood-vessels, especially 

 the arteries. The nerve thus altered passes out- 

 wards, dividing as it goes to send its ultimate 

 branches into the fibres of the muscles, into the 

 cells of th- skin and connective tissues, tendons, 

 and bones. In the dorsal region each nerve passes 

 to ito distribution without entering into connection 

 with it- neighbours, but in the cervical, lumbar, 



I ,_ <. 



and sacral regions the nerves split up and form new 

 junctions with each other, or plexuses as they are 

 called. (These are indicated in fig. 1, and on the 

 right-hand side of fig. 7, but the detailed descrip- 

 tion of them is impossible within the limits of this 

 article.) 



Function* of the Spinal Nerves. Sir Charles 

 Bell discovered 'that division of the anterior roots 

 was followed by Ions of |>wer of voluntary motion, 

 and that division of the posterior roots destroyed 

 the power of sensation. He termed the anterior 

 root motor, and the posterior sensory. It has since 

 been ascertained that the anterior roots carry out- 

 wards other impulses that do not result in motion, 



and that the posterior roots carry inwards impulses 

 w liirh limy not result in sensation. Therefore, it 

 is more correct to term these roots respectively 

 efferent and afferent. If the anterior root be divided 

 hetween the point of ii- origin from the cells of tin- 

 anterior horn of the spinal cord and ito junction 

 with the posterior root, the part unconnected with 

 the cord will waste along the whole length of the 

 nerve, and the muscles which it supplies will waste 

 also. The cells in connection with the anterior 

 roots, therefore, not only send out motor impulses, 

 but exert a nutritive or tro/ihir influence on th. 

 nerve and muscle. Division of the posterior root 

 beyond its ganglion is followed dy wasting of the 

 corresponding fibres of the nerve to their ultimate 

 termination. If the root be cut between the 

 ganglion ami the i-|iinal cord, the part attached to I 

 the ganglion remains unaltered, while that con- 

 nected with the spitial cord wastes. This shows 

 that the ganglion of the posterior root exerts a 

 trophic influence on the fibres connected with it. 

 If the nerve be divided after the junction of the two 

 roots, the whole of the nerve farthest from the 

 spinal cord will waste. 



The afferent nerve impulses which pass along the 

 posterior roots comprise those which give rise to 

 the sense of touch, pain, and temperature, and to 

 reflex movements of various kinds without neces- 

 sarily exciting our consciousness, such as those 

 concerned with the maintenance of the equilibrium 

 of the body, and with the functions of the internal 

 organs. 



Keflex Action. By this we mean an action 

 brought alxmt directly by the influence of an 

 all'erent impulse ouite indejietidently of voluntary 

 control. For such an action four elements are 

 necessary: (1) afferent fibres, (2) nerve cells or 

 centres, (3) efferent fibres, (4) muscle fibres. 

 The impulse travels up the 

 afferent fibres and stimulates 

 the nerve-cells to send an im- 

 pulse along the efferent fibre to 

 the muscles. If any of these 

 four factors is absent, the rellex 

 action cannot take place. A 

 familiar example is the mov- 

 ing of the foot as the result of 

 tickling the sole. The afferent 

 impulse passes up the nerves 

 to the nerve centres in tin- 

 spinal cord, which send out- 

 wards direct to the muscles 

 motor impulses, which often 

 cannot be controlled by the 

 will. 



Automatic Action. When 

 movement is brought about by 

 an impulse originated in a nerve 

 centre itself, without the influ- 

 ence of an afferent stimulus, it is called automatic 

 or spontaneous. Such net ions are apt to occur 

 rhythmically, such as the action of the heart. 



Voluntary Actions. In these the outgoing im- 

 pulses originate in the nerve-cells in the motor 

 area of the brain, and |>as.s down the opposite 

 side of the spinal cord to the nerve-cells in its 

 anterior horn. From thence they are transmitted 

 by the efferent nerves to the muscles. Fig. 9 

 will explain the relation of voluntary to reflex 

 action : c is a nerve-cell in the brain ; n, the 

 nerve-fibre in the spinal eon! which transmits 

 the nerve impulse originating in c to *, a cell 

 in the anterior horn of the spinal cord which for- 

 wards it through a nerve, e, to a muscle, m. 

 The reflex arc is represented by o, an end-organ ; 

 a, an afferent nerve ; , a nerve-cell ; e, an 

 efferent fibre ; m, a muscle. If c or n, or both, lie 

 destroyed by disease, the power of voluntary 



Fig. 9. 



