756 THE NERVOUS SYSTEM 



most rapid in regions corresponding to the mesodermic somites or primitive body segments, and 

 the result is that the neural crest becomes segmented also. The segments or cell-masses thus 

 formed are the beginning not only of the spinal ganglia, but also of the entire sympathetic sys- 

 tem. The cell-masses of the crest migrate to assume a more lateral position, and then occurs 

 a separation in their ranks. A portion of them remain in a dorso-lateral position near the wall 

 of the neural tube and develope into the neurones of the spinal ganglia (the cerebro-spinal sen- 

 sory neurones), but others wander further out into the periphery and become the neurones of the 

 sympathetic. Certain of those of this more nomadic group settle within the vicinity of 

 the vertebral column and form the gangliated cord or the proximal chain of sympathetic 

 ganglia; others migrate further, but in more broken rank, and become the ganglia of the prse- 

 vertebral plexuses or the intermediate chain, while still others wander into the very walls of the 

 peripheral organs and occur singly or in groups in such plexuses as those of Auerbach and Meiss- 

 ner, between the tunics of the walls of the alimentary canal. Scattered along between these 

 proximal, intermediate, and distal groups there are to be found small straggling ganglia, many 

 of which contain so few cell-bodies that they are indistinguishable with the unaided eye. These 

 sympathetic neurones, however, are always either directly or indirectly in connection with and 

 largely under the control of the neurones of the central system through efferent fibres passing 

 either via the ratni communicantes or in the peripheral distribution of the cerebro-spinal nerves. 



From the foregoing it will be seen that a ganglion may be defined as an aggre- 

 gation of cell-bodies of neurones whether sympathetic or cerebro-spinal. As growth 

 proceeds these cell-bodies send out their processes for the reception and transmis- 

 sion of nerve impulses. The cellifugal process, the axone, being in the majority of 

 cases the longer process, is more in evidence. Every ganglion, therefore, will have 

 connected with it bundles of nerve-fibres, some of which bear impulses to its cells 

 from neighbouring ganglia or from the tissues of neighbouring organs: others of 

 which arise from its cells and bear impulses to other ganglia or to the tissues of the 

 organs. Necessarily, the larger the ganglion, the larger will be these bundles of 

 fibres. In the peripheral system the larger of these bundles are given the general 

 name of nerves, the smaller are spoken of as branches, rami, etc. In the central 

 system the larger pathways, especially if mixed, are funiculi, the smaller and 

 those composed of axones of similar functional course are called fasciculi. 



THE CENTRAL NERVOUS SYSTEM 



The central nervous system or organ is an aggregation of ganglia a large gang- 

 lionatecl axis situated in the dorsal mid-line of the body and the bundles of fibres 

 connecting it with the tissues of other systems and with the peripheral ganglia are 

 of necessity correspondingly large. So numerous are the axones connecting these 

 ganglia and so intimately are they associated that a disturbance affecting any one 

 part of the system may extend to influence all other parts. The enlarged cephalic 

 extremity of this central axis, the encephalon, is a special aggregation of ganglia, 

 many of which are much larger than any others found in the body. 



In the study of the central nervous system its enveloping membranes or meninges 

 are met with first, and logically should be considered first, but since a comprehen- 

 sive description of these membranes involves a foreknowledge of the various struc- 

 tures with which they are related, it is more expedient to consider them after mak- 

 ing a closer study of the entire system they envelope. 



For convenience of study the central nervous system is separated into the 

 general gross divisions illustrated in fig. 564. Each of these divisions will be 

 subdivided and considered with especial reference to its anatomical and functional 

 relations to the other divisions and the inter-relations of its component parts. 



THE SPINAL CORD 



The spinal cord or medulla spinalis is the posterior and most attenuated portion 

 of the central nervous system. It is approximately cylindrical in form and termi- 

 nates conically. Its average length in the adult is 44 cm. (18 in.). It averages 

 about 2 cm. longer in the male than in the female. It weighs from 26 to 28 gm. or 

 about 2 per cent, of the entire cerebro-spinal axis, and has about T V the weight of 



