512 THE NERVOUS SYSTEM. 



5. The brain and spinal medulla, when studied by the naked eye, are seen to be 

 composed of white matter and gray matter. The white matter forms very nearly two- 

 thirds of the entire cerebro-spinal axis. It is composed of medullated nerve-fibres 

 embedded in neuroglial tissue. The gray matter is composed of nerve-cells with 

 their dendrites and axons. Some of the axons are in the form of naked axis 

 cylinders, whilst others have a coating of myelin. Intimately intermixed with 

 these parts is the neuroglia, which isolates them more or less completely from 

 each other. 



THE NATURE OF THE BRAIN. 



In the foregoing account it has been explained that the nervous system is 

 composed of a series of afferent nerves bringing information from every part of the 

 body into the central nervous system, from which efferent nerves pass out to the 

 muscular and other active parts of the body, providing the means for translating 

 such information into appropriate action. But it has been seen that the essential 

 part of the central nervous system is the intercalated cells, which provide the 

 means whereby the information brought in by any sensory nerve may be placed 

 at the service of the whole body, and the response which it excites may be controlled 

 and regulated by the condition of the rest of the body. The system of intercalated 

 cells links together into one co-ordinated mechanism the whole nervous system, 

 and, through it, every part of the body itself. 



In some very primitive and remote ancestor of man (and in fact of the vast 

 majority of animals) the front end of the nervous system became enhanced 

 in importance to form a brain, which assumed a dominant influence over the rest. 

 This was brought about in the first place by the fact that in an elongated prone 

 animal moving forwards, the front end would naturally come first into relation- 

 ship with any change in environment ; and this earlier acquisition of information 

 concerning the outside world would necessarily give the head end of the nervous 

 system exceptional opportunities for influencing the rest of the nervous system. 

 This predominance is further accentuated by the development in the head 

 region of the organs of special sense, which provide mechanisms specially adapted 

 to be influenced by light, sound, and such delicate chemical forms of stimulation 

 as excite in ourselves sensations of smell and taste. As the information conveyed 

 by these special senses, such as the scent of food or the visual impression of some 

 enemy, must be able immediately to influence the movements of the whole body, 

 it follows that a specially abundant system of intercalated elements link the 

 central ends of these nerves of the special senses with the rest of the central 

 nervous system. Moreover the predominant influence of the head end of the 

 central nervous system implies that it must be provided with a specially large 

 series of nerve-fibres, not only for the purpose of bringing this influence to beai 

 upon the rest of the nervous system, but also of being itself brought into intimate 

 relationship with the nervous system as a whole, seeing that sensory impulses art 

 constantly pouring into every part of it. 



Thus the head end of the central nervous system becomes the brain, whid 

 is characterised by a series of large irregular swellings, due to (a) the develop 

 ment around the insertion of each special sensory nerve of a mass, or group o 

 masses, of intercalated cells which will enable the effects of the visual, acoustic 

 olfactory, gustatory or other sensations to influence the whole nervous system 

 and (6) the evolution of complicated systems of intercalated cells, which receive 

 and in a sense blend, impressions coming from all parts of the nervous system 

 and emit fibres which pass, directly or indirectly, to the various groups of moto 

 nerve-cells and control their activities and, through them, the behaviour of th 

 animal. 



In the development of the human embryo this distinction between the hea< 

 end and the rest of the central nervous system is indicated even before th 

 medullary plate is completely folded up to form the neural tube. The widene< 



