498 THE NEEVOUS SYSTEM. 



The study of a simple scheme representing the relationship that obtains between 

 these three classes of elements in the extremely primitive animal, Hydra (Fig. 439), 

 will make these fundamental facts plain. Changes in the animal's environment 

 affect the extremities of the peripheral processes of the sensory cells (A, B, and 

 (7), which in Hydra are situated amongst the ordinary tegumentary cells: the 

 effect is transmitted by the central processes of such cells (A, for example), either 

 directly to the efferent cell, represented in the diagram by a motor nerve-cell, or 

 more usually to an intercalated nerve-cell (a, b, or e). Into this (a) impulses 

 stream from other intercalated cells (b and c), bringing the impulse from the 

 sensory cell A under the influence of those coming from B and from more distant 

 parts of the body through the intermediation of the intercalated cell c. The cells 

 a, c, and d are connected with the motor nerve-cell. Thus, there is provided a 

 mechanism whereby the conditions affecting other regions of the body, B and (7, 

 may influence the nature of the response which the stimulation of A evokes 

 either increasing or diminishing its effect or perhaps altering its character. 



In this way the intercalated nerve-cells form a great co-ordinating mechanism, 

 linking together all parts of the body in such a way that the activity of any part 

 of the organism may be influenced by the rest, and thus be enabled to act in the 

 interest of the whole. 



Hence the nervous system becomes the chief means whereby the various parts 

 of the body are brought into functional relationship one with the other, and co- 

 ordinated into one harmonious whole. 



Throughout the whole course of its subsequent evolution the nervous system is 

 formed of these three kinds of elements ; and the essential feature in its elaboration 

 and increasing complexity is the multiplication of the intercalated cells, and their 

 concentration, together with the motor nerve-cells, to form a definite organ, which 

 we call the central nervous system. 



During this process of development of the more complex forms of nervous 

 system, most of the sensory cells migrate from their primitive positions in the 

 skin (Fig. 439) ; and, as the free extremity of the peripheral process retains its 

 primitive relationship to the skin, such migration of the cell bodies necessitates 

 a great elongation of their peripheral processes. Although these sensory cells thus 

 move inwards into the deeper tissues of the body, the great majority of them do 

 not become incorporated in the central nervous system, but become collected into 

 groups, which form the ganglia of the sensory nerves. 



In addition to its primary functions of (a) providing the means whereby the 

 organism can be brought under the influence of its surroundings, and (b) co- 

 ordinating the activities of the whole body, the nervous system also comes tc 

 perform other functions of wider significance. 



In the course of its evolution the co-ordinating mechanism formed by the 

 intercalated cells becomes so disposed in each animal that an appropriate stimulus 

 applied to the sensory nerves can evoke a definite response, often of great com 

 plexity and apparent purposiveness. In other words, the nervous system become; 

 the repository of those inherited dispositions of its constituent parts whicl 

 determine the instincts : and in the course of time it eventually provides also th 

 apparatus by which individual experience and the effects of education can b 

 brought to bear upon and modify such instinctive behaviour. In other word; 

 from the nervous system is formed the instrument of intelligence ; and th 

 relatively great bulk and extreme complexity of that instrument the brain i 

 man are in a sense the physical expression of human intellectual pre-eminence. 



In conformity with its primary function of affording a means of communicatio 

 with the outside world, almost the whole nervous system in the human embryo, '< 

 in other animals, is developed from the ectoderm, as has already been explained i 

 the chapter dealing with General Embryology (p. 30 et seq.}. In the most primiti 1 

 Metazoa the sensory cells remain in the ectoderm (Fig. 439), but other ectoderm 

 cells become converted into motor nerve-cells and intercalated nerve-cells, whi< 

 wander into the underlying tissues (Fig. 439). In the human embryo there is <' 

 analogous process of development, but with the important difference that t 

 various nervous elements do not wander into the mesoderin individually. 



