September 5, 1912] 



NATURE 



15 



its individual functions. But unless there were some 

 sort of cooperation and subordination to the needs of 

 the body generally, there would be sometimes too little, 

 sometimes too much gastric juice secreted ; sometimes 

 too tardy, sometimes too rapid an absorption from the 

 intestine ; sometimes too little, sometimes too much 

 blood pumped into the arteries, and so on. As the 

 result of such lack of cooperation the life of the whole 

 would cease to be normal and would eventually cease 

 to be maintained. 



We have already seen what are the conditions which 

 are favourable for the maintenance of life of the 

 individual cell, no matter where situated. The prin- 

 cipal condition is that it must be bathed by a nutrient 

 fluid of suitable and constant composition. In higher 

 animals this fluid is the lymph, which bathes the tissue 

 elements and is itself constantly supplied with fresh 

 nutriment and oxygen by the blood. Some tissue-cells 

 are directly bathed by blood ; and in invertebrates, in 

 which there is no special system of lymph-vessels, all 

 the tissues are thus nourished. All cells both take 

 from and give to the blood, but not the same materials 

 or to an equal extent. Some, such as the absorbing 

 cells of the villi, almost exclusively give; others, such 

 as the cells of the renal tubules, almost exclusively 

 take. Nevertheless, the resultant of all the give and 

 take throughout the body serves to maintain the com- 

 position of the blood constant in all circumstances. 

 In this way the first condition of the maintenance of 

 the life of the aggregate is fulfilled by insuring that the 

 life of the individual cells composing it is kept normal. 



The second essential condition for the maintenance 

 of life of the cell-aggregate is the coordination of its 

 parts and the due regulation of their activity, so that 

 they may work together for the benefit of the whole. 

 In the anim'al body this is effected in two ways : first, 

 through the nervous system ; and second, by the action 

 of specific chemical substances which are formed in 

 certain organs and carried by the blood to other parts 

 of the body, the cells of which they excite to activity. 

 These substances have received the general designa- 

 tion of "hormones" (6p/«iia, to stir up), a term intro- 

 duced by Prof. Starling. Their action, and indeed 

 their very existence, has only been recognised of late 

 years, although the part which they play in the physio- 

 logy of animals appears to be only second in import- 

 ance to that of the nervous system itself ; indeed, 

 maintenance of life may become impossible in the 

 absence of certain of these hormones. 



Fart played by the Nervous System in the Maintenance 

 of Aggregate Life. — Evolution of a Nervous System. 



Before we consider the manner in which the nervous 

 system serves to coordinate the life of the cell-aggre- 

 gate, let us see how it has become evolved. 



The first step in the process was taken when certain 

 of the cells of the external layer became specially 

 sensitive to stimuli from outside, whether caused by 

 mechanical impressions (tactile and auditory stimuli) 

 or impressions of light and darkness (visual stimuli) 

 or chemical impressions. The effects of such impres- 

 sions were probably at first simply communicated to 

 adjacent cells and spread from cell to cell throughout 

 the mass. An advance was made when the more 

 impressionable cells threw out branching feelers 

 amongst the other cells of the organism. Such feelers 

 would convey the effects of stimuli with greater 

 rapidity and directness to distant parts. They may 

 at first have been retractile, in this respect resembling 

 the long pseudopodia of certain Rhizopoda. When 

 they became fixed they would be potential nerve-fibres 

 and would represent the beginning of a nervous 

 system. Even yet (as Ross Harrison has shown), in 

 the course of development of nerve-fibres, each fibre 

 makes its appearance as an amoeboid cell-process which 



NO. 2236, VOL. 90] 



is at first retractile, but graduallv grows into the 

 position it is eventually to occupy and in which it 

 will become fixed. 



In the further course of evolution a certain number 

 of these specialised cells of the external layer sank 

 below the general surface, partly perhaps for protec- 

 tion, partly for better nutrition : they became nerve- 

 cells. They remained connected with the surface by a 

 prolongation which became an afferent or sensory 

 nerve-fibre, and through its termination between the 

 cells of the general surface continued to receive the 

 effects of external impressions ; on the other hand, 

 they continued to transmit these impressions to other, 

 more distant cells by their efferent prolongations. In 

 the further course of evolution the nervous system 

 thus laid down became differentiated into distinct 

 afferent, efferent, and intermediary portions. Once 

 established, such a nervous system, however simple, 

 must dominate the organism, since it would furnish 

 a mechanism whereby the individual cells would work 

 together more effectually for the mutual benefit of 

 the whole. 



It is the development of the nervous system, 

 although not proceeding in all classes along exactly the 

 same lines, which is the most prominent feature of the 

 evolution of the Metazoa. By and through it all 

 impressions reaching the organism from the outside 

 are translated into contraction or some other form 

 of cell-activity. Its formation has been the means of 

 causing the complete divergence of the world of 

 animals from the world of plants, none of which 

 possess any trace of a nervous system. Plants react, 

 it is true, to external impressions, and these impres- 

 sions produce profound changes and even compara- 

 tively rapid and energetic movements in parts distant 

 from the point of application of the stimulus — as in 

 the well-known instance of the sensitive plant. But 

 the impressions are in all cases propagated directly 

 from cell to cell — not through the agency of nerve- 

 fibres ; and in the absence of anything corresponding 

 to a nervous system it is not possible to suppose that 

 any plant can ever acquire the least glimmer of 

 intelligence. In animals, on the other hand, from a 

 slight original modification of certain cells has directly 

 proceeded in the course of evolution the elaborate 

 structure of the nervous system with all its varied and 

 complex functions, which reach their culmination in 

 the workings of the human intellect. "What a piece 

 of work is a man ! How noble in reason ! How 

 infinite in faculty ! In form and moving how express 

 and admirable ! In action how like an angel ! In 

 apprehension how like a god ! " But lest he be elated 

 with his physical achievements, let him remember that 

 they are but the result of the acquisition by a few 

 cells in a remote ancestor of a slightly greater ten- 

 dencv to react to an external stimulus, so that these 

 cells were brought into closer touch with the outer 

 world ; while, on the other hand, by extending beyond 

 the circumscribed area to which their neighbours 

 remained restricted, they gradually acquired _ a 

 dominating influence over the rest. These dominating 

 cells became nei-ve-cells ; and now not only furnish the 

 means for transmission of impressions from one part 

 of the organism to another, but in the progress of 

 time have become the seat of perception and conscious 

 sensation, of the formation and association of ideas, 

 of memory, volition, and all the manifestations of the 

 mind ! 



Regulation of Movements by the Nervous System. — 

 Voluntary Movements. 

 The most conspicuous part played by the nervous 

 system in the phenomena of life is that which pro- 

 duces and regulates the general movements of the 

 bodv — movements brought about bv the so-called 



