lO 



NATURE 



[May 5, 



to any particular group of nerve-cells. Moreover, the 

 anatomical evidence of such changes taking place is at 

 present of the flimsiest character. 



If these theories be true, what, it may be asked, is the 

 agency that causes the dendrites to contract or the 

 neuroglia cells to expand ? Is there really a soul sitting 

 aloof in the pineal gland, as Descartes held ? When a 

 man like Lord Brougham can at any moment shut him- 

 self away from the outer world and fall asleep, does his 

 soul break the dendritic contacts between cell and 

 cell ; and when he awakes, does it make contacts and 

 switch the impulses evoked by sense stimuli on to one 

 or other tract of the axons, or axis cylinder processes, 

 which form the association pathways ? Such an hypo- 

 thesis is no explanation : it simply puts back the whole 

 question a step further, and leaves it wrapped in mystery. 

 It cannot be fatigue that produces the hypothetical inter- 

 ruptions of the dendritic synapses and then induces 

 sleep, for sleep can follow after fatigue of a very limited 

 kind. A man may sleep equally well after a day spent 

 in scientific research, as after one spent in mountain- 

 climbing, or after another passed in idling by the sea- 

 shore. He may spend a whole day engaged in mathe- 

 matical calculation, or in painting a landscape. He 

 fatigues — if we admit the localisation of function to de- 

 finite parts of the brain — but one set of association 

 tracts, but one group of cells, and yet, when he falls 

 asleep, consciousness is not partially, but totally 

 suspended. 



We must admit that the withdrawal of stimuli, or their 

 monotonous repetition, are factors which do undoubtedly 

 stand out as primary causes of sleep. We may suppose, 

 if we like, that consciousness depends upon a certain rate 

 of vibration which takes place in the brain structure. 

 This vibration is maintained by the stimuli of the pre- 

 sent, which awaken memories of former stimuli, and are 

 themselves at the same time modified by these. By each 

 impulse streaming into the brain from the sense organs, 

 we can imagine the structure of the cerebral cortex to be 

 more or less permanently altered. The impulses of the 

 present, as they sweep through the association pathways, 

 arouse memories of the past ; but in what way this 

 is brought about is outside the range of explan- 

 ation. Perhaps an impulse vibrating at a certain 

 rate may arouse cells or fibrils tuned by past 

 stimuli to respond to this particular rate of vibra- 

 tion. Thus may be evoked a chain of memories, 

 while by an impulse of a different rate, quite another 

 set of memories may be started. Tracts of association 

 are probably formed in definite lines through the nervous 

 system, as during the life of a child repeated waves of 

 sense-impulses beat against and overcome resistances, 

 and make smooth pathways here and there through the 

 brain structure. Thus may be produced growth of axons 

 in certain directions, and synapses of this cell with that. 

 If the same stimulus be often repeated, the synapses 

 between groups of cells may become permanent. A 

 memory, a definite line of action which is manifested 

 by a certain muscular response, may thus become 

 structurally fixed. If the stimulus be not repeated, the 

 synapses may be but temporary, and the memory fade 

 as the group of cells is occupied by a new memory of 

 some more potent sense stimulus. Many association 

 tracts and synapses are laid down in the central nervous 

 system when the child is born. These are the fruits of 

 inheritance, and by their means, we may suppose, in- 

 stinctive reflex actions are carried out. 



So long as the present stimuli are controlled by past 

 memories and are active in recalling them, so long does 

 consciousness exist, and the higher will be the conscious- 

 ness the greater the number and the more intense the 

 character of the memories aroused. We may suppose 

 that when all external stimuli are withdrawn, or the brain 

 soothed by monotony of gentle repetition, and when the 



NO. 1488, VOL. 58] 



body is placed at rest, and the viscera are normal and 

 give rise to no disturbing sensations, consciousness is then 

 suspended, and natural sleep ensues. Either local fatigue 

 of the muscles, or of the heart, or ennui, or exhaustion of 

 some brain centre usually leads us to seek those conditions 

 in which sleep comes. The whole organism may sleep 

 for the sake of the part. To avoid sleeplessness, we 

 seek monotony of stimulus either objective or subjective. 

 In the latter case, we dwell on some monotonous memory 

 picture, such as sheep passing one by one through a 

 gap in the hedge. To obtain our object, we dismiss 

 painful or exciting thoughts, keep the viscera in health, 

 so that they may not force themselves upon our atten- 

 tion, and render the sense-organs quiet by seeking 

 darkness, silence and warmth. L. H. 



A PROPOSED REVOLUTION IN NAUTICAL 

 ASTRONOMY. 



DURING the last two years a movement has been set 

 on foot, which seems likely to be attended by some- 

 what important results in the simplification of the formula: 

 of astronomical navigation for every-day use. Any one 

 who has looked even cursorily into a text-book of naviga- 

 tion of the Raper type, can hardly fail to have been im- 

 pressed by the multiplicity and variety of the precepts, 

 and can easily understand how complicated the various 

 rules must appear to the unlearned men, upon whom, for 

 the most part, the daily routine of practical navigation at 

 sea must devolve. 



And the difficulty of comprehending and putting into 

 practice the various rules, is undoubtedly increased by 

 the fact that at one time or another all the trigonometrical 

 functions of an angle are brought into play. Sines, 

 cosines, tangents, cotangents, secants and cosecants, 

 versed sines and half-versed sines, all make their appear- 

 ance, adding to the bewilderment of the unskilled com- 

 puter, and introducing the hability to take a required 

 function from a wrong column as a very frequent source 

 of error. 



Nautical astronomy, for the most part, maybe regarded 

 as simply a practical application of the formula; em- 

 ployed in the solution of spherical triangles, so that the 

 object to be attained by those who would simplify the 

 various problems, is to devise a system of formula; in 

 logarithmic shape, which, without materially adding to 

 the amount of arithmetic employed, should introduce but 

 one function of an angle throughout, such as the sine, 

 the cosine, or the tangent. In the verbal precepts, into 

 which, for the benefit of those possessing no knowledge 

 of mathematics, the formula; have to be translated, the 

 simple word "logarithm" would then take the place of 

 " log sine," " log cosine," &c., and a single table of a few 

 thousand logarithms would do the work formerly effected 

 by the aid of a large collection of different tables. 



To M. E. Guyou, an officer of the French navy, belongs 

 the credit of having first devised such a system. As 

 far back as the year 1885 he published in a small 

 pamphlet entitled " Tables de Poche," methods of finding 

 hour angle and azimuth of a heavenly body by means 

 of a single table of logarithms. During the next ten 

 years he employed himself in further researches, and 

 early in 1896 there appeared in connection with the 

 "Annales Hydrographiques," published periodically by 

 the Hydrographic Department of the French navy, a 

 more exhaustive account of his methods, with a special 

 arrangement of the required table, intended to enable his 

 processes to be more easily and effectively carried out.' 



The particular table employed by M. Guyou does not 

 give logarithms for one of the ordinary functions of the 



1 " Les problemes de Navigation et la Carte Marine. Types de calcul 

 et tables completes." Par M. le capitaine de frigate E. Guyou, Membre 

 de I'Academie des Sciences. (Paris : T nprimerie Nationale, 1895 ) 



