yuly 19, 1877] 



NATURE 



23T 



from observations on Jupiter's satellites, and the radius of 

 the earth 6378'233 kilometres. 



Again, the sun's parallax deduced from M. Comu's 

 values of the velocity of light in conjunction with the 

 value of aberration is, with Bradley's estimate of 2o""25, 

 8"-8S2, and with Struve's, of 2o"-445, 8"798. These values 

 of parallax compare favourably with determination by 

 other methods, of which we give a few examples. The 

 value given by the transits of Venus in 1761 and 1769 

 was 8"'5776 computed by Encke, but increased to 8"-89i 

 by Mr. Stone on a redetermination. By the record of an 

 observation of the occultation of '^r.^ Aquarii on October 

 I, 1672, M. Leverrier obtained 8"'S66 ; by meridian obser- 

 vations of Mars at Greenwich iu 1862, 8"932 ; by the 

 latitude of Venus obtained from transits of 1 761 and 1769, 

 combined with present latitudes, ;\I. Leverrier finds 8"853 ; 

 from the discussion of meridional observations of Venus 

 fn an interval of 106 years S" 859 ; by the opposition of 

 Mars in i860 by M. Liais 8"76o ; by opposition of Flora 

 in 1873 by Prof. Galle 8"S73. Judging from these results 

 the velocity of 186,638 miles per second is not very far 

 from the mark, and the care in selection of methods and 

 in computing results can scarcely be surpassed. 



G. M. S. 



EVOLUTION OF NERVES AND NERVO- 

 SYSTEMS'- 

 "\T ERVE-TISSUE universally consists of two elemen- 

 -■-^ tary structures, viz., very minute nerve-cells and 

 very minute nerve-iibres. The nerve-fibres proceed to 

 and from the nerve-cells, thus serving to unite the cells 

 with one another, and also with distant parts of the 

 animal body. Moreover, nerve-cells and fibres, wherever 

 we meet with them, present very much the same appear- 

 ances. Here, for instance, is a sketch of highly magni- 

 fied nerve-tissue as we find it in the human brain, and 

 here is one of my own drawings of nerve-tissue as I have 

 found it in the jelly-fish ; and you see how similar the 

 drawings are — notwithstanding they are taken from the 

 extreme limits of the animal kingdom within which nerve- 

 tissue is known to occur. 



Nerve-cells are usually found collected together in 

 aggregates which are called nerve-centres or ganglia, 

 to and from which large bundles of nerve-fibres come 

 and go. These large bundles of nerve- fibres are whr.t we 

 see with the naked eye as nerves, permeating the body in 

 all directions. When such a bundle of nerve-fibres 

 reaches a ganglion, or collection of nerve-cells, it splits 

 up like the end of a rope which has been teased out, and 

 the constituent fibres pass into and out of the nerve-cells, 

 so interlacing with one another in all directions, as here 

 diagrammatically represented. More true to nature is 

 this diagram, which represents a magnified section of 

 human brain — the human brain being itself nothing more 

 than a collection of very large ganglia. 



To explain \.'he funtlioH of nerve-cells and nerve-fibres, 

 I must begin by explaining what physiologists mean by the 

 word " excitability." Suppose this to represent a muscle 

 cut from the body of a freshly-killed animal. So long as 

 jou do not inteifere with it in any way, so long will it 

 remain quite passive. But every time you stimulate it 

 either with a pinch, a burn, or, as represented in the dia- 

 gram, with an electrical shock, the muscle will give a 

 single contraction in response to every stimulation. Now 

 it is this readiness of organic tissues to respond to a 

 stimulus that physiologists designate by the term ex- 

 citability. 



Nerves, no less than muscles, present the property of 

 being excitable. Suppose, for instance, that this is an- 

 other muscle prepared in the same way as the last one, 

 except that together with the muscle there is cut out the 



■ Abstract of a Lecture delivered at the Royal Inslilu'icn on Friday 

 evening, May 25, 1877. By George J. Roman;s, M.A., F.L.S., &c. 



attached nerve. Every time you pinch, burn, or electrify 

 any part of the nerve, the muscle will contract. But you 

 will carefully observe there is this great difference between 

 these two cases of response on the part of the muscle ; 

 viz., that while in the former case the muscle responded 

 to a stimulus applitd directly to its own substance, in the 

 latter case the muscle responded to a stimulus applied at 

 a distance prom its own substance, which stimulus was 

 then conducted \.o the muscle by the nerve. And here we 

 perceive the characteristic function of nerve-fibres, viz., 

 that of conducting stimuli to a distance. This is the 

 function of nerve-yf/'/rj- ; but the function of nerve- rc'/Zj- is 

 different, viz , that of accumulating nervous energy, and 

 at fitting times of discharging this energy into the 

 attached nerve-fibres. The nervous energy when thus dis- 

 charged from the nerve-cells acts as a stimulus to the nerve- 

 fibre ; so that if a muscle is attached to the end of the 

 fibre it contracts on receiving this stimulus. I may add 

 that when nerve-cells are collected into ganglia they often 

 appear to discharge their energy spontaneously, without 

 any observable stimulus to cause the discharge ; so that in 

 all but the lowest animals, whenever we meet with appa- 

 rently spontaneous action, we infer that ganglia are pro- 

 bably present. But the point which most of all I desire you 

 to keep well in mind this evening is the distinction which 

 I here draw between muscle and nerve. A stimulus ap- 

 plied to a nerveless muscle can only course through the 

 muscle by giving rise to a visible wave of contraction, 

 which spreads in all directions from the seat of stimulation 

 as from a centre. A nerve, on the other hand, conducts 

 the stimulus without undergoing any change of shape. 

 Now in order not to forget this all-important distinction, 

 I shall always to-night speak of muscle as conducting a 

 visible wave oi contraction, and of nerve as conducting 

 an invisible or molecular wave of stimulation. Nerve- 

 fibres, then, are functionally distinguished from muscle- 

 fibres — and also, I may add, from protoplasm — by 

 displaying the property of conducting invisible or molecu- 

 lar waves of stimulation from one part of an organism to 

 another — so establishing physiological continuity between 

 such parts without the necessary passage of contractile 

 waves. 



I will now conclude all that it is necessary to say about 

 the function of nervous tissue by describing the me- 

 chanism of reflex action. Suppose this to represent any 

 peripheral structure, such as a part of the skin of some 

 animal, this a collection of nerve- cells or ganglion, and 

 this a muscle. The part of the skin represented is united 

 to the nerve-cells composing the ganglion by means of 

 this in-coming nerve-trunk, while the nerve-cells in 

 the ganglion are united to the muscle by means of this 

 out-going nerve-trunk. Therefore when any stimulus 

 falls on the skin where this in-coming nerve-trunk takes 

 its origin, the nerve-trunk conveys the stimulus to the 

 nerve-cells in the ganglion. When the nerve-cells receive 

 the stimulus they liberate one of their characteristic dis- 

 charges of nervous energy, which discharge then passes 

 down this out-going nerve and so causes the muscle to 

 contract. Now this particular kind of response is called 

 response by reflex action, because the stimulus wave does 

 not pass in a straight line from the seat of stimulation to 

 the muscle, but passes in the first instance to the gan- 

 glion, and is from it replected to the muscle. This, at first 

 sight, appears to be a roundabout sort of a process, but 

 in reality it is the most economic process available ; for 

 we must remember the enormous number and complexity 

 of the stimuli to which every animal is more or less 

 exposed, and the consequent necessity that arises in 

 the case of highly organised animals of there being 

 some organised system whereby these stimuli shall be 

 suitably responded to. Or, to adopt a happy illustration 

 of Prof. Bain, the stimuli are systematised on the same 

 principle as the circulation of letters by post is system- 

 atised ; for just as in the case of the letters there is no 



