Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/1004

 L I M

L I M '

LIMARIA, in ichthyology, a name given by Gaza and fame other writers to the thynnus or tunny nfh, called the Spanifh mackrel. See the article Thynnus. LIMAX marinus, in zoology, a name given by fome to the lipparis, or as it is commonly termed in Englifh the fea fnail, caught in plenty at the mouths of rivers in Yorkfhire and fome other places. Ray's Ichthyogr. Append, p. 17. LIMB (Cyd.)— Motions of the Limes. It cannot but have been obferved, by every one, that it is much more natural and eafy to make certain motions with both hands, or both feet at once, in the two contrary or oppofite directions, than in the fame direction. We may trace in the air, for inftancc, two fpiral lines with our two hands, with much more eafe, in the contrary directions, than if we attempt to do them the fame way ; and it will be eafy to find that the cafe is exactly the fame with the feet, if wc attempt to do the fame thing with them when fitting ; and the very ■fame difficulty on one hand and facility on the other will be found, if we attempt the fame motion with one hand only, and with the oppofite foot at the fame time : yet it is ex- tremely difficult, and almoft impoffible, to make thefe mo- tions in a contrary direction, at the fame time with the arm and leg of the fame fide. We muft therefore alfo rcfolve into this principle, the power which we have by a fort of natural mechanifm, .and that without any acquired habitude, of ealily making with one hand, or even with both hands at once, exactly the counter action to that motion, which the other band naturally and mqft eafily is found to make, and the contrary of which it cannot itfclf make without difficulty, and repeated trials and cuftom. It is owing to this, that we can with the left hand eafily make the very contrary ffrokes with the pen, that we are accuftomed to make with the right j if we fuffer that left hand to move, without giving our particular atten- tion to what it is doing, as in the playing on fome inftru- ments of mufic, the fingers being after practice left as it •were to themfelves, fucceed more happily than when the attention is more engaged in what they are doing. The letters or figures thus made by counter ftrokes from the unforced action of the left hand, though they appear ftrange and confuted at firff. fight, yet if held up againft a looking- giafs, they will be found fufficiently. determinate and ex- prefs.

In the fame manner that the counter actions to thofe, which feem in fome degree natural to our peculiar limbs, are 'with difficulty performed, there are a great many motions, which we continually do by the mere natural dif- pofition of our limbs freely, without art and without exer- cife ; nay, even without cuftom, either of which, with either hand or foot is equally eafy, and yet we are not with- out great difficulty able to do both, when in contrary directions one to another. Of this nature is the attempt to make two different motions with the hands, in right lines, the one of which mall be perpendicular to the other; the moving one hand, or one foot, perpendicularly up or perpendicu- larly down, at the fame inftant that the other hand or foot is moving, directly from right to left, or from left to right, is, according to this principle, very difficult ; efpecially to make them both perfectly, regularly, and tolerably quick, as is eafily found by any one, in attempting to rub the body upwards and downwards with one hand, and tranfverfly, or from fide to fide with the other ; and much more plainly, if it be attempted to make thefe contrary motions in the air, without touching any body, or touching only very lightly, and performing the motion quickly. This is no where more obvious than in thofe, who are learning to play upon the trumpet marine, a firing inftrument, with a re- markably long neck, where the running up and down the hand to make the notes is found extremely aukward, while the other is employed in an exactly contrary motion Tideways, in directing the bow.

This difficulty of making motions, at the fame time, con- trary to one another with two limbs, either of which motions fingly would be no difficulty to either, is not peculiar to the arms ; if we fit down and try the experiment with the feet the fuccefs is found the fame, and even the moving differently the like fingers or toes of the different hands or feet, is found of proportionate difficulty. Nor is the difficulty of thefe motions peculiar to the perpendicular or tranfverfe ones ; the making circles, or fpiral or fcrpen- tine lines, with the two hands, or two feet, at the fame time, in the fame contrary directions, being equally difficult : yet it is equally true, that though it is fo extremely difficult, and almoft impoffible, to make thefe motions in contrary direc- tions at the fame time, yet tbey arc all motions, which we fcparately make, with great facility, and with great quick- nefs. We, in the fame manner, are not able, by any means, to direct our two eyes to two different objects, placed in contrary fituacions at the fame time ; but in this the difficulty, though great in the ftructure of our eyes, is not univerfal to all creatures. The chamasleon being able to do this, and to look before and behind, or in any other ever fo different directions, both at the fame time. Mr. Winflow was led by a great number of observations of this

kind, diligently to examine the difpofitions of the nerves* and their different origins in the brain, the cerebellum, the medulla oblongata, and the fpinal marrow. The firft thing on which this accurate anatomiit made his obfervation, was the crucial interlacement of the nerves in the medulla ob- longata, firft difcovercd by Mr. Petit, and accurately defcribed and figured by him ; and the obfervations of the fame au- thor, of different perfons, who, within Ins own knowledge, bad become paralytic in the limbs on the contrary fide of their body to the affected part of the brain alfo. The being able determin.-'ti.-ly to bring on the like fymptoms in brutes, from injuries to the contrary fides of the brain, and the re- gular croflmg of the filaments which pafs tranfverfely on the medullary arch, commonly called the callous body of the brain, and of other of the nervous fibres, feemed to promife to give fome light into the explication of the diffi- culty of thefe contrary motions being performed at the fame time. When this accurate obferver had fufficiently informed himfelf of thefe croffings of the nerves, and the other re- ciprocal communications of the feveral parts of the brain, or its ^two lateral halves, as well by the two large tranfverfe cords, as by the thick tranfverfe fibres of the final 1 inferior, and triangular furface of the arch ; he beftowed his parti- cular obfervation on the ftructure of the fpinal marrow. This he carefully examined by the microfcope, and befide the tranfverfe fibres, which make the communication be- tween the two columns of this marrow, and which Mr. Petit had before nicely defcribed, there appeared to Mr. Winflow, in the thicknefs of each of thefe columns, a long train very diftinct, and made up of a lefs white and more cloudy matter than the reft j or a medullary cord, which had not been before obferved by any body, any more than the fmall cords and tranfverfe fibres of the fummit of the arch of the brain, or the croffing of its tranfverfe fibres. By thefe feveral croffings, and reciprocal communications of the medullary filaments, which are, as it were, the roots of the nerves of each half, or each feveral fide of the human body ; it feems not impoffible to explain the reafon of the before obferved difficulty of making the contrary motions with the two oppofite limbs at the fame time ; efpecially when it is obferved, that the two fmall parcels of medullary filaments, which compofe the primitive trunks of every pair of nerves that are propagated from the fpinal marrow, have their paflage through the lateral apertures of the vertebrae, and before that feem to proceed partly from the neighbouring column or moiety of the fpinal marrow, and partly from the other column or moiety, by means of the croffing before- mentioned ; and confequently, that by this reciprocal crof- fing, or interlacing, one of the primitive parcels or bundles of filaments of each primitive trunk of the nerves of one fide of the body, muft communicate with the fellow parcel or bundle of filaments of the nerves of the other fide. Every body almoft knows that the motion of the mufcles, and fenfation by the touch, are performed by means of the nerves and their filaments ; but no one has ever yet dif- covercd the true means : this however is certain, that when we would put in motion, at one time, the two legs, or the two arms, or the fingers or toes of each hand or foot ; at that time the crofted filaments of the primitive trunks of the nerves, which go to their correfpondent mufcles on each fide, immediately come mutually into contact, and confent with one another ; and their fo meeting, in the time of their motion, communicates certain impreffions reciprocally to the feveral nerves, which from them are propagated to the two different fides of the body, and continue them to act in concert on the feveral mufcles of both the arms or legs at the fame time, and communicate to both that prin- ciple of motion, which we meant to give alone to one. Mem. Acad. Scien. Par. 1739. Limb of a fewer, among botanifts. See Petal, LIMBERS, in artillery, a fort of advanced train joined to the carriage of a cannon, upon a march. It is compofed of two fliafts wide enough to receive a horfe betwixt them, which is called the filet horfe ; thefe fhafts are joined by two bars of wood, and a bolt of iron at one end, and have a pair of fmall wheels ; upon the axle-tree rifes a ftrong iron fpike, upon which the train of the carriage is put. But when a gun is upon action, the limbers are run out behind her. Limber Ww, in afhip, little holes cut through her floor tim- bers, ferving to let the water to the well of the pump, which otherwife would lie between thofe timbers, where the keel rope runs. LIME {Cyd.) — Lime affords a great variety of figures in its falts. The moft perfect concretions among them feem to be double pointed figures, thick toward one end and growing thin toward the other ; fome are flat, and of a quadrilateral figure, and others flat and very thin but hexangular ; fome are of the fhape of a wedge, being cut off taper near one end, and blunt at the other. Soda, or pot-afh, made of the plant kali, is very elegant and various in its moots. Some are beautiful prifms, with the angles cut off" near each end ; others are oblong, and lefs regularly formed, fome fquare and verv thin ; others oblong, with a tendency to a rhom- boidal fh;ipe, and fome roundifb. Thefe are a few, out

of