causes muscular contraction, passes along a nerve in one direction; and that which causes sensation, in a contrary direction. The question is, can the same nerve perform both functions at once?

:

It was after years of intense study and application that Sir Charles Bell answered this question. His idea is, that "the nerves of the body possess distinct and appropriate functions, corresponding with the parts of the brain and spinal marrow with which they are connected at their ROOTS; and when a nerve, which appears simple, is found to bestow more than one endowment, it is a sign that that nerve has more than one origin from the brain, and consists, in reality, of several nerves joined together." He maintained that each nerve of sense is limited to receiving a distinct and appropriate impression that the nerve of vision can only give ideas of light and colour; the nerve of hearing, impressions of sound; the nerve of smelling, the perception of odours, and so on; and that these special properties depended on each of the nerves of sense having its root in a distinct portion of the brain, provided and adapted for receiving its own peculiar impression. To establish his theory, he took a nerve of the arm, and, tracing it from the arm towards its origin in the spinal marrow, he found that, as it approached that origin, it subdivided into two parts or roots; that one of these roots entered a division of nervous substance distinct from the other; that the root which passed to the posterior division of the spinal marrow had a ganglion or an accumulation of nervous substance upon it, and bestowed sensation alone; while the root which went to the anterior division gave motion alone. While the great fact was thus brought out that the nerves of sensation are distinct from those of motion, Dr. Marshall Hall laid claim to the discovery of another set of nerves, which he calls the EXCITO-MOTORY, and which have their root or origin in the true spinal marrow; whereas, according to his theory, the nerves of sensation and volition only run along the course of the spinal cord. These excitor nerves have peculiar excitabilities, and pursue their course principally from internal surfaces to the MEDULLA OBLONGATA, or termination of the true spinal marrow; while the motor nerves pursue a reflex course from that medulla to certain muscles. Hence their name. The nerves of sense may act spontaneously, but these are always excited. Besides these two grand divisions, there is a third distribution known by the name of the ganglionic or sympathetic nerves, which are peculiar to the viscera of the chest and abdomen. Let us try and simplify this a little.

We have the spinal cord, the top of which is called THE MEDULLA OBLONGATA, or the spinal marrow lengthened in its upward course till it becomes united with the brain.

In the brain we have the CEREBRUM, or that portion of it which is situated in the front and top of the skull, and which is the seat of sensation. We have also the CEREBELLUM, or that portion which is situated in the back of the skull, which is divided into two lobes, and which is inseparable from the power of volition.

among the viscera of the abdomen, and intended to unite in sympathy those parts by which the various organic functions are performed; such as secretion, absorption, circulation, assimilation of the food, the growth and decay of the body, and so on. But this is a point which we leave for the present. Sir Charles Bell was of opinion, that there are certain nerves which pass off from a circumscribed central portion of the nervous system-the medulla oblongata-and diverge to different parts of the head, neck, throat, and chest,-or those stru tures which together form a mechanism for respiration-that such an arrangement is not to be found in the lowest animals, but would seem to be a gradual development in the animal kingdom, until in man it becomes the organ of Voice and Expression. These he named RESPIRATORY NERVES. Of his numerous examples, we select the two following: :"Observe the condition of a man convulsed with laughter, and consider what are the organs or system of parts affected. He draws a full breath, and throws it out in interrupted, short, and audible cachinnations; the muscles of his throat, neck, and chest, are agitated; the diaphragm is especially convulsed. He holds his sides, and, from the violent agitation, he is incapable of a voluntary act. It is impossible to avoid the conclusion, that it is the respiratory organs and their muscles which are affected during the paroxysm of laughter. "There is no state of suffering from which we can so well infer the nature of the agitation of the frame as from hydrophobia. The patient being sensible of his condition; and calm, and aware of the experiment which is to be made upon him by his physician when he calls for a glass of water, cannot resist the inHuence of the disease. He shudders-his face assumes an expression of extreme horror and alarm-convulsive gulpings take place in his throat;-he flies to some support, and clings to the bedpost in an agony of suffocation. This I have witnessed in a powerful man. I have had the pain of seeing the disease in a girl of eighteen. The irritability of the skin being increased to an awful degree, so that the touch of her long hair falling on the naked body excited the paroxysms. These recurred with a sense of choking, with sudden and convulsive heavings of the chest, a shuddering, and catching of the muscles of breathing, and an appalling expression of suffering. The paroxysms in such a case becoming more frequent and severe, finally exhaust the powers of life. In these convulsions, it is the nervous and muscular systems belonging to the natural function of respiration which are affected; and as they are also the organs of expression, the condition is seen, not only in the countenance, but in the throat and chest, to be that of extreme horror."

other for sensation.

In addition to these, there is THE FIFTH CEREBRAL NERVE, whose larger root confers sensation on all parts of the head not supplied by the superior spinal nerves; while its lesser root gives the power of action to those muscles which we employ in mastication or the chewing of our food.

Then come THE EXCITO-MOTORY NERVES, which arise from the medulla oblongata, or true spinal marrow. The excitement passes from some internal surface to the medulla or marrow, and from thence the motive-power is conveyed to the muscles to be affected and moved.

Lastly, comes THE SYMPATHETIC SYSTEM. This consists of ganglia, or accumulations of nervous substance, scattered

Viscera, from the Latin VISCUS, the bowels or entrails, but including the heert, liver, and lungs. ABDOMEN is also from the Latin-and signifies the belly.

stimuli.

These nerve-fibres are full of nervous matter, are arranged in parallel or interlacing bundles, and the whole nerve is enclosed in a sheath or tube, which is composed externally of a very delicate transparent membrane. Within this tube is a hollow cylinder, of a substance which differs from the matter that occupies the centre of the tube-the centre being occupied by a transparent substance, which is the essential component of the nervous fibre. The diameter of these little tubes is between both and 6th part of an inch. It may, however, be as much as both, and as little in some few instances as 16th. They are larger in the nerve-trunks than in the brain.

Now what is the purpose, design, or end of this grand nervous system? It may be that it has little to do with those operations or functions which go to make up the vegetative or organic life of the animal. For example:-it may have little to do with the reduction of food in the stomach, since that is a purely chemical operation produced by the solvent power of the gastric juice. It may have nothing more to do with the process of absorption, this being a purely vegetative operation,

Dis, that is, dia in another form, may be rendered by the phrase, in two directions, or in different ways, as in distract (from dis and traho, I draw); to distract is to draw a person's mind in two or more directions so as to produce confusion and pain. Dis is found in these forms; namely, di, dif, div.

Di, dif, &c., as in diverse (from di and versus, turned), turned in opposite directions, different, opposed;

"And for there is so great diversitie

In English, and in writing of our tong,
So pray I God that none miswrite thee,
Ne misse the metre for defaut of song."

Chaucer, "Troilus."

Dif, as in dificult, where the dif (dis) has a reversing force; difficult comes from dis and facilis; facilis is the Latin for easy, the a being changed into i as is customary in compounds of facio; so that difficult is equivalent to our uneasy; that is, not easy.

Dir (of Latin origin), as in dirge, a sacred song, so called from the beginning of the Psalm, "Dirige nos, Domine" (Direct us, O Lord), and accustomed to be sung at funerals.

"The raven croak'd, and hollow shrieks of owls,
Sung dirges at her funeral."

Ford, "Lover's Melancholy."

Down, of Saxon origin, is the expression of descent; hence motion from a higher to a lower level; and hence, perhaps the application to "the downs; that is, hillocks viewed in relation to their declivities. Down was formerly used as a verb.

"The hidden beauties seem'd in wait to lie,

To down proud hearts that would not willing die."
Sir P. Sidney, "Arcadia."

Dun, in Saxon, signifies an elevation, a hill, and even a mountain; it may be the origin of our ton as in Broughton a fortified height. Downs may be hence derived. In Webster's Dictionary Downs are defined as "ridges of high land, such as lie along the coasts of Essex and Sussex, in England; hence roads in which ships lie What is called "Salisbury off these hilly coasts at anchor." Plain" is, in the parts near the city, a chalky down, famous for feeding sheep.

The wretched parents all that night
Went shouting far and wide;

But there was neither sound nor sight
To serve them for a guide.

At daybreak, on a hill they stood
That overlook'd the moor;

And thence they saw the bridge of wood,
A furlong from the door.

They wept, and turning homeward, cried,
"In heaven we all shall meet :"
-When in the snow the mother spied

The print of Lucy's feet.

Half breathless from the steep hill's edge
They track'd the footmarks small;
And through the broken hawthorn-hedge
And by the long stone-wall;

And then an open field they cross'd;
The marks were still the same;
They track'd them on, nor ever lost;
And to the bridge they came.
They follow'd from the snowy bank,
Those footmarks, one by one,
Into the middle of the plank;

And further there were none !

Yet some maintain that to this day
She is a living child;

That you may see sweet Lucy Gray
Upon the lonesome wild.

O'er rough and smooth she trips along,
And never looks behind;

And sings a solitary song

That whistles in the wind.

EXERCISES IN COMPOSITION.
HISTORICAL THEME.

Jacob's Journey to Padan-aram.

EXERCISES FOR PARSING.

LUCY GRAY.

Oft I had heard of Lucy Gray:
And, when I goss'd the wild,

I chanced to see at break of day
The solitary child.

No mate, no comrade, Lucy knew;
She dwelt on a wide moor,
The sweetest thing that ever grew
Beside a human door!

You yet may spy the fawn at play,
The hare upon the green;
But the sweet face of Lucy Gray
Will never more be seen.
To-night will be a stormy night-
You to the town will go

And take a lantern, child, to light
Your mother through the snow."
That, father! will I gladly do:
'Tis scarcely afternoon-

The Minster-clock has just struck two,
And yonder is the moon."

At this the father raised his hook,
And snapp'd a fagot-band;

He plied his work;-and Lucy took,
The lantern in her hand.
Not blither is the mountain roe:
With many a wanton stroke
Her feet disperse the powdery snow,
That rises up like smoke.

The storm came on before its time:
She wandered up and down;
And many a hill did Lucy climb;
But never reach'd the town.

LESSONS IN GEOLOGY.-No. VIII. By THOMAS W. JENKYN, D.D., F.G.S. CHAPTER I.

THE ACTION OF VOLCANOES ON THE EARTH'S CRUST.

SECTION IV.

ON CHANGES IN THE ASPECT OF VOLCANIC MOUNTAINS.

YOUR last lesson will have taught you that, in the progress of centuries, very great changes must necessarily take place in the configuration, or in what may be called, the physiognomy Think-how the expansive power of of volcanic mountains. the heat below may be cracking the sides of the mountains into fissures through which streams of lava flow;-how the cliffs or the walls of the crater may be falling into the tremendous chasm as the result of the dilapidating action of fire;-how volumes of lava may be filling up gullies and ravines, and by this means check or change the course of rivers ;-how rains, melted snow, and rivulets may be wearing away and removing to a distance, the sand, the dust, and the soil which had settled on the sides of the hill;-and, how these and other agencies may be annually and constantly changing the outward character of mountain ridges. The knowledge of these changes is an important element in the study of geology.

To assist you in the knowledge of these changes, perhaps the best way, instead of distracting you with a variety of illustra tions from several volcanoes of the globe, is to fix your attention upon the changes in the aspect of one mountain-such as VESUVIUS.

In the last lesson you were placed upon a safe ledge of volcanic rock which overhung the tremendous crater, and from which you could command a view of the burning lake, and of

the conical formation of fumeroles. In the present lesson, imagine that some years or generations shall have passed away, and that then you revisit that same cliff. The whole scene is changed. The lava does not boil. The fumeroles emit no volumes of vapour, or jets of cinder. The eternal fires have retired to their retreats in the deep caverns of Vulcan. The surface, where the lava burned and boiled, is cooled and consolidated into a firm plain-if plain may be called what is so jagged, rugged, and ruinous, as the scene presented in tig. 17.

This plain or bottom is everywhere covered with massive blocks of lava, and studded with the peaks of extinguished cones or fumeroles.. In the foreground you have blocks of cooled lava: in the centre, the vent of a cone which has been fissured along the whole of its length; on the left an extinguished cone, with a cavernous gash near its base; and, all around, peaks of cones within the walls of the crater.

Leave that rugged scene in a state of rest. In the course of years, the volcano again stirs up deep foundries below, and awakes all its smithies into activity. Volumes of lava boil up.

They

fill the spaces between the conical peaks.

his Geography, he narrates the terrific earthquakes and convulsions which had taken place several times in the Island of Pithecusa, now called Ischia, a little to the north of the Bay of Naples. Of any disturbance in Vesuvius he says nothing. Fig. 18, gives a view of Vesuvius as it was known in the time of Strabo.

According to the description which Strabo has given of the figures of Vesuvius, it seems to have been a truncated cone, with a depression at the summit, which was the remains of an extinguished crater. When Campania, or southern Italy,

They flow into the empty vents and hollow fissures, fill them | up, become hardened into masses or dikes, and make the surface appear almost a perfect level. After many centuries or ages, a section of this part of the mountain comes, by some means or other, to be exposed to the view of a geologist; and then, the multifarious formation of the rock is accounted for, by him, on the principles of the intermittent activities of volcanoes. The changes which I have just described, are alterations which are pro

duced in the internal structure of the mountain. There are also other changes which take place in a volcano's outward physiognomy, or external aspect, so as to make the mountain look different in the landscape. The character of these variations, also, will help you in the study of Geology.

culaneum.

was first colonised by

the Greeks, Vesuvius afforded no marks of a volcanic character, except such as a naturalist, accustomed to the examination of rocks, might have inferred: and these were recognised by Strabo. In his days, the vast cone of the

entire mountain appeared regular in its outline, and crowned with a rounded summit, having edges which encompassed a hollow, nearly filled up, and covered with wild vines. The outside declivities of the hill were clothed with fields highly cultivated, and beautified with fertile orchards and vineyards. At the base of the mountain lay the populous and flourishing cities of Pompeii and Her

In A.D. 63 Vesuvius gave its first notice of action. It convulsed the whole district, and did much injury to houses, villages and towns upon its flanks. From A.D. 63 to 69 the shocks of the mountain were frequent; and, in August of that year, occurred that awful eruption which destroyed the cities of Pompeii and Herculaneum, and proved fatal to the elder PLINY.

The volcanic region best known to the ancients is that of Sicily, and Campania in Italy-but especially that of Naples; for they have handed down to us tolerably distinct and wellconnected records of the history of Vesuvius, which the Italians of the present day call by the name of SOMMA.

Before the Christian era there is no record-there is not even a tradition or a poetical myth, of Vesuvius having been in a state of activity. If such had ever been known, STRABO would have given an account of it; for in the Fifth Book of

The best geologists of the present day think that the eruption which took place in A.D. 79, and subsequent ones, destroyed, or wore away, the side of the cone which is nearest the sea, see fig. 18, leaving the high cliff, now especially called Somma, encircling a new cone

as represented in fig. 19.

After the death of the elder Pliny, his nephew, called the younger Pliny, wrote to the historian Tacitus, a brief but lively account of the phenomena of this eruption. At first, a thick volume of smoke rose vertically from the ancient crater, now ruptured by elastic gases. The top of this column spread itself on all sides like the head of a wheat-sheaf, or the upper boughs of the pine-tree. It was occasionally fired by flashes like lightning, and each flash was succeeded by profound and terrible darkness. Ashes fell on the sea, far from land, and the sea