is, indeed, another element which, although it was not entirely overlooked by this great philosopher, certainly was not duly appreciated by him, that is, naming; an element, the nature of which Mr. Mill has investigated at great length, and the operation of which he has taken great pains to trace and to elucidate, often with the happiest result. In a word, sensation, association, and naming, are the three elements which are to the constitution of the mind what the four elements, carbon, hydrogen, oxygen, and azote, are to the composition of the body.

Instead of entering into any analysis of Mr. Mill's work, of which the brief space that remains to us will not admit, we shall conclude with an observation or two on a point on which Mr. Mill does not enter. Fully aware of the importance of having in the commencement of the study of the mental phenomena a distinct conception of the simple elements of which the compound mental states are composed, Mr. Mill begins his investigation with sensation, and with those cases of sensation which are most familiar, and which can be most easily thought of by themselves; that is, of which a conception free from the mixture of any extraneous ingredient can be most certainly formed. We recommend this part of the work to the special attention of the student. He will find his subsequent analysis of the complex mental phenomena exceedingly facilitated by acquiring in the commencement a familiar acquaintance with the simple mental states, that are the immediate result of sensation. And for the more clear and perfect understanding of sensation itself, it is desirable that some account should be given of the physical apparatus, and of the physiological process upon which it depends. It is true that the most perfect knowledge of the anatomy and physiology of the organs concerned in sensation, do no more than make us acquainted with the ultimate facts, the exact events that happen: but still, since a knowledge of those events enables us to understand with greater precision the simple states of sensation, it may be worth while to expound in a few brief words the sum of what physiologists, up to the present time, have succeeded in making out. The physical phenomena to which it is necessary to attend, are those which relate both to sensation and to motion. Often sensation and motion are closely related, being reciprocally antecedents and sequents: at other times there is no relation whatever between them; and it is important, as will be seen immediately, to have a clear perception of the cases in which that relation does or does not subsist.

Sensation is a function of the nervous system; motion is a function of the muscular fibre. The nervous system consists of

brain, spinal cord, and nerves. Of the structure of the brain and spinal cord it is not necessary to our present purpose to say any thing; it is only requisite to state that there is the most satisfactory evidence, that the seat of sensation is exclusively in these portions of the nervous system.

Nerves are attached by one extremity to the brain or spinal cord, and by the other extremity to certain instruments, which are called organs. A nerve is composed of nervous matter, and of membrane. The nervous matter is a soft pulpy substance, perfectly analogous to that of the brain and spinal cord; the membrane forms a sheath or tube around this matter, so as completely to enclose it. From the extremity of a nerve, which is attached to the brain or spinal cord, to that which terminates in an organ, this nervous matter is uninterruptedly continuous, and this continuity is essential to its function.*

An organ of sense consists of organized substance and of nervous matter. The conformation of the organized substance is such as specifically to adapt it to receive and modify certain impressions produced by external bodies; the conformation of the nervous matter is such as suitably to dispose it for the reception of those impressions after they have been thus modified: to this nervous matter one extremity of the nerve is attached, while the other extremity, as has been stated, is united to the brain or the spinal cord; whatever impression is received by the nervous matter of the organ of sense is conveyed by it to the nerve, and by the nerve is transmitted to the brain or the spinal cord. We are now prepared to understand the events that take place in the operation of sensation. These events are three :First, the communication of an impression by the organ of sense to the nerve which is in connection with it; secondly, the transmission of that impression along the trunk of the nerve to the brain or spinal cord; thirdly, the reception of that impression by the brain or spinal cord. By the most ample deduction of facts it is proved, that an external impression cannot produce sensation unless each of these events take place, and in the exact order here stated. The effect produced in the organ of sense, that is the impression it receives, must be communicated to the nerve; the nerve must transmit it to the

* From some curious experiments which have been lately performed, it would seem that the function of a nerve can be performed without an absolute continuity of its nervous matter: if the trunk of a nerve be cut, and its cut ends be placed within the distance of half an inch of each other, the function of the nerve goes on; if they are removed to a greater distance the function of the nerve is wholly at an end,

brain or spinal cord; the brain or spinal cord must receive it: that reception is sensation.

Motion is of two species, voluntary and involuntary. The distinction with reference to our present subject is important, because sensation is connected with the first, but it is not connected with the second.

Voluntary motion includes those actions of the muscles by which locomotion is performed, or any desire or purpose of the animal accomplished. Involuntary motion includes those actions of the muscles by which the conservative functions are carried on ; that is, the greater part of those minute operations that compose the functions of digestion, respiration, circulation, secretion, excretion, and so on-functions which maintain the life of the animal.

Both species of motion are performed by the action of the muscular fibre; by that action which is called contraction, and which consists in the shortening of the fibre. No muscular fibre can contract of itself, it must be excited to contraction by some external agent; that agent, whatever it be, is called a stimulant. To all voluntary muscles there belongs but one proper stimulant, that is, volition, or rather some nervous influence sent by volition into these muscles. To involuntary muscles there are numerous stimulants. In general each involuntary muscle has its own specific stimulus. Thus the specific stimulus of the heart is blood of the right side of the heart venous blood, of the left side of the heart arterial blood. The specific stimulus of the stomach is aliment: of the small intestines chyle: of the large intestines the refuse matter of the aliment which has not been converted into chyle, and so on. Hence we learn why the action of voluntary muscles is only occasional: why on the contrary the action of involuntary muscles, is for the most part incessant, continuing day and night without intermission during the whole period of life. Muscles act only when their proper stimulus is applied to them. The stimulus of the voluntary muscles is applied to them only at intervals; namely when the will of the animal excites them to action: on the other hand the stimulus of involuntary muscles is applied to them incessantly, the nutritive processes of the economy requiring their unintermitting action. The volition of the animal can at any time determine to the voluntary muscles their proper stimulus,hence they must at all times be at the command of the animal, and this it is which renders them voluntary. But the proper stimulus of the involuntary muscles cannot be determined to them by the will of the animal: it is determined by peculiar vital laws over which the volition of the animal has no control;

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and, in consequence of this arrangement, they must necessarily be placed wholly beyond the command of the animal, and for that very reason rendered involuntary. The wisdom of the constitution of this part of the animal economy is admirable. The voluntary muscles are the instruments by which the animal accomplishes its desires of these instruments it is necessary that it should have the command. But the involuntary muscles are the instruments by which the physical processes that sustain life are carried on: these operations inconceivably minute, countless in number, and requiring for the conservation and integrity of the whole machinery that their action should be unceasing, were these operations placed in any degree within the control of the animal, might be greatly impeded by his volition, but could not possibly be promoted by it: they are therefore placed beyond it.

In voluntary motion the events that form the train belonging to the operation, take place in the inverse order of those of sensation. In voluntary motion there is first an influence communicated from the brain or spinal cord to the nerve; this influence is propagated, down the trunk of the nerve, to the muscle; and when received by the muscle it produces an effect upon its fibres which causes them to contract. The proof is even more full and complete that all these events are essential to the action of the muscle, than it is that each event in the train of sensation is indispensable to the result. In motion then the influence begins in the brain or spinal cord, and ends in the instrument: in sensation, on the contrary, it begins in the organ or instrument, and ends in the brain or spinal cord.

It is a curious fact for the establishment of which we are indebted to the accurate experiments of modern physiologists, that the nerve of sensation is perfectly distinct from the nerve of motion, and that each has its own peculiar and appropriate seat in the brain or spinal cord.

We have stated that a nerve is composed of a quantity of nervous matter enclosed in a tube of membrane. But when a nerve is particularly examined, it is found to consist of a number of smaller nerves: to be, in fact, an aggregate of a vast multitude of threads of nervous matter, each enveloped in its own membrane: these threads are called filaments. A glance at the accompanying wood-cut will convey to the reader a more distinc'. conception of this structure than any description. A represents a nerve enveloped in its membrane; BB the distinct filaments of which it is composed; C one of

B

A

B

these filaments dissected out. Now these filaments, taken altogether, may perform one function, that of sensation for example, or some may perform the function of sensation and some that of motion; but the filaments of sensation, though bound up in the same bundle with those of motion, apparently for the convenience of distribution to their respective organs, always remain perfectly distinct; they are merely in juxta-position: they never intermix in substance, never interfere in function. And the important fact is, that if all the filaments of a nerve perform the same function, they are all united to the same part of the brain or spinal cord; but if some perform one function and some another, some filaments are united to one part of the brain or spinal cord and others to another part. The spinal nerves, for instance, are compound nerves, that is, some of the filaments of which they are. composed are for sensation, and others for motion. The arrangement of these different filaments at their junction with the spinal cord may be distinctly seen by referring to the annexed figure: AA represent the spinal cord seen in front; BBBB,

the membrane in which it is enveloped reflected back, C a spinal nerve with all the filaments of which it is composed mixed together, D E two masses of these filaments separating from each other, the one E going to be attached to the anterior surface of the spinal cord F, and the other D going to be attached to the posterior surface. Now it is established by the . most satisfactory evidence, that the filaments attached to the anterior surface are for motion, while those attached to the posterior surface are for sensation. If in a living animal the anterior filaments are pricked or irritated, the animal gives no indication of feeling, but the muscles to which the injured filaments are distributed, are thrown into violent contractions: on the contrary,