minister. If the spinal cord of a Frog be divided in its back, above the crural plexus, so as entirely to cut off the nerves of the lower extremities from connexion with the brain, the animal loses all voluntary control over these limbs, and no sign of pain is produced by any injury done to them. But they are not thereby rendered motionless; for various stimuli applied to the limbs themselves will cause movements in them. Thus if the skin of the foot be pinched, or if a flame be applied to it, the leg will be violently retracted. Or, if the cloaca be irritated by a probe, the feet will endeavor to push away the instrument. We have no reason hence to believe, that the animal feels the irritation, or intends to execute these movements in order to escape from it; for motions of a similar kind are exhibited by men, who have suffered injury of the lower part of the spinal cord, and who are utterly unconscious, either of the irritation which their limbs receive, or of the actions which they perform.

393. We are not to suppose, however, that the stimulus acts at once upon the muscles, without the nervous system being concerned at all; throwing them into contraction by its direct influence. For it is quite certain, that unless the nervous trunks remain continuous with the spinal cord, and unless the part of the spinal cord with which they are connected remains sound (although cut off from connexion with the parts above, and with the brain), no action will result. If the trunks be divided, or either of the roots by which they are connected with the spinal cord be severed, or the lower portion of the spinal cord itself be injured, no stimulation will cause the muscular movements just described. A very good example of this necessity for the completeness of the nervous trunks, which convey impressions to and from the central organ, is found in the movements of the iris, for the contraction and dilatation of the pupil. Here the stimulus of light upon the retina gives rise to a change in the condition of the optic nerve; which, being transmitted to a certain portion of the encephalon with which that nerve is connected, excites there a motor impulse; and this impulse is conveyed through a distinct nerve (a branch of the third pair) to the iris, occasioning contraction of the pupil. Every one knows that this adjustment of the size of the pupil to the amount of light, is effected without any exertion of the will on his own part, and even without any consciousness that it is taking place. It is performed, too, during profound sleep; when the influence of light upon the retina excites no consciousness of its presence,-when no sensation, therefore, is produced by it.

394. The class of actions thus performed, is termed reflex; and we see that every such action involves the following series of changes. In the first place, an impression is made upon the extremity of a nerve, by some external agent; just as when sensation is to be produced. Secondly, this impression is transmitted by a nervous trunk to the spinal cord in Vertebrata, or to some ganglionic mass which answers to it in the Invertebrata. But instead of being communicated by its means to the mind, and becoming a sensation, it immediately and necessarily executes a motor impulse; which is reflected back as it were to certain muscles, and, by their contraction, gives rise to a

movement. We shall hereafter see, that nearly all those movements in the animal body, which are immediately connected with the maintenance of the organic functions, such as those of respiration, deglutition (or swallowing), the expulsion of the fæces, urine, and foetus, &c., -are performed in this manner.

395. Now there is strong reason to believe that the changes which take place in the nervous trunks are of the same nature, whatever may be the source from which they proceed,-whether, for example, the movement is simply reflex, whether it proceed from a mental emotion, or whether it be executed in obedience to an act of the will. It was formely supposed that all the afferent or centripetal fibres pass up to the Brain, and that all the efferent or centrifugal fibres pass down from the same organ; the Spinal Cord being looked upon as little else than a bundle of nerves. It is now known, however, that by far the greater part of the fibres of any trunk terminates in the central organ, to which that trunk at first proceeds; and that the Spinal Cord may be considered as a series of such ganglionic centres, each receiving the afferent fibres, and giving origin to the efferent of its own segment. So, again, the special sensory nerves, the olfactive, optic, auditory, and gustative, terminate in their own ganglionic centres, which lie at the base of the brain, in immediate connexion with the summit of the spinal cord, and which are quite independent of the cerebrum. The apparatus for receiving impressions, and for originating motions, is thus complete in itself; and the addition of the cerebrum does not make any essential difference in its operations, save that this sensori-motor apparatus (as it may be termed) is made to act through its means as the agent of the mind, in addition to its functions as the instrument of the automatic movements. We shall hereafter see (CHAP. XII.), that the difference between Instinct and Intelligence is closely connected with the development of the cerebrum; but that this organ, even in that highest grade of development which it possesses in Man, has no other connexion with the sensory organs than that which it acquires through its relation with the sensory ganglia, and has no more power of exciting muscular movement, than by playing (so to speak), upon the spinal cord, whose efferent fibres respond to its mandates, just as they would do to the stimulus, of an impression primarily acting through that organ.

396. Of the mode by which the effects of changes in one part of the Nervous system, are thus instantaneously transmitted to another, nothing whatever is known. There is evidently a strong analogy between this phenomenon, and the instantaneous transmission of the Electric power along good conductors; but the relation is much more intimate than this, for Electricity is capable of exciting Nerve-force whilst, conversely, Nerve-force can excite Electricity. Thus a very feeble galvanic current transmitted along a motor nerve, serves to excite contractions in the muscles supplied by it; and in like manner, a galvanic current transmitted along any of the sensory nerves, give rise to a sensation of the kind to which the nerve ministers. Moreover we shall hereafter see, that certain animals are capable of generating Electric power in a very remarkable manner (CHAP. X.); and that the

nervous force is essentially concerned in this operation. But, on the other hand, it is quite certain that the influence transmitted along the nerves of the living body is not ordinary electricity; for all attempts to procure manifestations of electric changes in the state of nerves, that are acting most energetically on muscles, have completely failed; and a nerve remains capable of conveying the influence of electricity, when it has been rendered unable to transmit the influence of the brain, as by tying a ligature round it, or by tightly compressing it between the forceps, which gives no interruption to the one agency, while it completely checks the other.-Notwithstanding, then, the strong analogy which exists between these two powers, we are not warranted in regarding them as identical; but they have towards each other that relation of reciprocity, which exists between Electricity and Heat, or between Electricity and Magnetism, each being convertible into the other in a certain definite ratio (§ 53).

397. It is more desirable, however, that we should understand the conditions under which the phenomena of the Nervous System take place, than that we should spend much time in discussing the identity of its peculiar powers with any others in Nature. The conducting power of the nervous fibres appears to remain with little decrease for some time after death, especially in cold-blooded animals; for we can, by pinching, pricking, or otherwise stimulating the motor-trunk, give rise to contractions in the muscles supplied by them, exactly as during life. This power is much lessened by the influence of narcotics; so that if a nervous trunk be soaked in a solution of opium, belladonna, or other powerful narcotic, it ceases to be able to convey the effects of stimuli to the muscles, some time before the muscles themselves lose their contractile power. On the other hand, it seems to be exalted by various irritating influences; so that, when the nervous trunk has been treated with strychnia, or when it has been subjected to undue excitement in other ways, a very slight change is magnified (as it were) during its transmission, and produces effects of unusual intensity.

398. Now although the conducting power of the fibrous structure will continue for a time, after the circulation through it has ceased, the peculiar endowments of the vesicular substance, by which it originates the changes which the former transmits, are only manifested, when blood is moving through its capillaries. Thus if the circulation through the brain cease but for a moment, total insensibility, and loss of the power of voluntary motion, immediately surpervene. The brain is supplied with blood through four arteries, the two internal carotids, and the two vertebrals; and by the communication of these with each other through the circle of Willis, the circulation will still be kept up, if only one of them should convey blood into the cavity of the cranium. Hence it is necessary that the flow of blood should be checked through all of them, in order that the functions of the brain should be suspended; and the suspension is then complete and instantaneous. The best method of effecting this was devised by Sir Astley Cooper. He tied both the carotid arteries in a dog: which, for the reasons just mentioned, did not produce any decided influence on the functions of the brain, the circulation being kept up through the vertebrals. But upon compress

ing the latter, so as to suspend the flow of blood through them, immediate insensibility, and loss of voluntary power, were the result. When the compression was taken off, the animal immediately returned to its usual state; and again become suddenly insensible, when the pressure was renewed. Although the functions of the brain were thus suspended, those of the spinal cord were not; as was shown by the occurrence of convulsive movements. But in the state called Syncope, or fainting, the suspension of the circulation, by a failure in the heart's action, causes an entire loss of power in both these centres; and a complete cessation of muscular movement is the result. This condition may come on instantaneously, under the influence of powerful mental emotion, or of some other cause, which acts primarily in suspending the heart's action, and consequently in checking the circulation; the insensibility, and loss of muscular power, are secondary results, depending upon the suspension of the powers of the nervous centres, consequent upon the cessation of the flow of blood through them.

399. The due activity of the vesicular nervous matter is not only dependent upon a sufficient supply of blood, but it requires that this blood should be in a state of extreme purity; for there is no tissue in the body, whose functions are so readily deranged, by any departure from the regular standard in the circulating fluid,-whether this consist in the alteration of the proportions of its normal ingredients, or in the introduction of other substances which have no proper place in it. One of the most fertile sources of disturbance in the action of the brain, consists in the retention of substances within the blood, which ought to be excreted from it. We shall hereafter see, that three of the largest and most important organs in the body,-the lungs, the liver, and the kidneys, have it for their special office, to separate from the circulating fluid the products of the decomposition, which is continually taking place in the body; and thereby to maintain its purity and its fitnesss for its important functions. Now if these, from any cause, even partially fail in their office, speedy disturbance of the functions of the nervous centres is the result. Thus if the lungs do not purify the venous blood of its impregnation of carbonic acid, or restore to it the proper proportion of oxygen, the functions of the brain are seriously affected. The sensations become indistinct, the will loses its control over the muscles, giddiness and faintness come on, and at last complete insensibility supervenes. Corresponding symptoms occur, though to a less serious degree, when the excretion of carbonic acid is but slightly impeded. Thus when a number of persons are shut up in an ill-ventilated apartment, for a sufficient length of time to raise the proportion of carbonic acid in the air to 1 or 2 per cent., the continued purification of their blood by respiration is but insufficiently performed, for reasons which will be stated hereafter (CHAP. VIII.); and the carbonic acid accumulates in their blood in a sufficient degree, to produce headache and obtuseness of the mental powers.-Similar results take place, as will be shown hereafter, from the retention of the substances which ought to be drawn off by the liver and kidneys; tl ese, when they accumulate in even a trifling degree, produce torpor of the functions of the brain; and when their proportion increases, complete cessation of

its powers is the result, their action being precisely that of narcotic poisons. Various substances introduced into the blood may exert similar influences; depressing the activity of the vesicular substance of the nervous centres, and consequently producing torpidity, not merely in regard to the reception of impressions, and the performance of voluntary motions, but also in the mental operations generally.

400. On the other hand, various conditions of the blood, especially those depending on the presence of certain external agents produce an undue energy in the functions of the nervous centres; which energy, however, is almost invariably accompanied by irregularity, or want of balance among the different actions. Of this we have a familiar example in the operation of alcohol. Its first effect, when taken in moderate quantity, is usually to produce a simple increase in the activity of the cerebral functions. A further dose, however, occasions not merely an increase, but an irregularity; destroying that power of self-control, which is so important a means of balancing the different tendencies in the healthy condition of the mind. And a still larger dose has the effect of a narcotic poison; producing diminution or suspension of activity in all the functions of the brain. In some persons, this is the mode in which the alcohol acts from the first, its stimulating effects being altogether wanting. A similar activity is usually produced by the respiration of the Nitrous Oxide, which seems to increase all the powers of the mind, save that of self-control, which it diminishes; the individual, while under its influence, being the slave of his impulses, which act on his muscular system with astonishing energy. Very analogous to this, is the incipient stage of mania; which is simply an undue energy of the cerebral functions at first in some degree under the control of the will, but afterwards, increasing to an extent that renders the individual completely powerless over himself; and showing itself in the intensity of the sensations produced by external objects, in the vividness of the trains of thought (which, being entirely uncontrolled, succeed each other with apparent irregularity, though probably according to the laws of association and suggestion), and in the violence of the muscular actions. Such a state may continue for some time, without the intervention of sleep; but the subsequent exhaustion of nervous power is proportioned to the duration of the excitement; and frequent attacks of mania almost invariably subside at last into imbecility.

401. In these cases of undue excitement, there is obviously an increase in the supply of blood to the head; as indicated by the suffusion of the face, the injection of the conjunctiva, the throbbing at the temples, the pulsation of the carotids; and we find that measures which diminish the activity of the circulation through the brain, are those most effectual in subduing the excitement. But it does not at all follow, that this undue action of the brain should be connected with an excess in the whole amount of nutritive material, and should require general depletion for its treatment. In fact, a very similar class of symptoms may present itself under two conditions of an entirely opposite kind,inflammation, accompanied with an increase in the proportion of fibrine in the blood, and requiring treatment of a lowering kind,—and irritation, depending on a state of blood in which there is a deficiency of solid