materials, and requiring a strengthening and even a stimulating regimen. The skill of the practitioner is often put to the test, in the due discrimination between these states.

402. The preceding examples mark the influence of various causes upon the actions of the vesicular matter of the brain; others might be adduced to show, that the vesicular substance of the spinal cord is also liable to have its powers depressed or excited; but these will be best adverted to hereafter, when the distinct functions of that organ are under consideration (CHAP. XII.) We may simply notice, that the stimulating effect of Strychnia is peculiarly and most remarkably exerted upon the vesicular substance of the spinal cord; and that a corresponding state, in which violent convulsive actions are excited by the most trifling causes, sometimes presents itself as a peculiar form of disease, named Tetanus, which may be either idiopathic, depending probably upon a disordered condition of the blood, or traumatic, consequent upon the irritation of a wound.

403. But, as formerly remarked, it is not in the Nervous centres only, that changes originate. Whenever an impression is made upon the surface of the body, or upon the organs of special sense, which, being conducted to the nervous centres, either excites a sensation in the brain (§ 390), or a reflex action through the spinal cord (§ 392), the reception and propagation of such impression at the extremities of the sensory nerves requires a set of conditions of the same kind with those, which we have seen to exist in the nervous centres. In fact, if we regard the course of the motor nerves as commencing in the nervous centres and terminating in the muscles, we may with equal justice consider that of the sensory nerves as originating in their peripheral extremities, and terminating in the sensorium. And, as already stated (§ 381), precisely the same kind of vesicular structure exists in some (probably in all) of the peripheral expansions of the sensory nerves, as makes up the gray substance of the brain and spinal cord. Now it is easily shown, that the circulation of the blood through these parts is just as necessary for the original reception of the impressions, as is the circulation through the brain to their reception as sensations, and to the origination of motor impulses by an act of the will. We find that anything which retards the circulation through a part supplied by sensory nerves, diminishes its sensibility; and that if the flow of blood be completely stagnated, entire insensibility is the result. A familiar example of this is seen in the effects of prolonged cold; which, by diminishing, and then entirely checking, the flow of blood through the skin, produces first numbness, and then complete insensibility of the part. This result, however, may be partly due to the direct influence of the cold upon the nerve-vesicles themselves; depressing their peculiar vital powers (§ 97). The same effect is produced, however, when the supply of blood is checked in any other way; as, for example, by pressure on the artery, or by obstruction in its interior. Thus when the main artery of a limb is tied, numbness of the extremities is immediately perceived; and this continues, until the circulation is re-established by the collateral branches, when the usual amount of sensibility is restored. Again, in the gangrene which depends upon obstruction of the arterial trunks by a

fibrinous clot in their interior, diminution of sensibility, consequent upon the insufficient circulation, is one of the first symptoms.

404. On the other hand, increased circulation of blood through a part produces exaltation of its sensibility; that is, the ordinary impressions produce changes of unusual energy in its sensory nerves, This is particularly evident in the increased sensibility of the genital organs of animals during the period of heat; and in those of Man, when in a state of venereal excitement. Moderate warmth, friction, exercise, and other causes which increase the circulation through a part, also augment its sensibility; and this augmentation is one of the most constant indications of that state of determination of blood, or active congestion, which usually precedes inflammation, and which exists in the parts surrounding the centre of inflammatory action. But it must be borne in mind, here as elsewhere (§ 401), that such exaltation of function in a limited part, is quite consistent with general debility; and in fact we may often observe, that the tendency of such local affections is particularly great, when the blood is in a very poor condition. (See CHAP. V.)

405. To sum up, then, we may compare the vesicular substance, wherever it exists, to a galvanic combination: the former being capable of generating nervous influence, and transmitting it along the fibrous structure, to the part on which it is to operate; in the same manner as the latter generates electric power, and transmits it along the conducting wires, to the point at which it is to effect a decomposition or any other change. In one of the most perfect forms of the galvanic battery (that invented by Mr. Smee), although the metals remain inserted in the acid solution, and are consequently always ready for action, no electricity is generated until the circuit is complete; and the waste of the zinc, produced by its solution in the acid, is therefore exactly proportional to the electric effects to which it gives rise. The condition of the nervous system, in the healthy and waking state, bears a close analogy to this; for it is in a state constantly ready for action, but waits to be excited; and its waste is proportional to the activity of its function. The vesicular matter, diffused over the surface of the body, is inactive, until an impression is made upon it by some external agent; but a change then takes place in its condition (of which we know no more, than that the presence of arterial blood and a certain amount of warmth are necessary for it), which is transmitted to the central organs by the sensory trunks. It would appear that the excitement of this change has a tendency to increase the afflux of blood to the part; thus when a lozenge or some similar substance is allowed to lie for a time in contact with the tongue, or with the side of the mouth, a roughness is produced, which is due to the erection of the sensory papillæ, by the distension of their blood-vessels.-On the other hand, the change in the vesicular matter of the central organs, by which motion is produced in the distant muscles, may be excited either by the stimulus conveyed by the afferent nerves (as in reflex action, § 392), or by an act of Mind. This act may be voluntary, originating in the will; or it may be instinctive or emotional, resulting from certain states of

mind excited by sensations, and altogether independent of the will. Of the mode in which the mind thus acts upon the nervous system, we know nothing whatever, and probably never shall be informed in our present state of being. But it is sufficient for us to be aware of the physiological fact of the peculiar connexion between the mind and the brain; a connexion so intimate, as to enable the mind to receive through the body a knowledge of the condition of the Universe around it, and to impress on the body the results of its own determinations; and of such a nature, that the regularity of the working of the mind itself is dependent upon the complete organization of the brain in the first instance, upon the constant supply of pure and well elaborated blood, and upon all those influences which favor the due performance of the nutritive operations in general.

BOOK II.

SPECIAL PHYSIOLOGY.

CHAPTER IV.

OF FOOD, AND THE DIGESTIVE PROCESS.

1. Sources of the Demand for Aliment.

406. THE dependence of all Organized beings upon food or aliment, must be evident from the facts stated in the preceding portion of this Treatise. In the first place, the germ requires a large and constant supply of materials, with which it may develope itself into the perfect being, by the properties with which it is endowed. In all but the lowest tribes of Plants, we find the materials required for the earliest stages of the process prepared and set apart by the parent. Thus in the seed, the germ itself forms but a small proportion of the whole substance, the principal mass being composed of starchy matter which is laid up there for its nutrition; and the act of germination consists in the appropriation of that nutriment by the germ, and the consequent development of the latter, up to the point at which it becomes independent of such assistance, and is able for itself to procure, from the soil and atmosphere that surround it, the materials for its continued growth. So in the egg of the Animal, the principal mass is composed of Albumen and oily matter; the germ itself being, at the time the egg is first deposited, a mere point invisible to the naked eye; these materials serve as the food or aliment of the germ, which gradually draws them to itself, and converts them into the materials of its own structure; and at the end of a certain period, the young animal comes forth from the egg, ready to obtain for itself the food which is necessary for its continued increase in size.

407. In many instances among the lower animals, the form in which the young animal emerges from the egg is very different from that which it is subsequently to assume; and the latter is only attained by a process of metamorphosis. This change has been longest known, and most fully studied, in the case of Insects and Frogs; whic merly thought to constitute an exception to all general respect, the Insect coming forth from the egg in the st

and the Frog in the condition of a Fish. But it is now known that changes of form, as complete as these, occur in a large proportion of the lower tribes of Animals; so that the absence of them is the exception. The fact seems to be, that the supply of nutriment laid up within the egg, among the lower classes, is by no means sufficient to carry on the embryo to the form it is subsequently to attain; and its development is so arranged, that it may come into the world in a condition which adapts it to obtain its own nutriment, and thus to acquire for itself the materials for its further development. Thus the Insect, in its larva or Caterpillar state, is essentially a foetus in regard to its grade of development; but it is a foetus capable of acquiring its own food. In this condition it attains its full growth as regards size, though its form remains the same; but it then, in passing into the Chrysalis state, reassumes (as it were) the condition of an embryo within the egg,-the development of various new parts takes place, at the expense of the nutriment stored up in its tissues, and it comes forth as the perfect Insect. In many of the lower tribes, the animal quits the egg at a still earlier period in comparison; thus it has been lately shown by M. Milne Edwards, that some of the long marine worms consist only of a single segment, forming a kind of head, when they leave the egg; and that the other segments, to the number (it may be) of several hundred, are gradually developed from this, by a process that resembles the budding of Plants.

408. Up to the period, then, when the full dimensions of the body have been attained, and the complete evolution of all its organs has taken place, a due supply of food is necessary for these purposes. In the Plant, nearly the whole of the alimentary materials taken into the system, are thus appropriated; the extension of its structure going on almost indefinitely, and the waste occasioned by decay being comparatively small. Thus the carbon, which is given out by the respiratory process in the form of carbonic acid, bears but a small proportion to that which is introduced by the decomposition of that same gas, under the influence of light (§ 81). And the fall of the leaves, which takes place once a year or more frequently, and which gives back a large quantity of the matter that has undergone the organizing process, does not occur, until by their means a considerable addition has been made to the solid and permanent substance of the tree.

409. This is not the case, however, with the animal. Its period of increase is limited. The full size of the body is usually attained, and all the organs acquire their complete evolution, at a comparatively early period. The continued supply of food is not then requisite for the extension of the structure, but simply for its maintenance; and the source of the demand lies in the constant waste, to which, during its period of activity, it is subjected. We have seen that every action of the Nervous and Muscular systems involves the death and decay of a certain amount of the living tissue, as is indicated by the appearance of the products of that decay in the Excretions, and a large part of the demand for food will be consequently occasioned by the necessity for making good the loss thus sustained. Hence we find that the demand for food bears a close relation to the activity of the animal functions; so that a diet, which would be superfluous and injurious to an individual