have much less disposition to pass off by this secretion; and they are consequently to be preferred during lactation.

839. From the close correspondence which exists between the elements of the Milk and those of the Blood, it is evident that we cannot expect to trace the existence of the former, as such, in the circulating current. It is interesting, however, to remark, that a preparation appears to be taking place in the laboratory of the system, for the production of this secretion, long before the period of parturition. The Urine of pregnant women almost invariably contains a peculiar substance termed kiestine, which is nearly related to caseine, and which disappears from the urine as soon as lactation has fully commenced. It would seem, therefore, that a compound of this nature is in course of preparation during pregnancy; and that it is eliminated by the kidney, until the Mammary Gland is prepared for the 'active performance of its functions.-That the kidney may relieve the system from the accumulation of other constituents of the mammary secretion, appears from a case recently put on record; in which the urine of a parturient female, who did not suckle her infant, was found to contain a considerable quantity of butyric acid, during several days. There can be no doubt that, in ordinary states of the system, this secretion cannot be required for the depuration of the blood, since it does not occur in the male at all, and is present in the female at particular times only. But these facts afford ground to believe that, when the process is going on, certain products are generated in the system, which are not found there at other times. And it is quite certain that the sudden checking of the secretion, or the reabsorption of the fluid already poured out, occasioning an accumulation of these substances in the circulating current, may give rise to very injurious consequences. Some very curious instances are on record, in which a transference of the secreting power to some other surface has taken place under such circumstances; so as to relieve the system from the accumulation in question.

CHAPTER XII.

OF THE NERVOUS SYSTEM AND ITS ACTIONS.

1. General View of the operations, of which the Nervous System is the

instrument.

840. We have now considered the entire series of those operations, which make up the vegetative or organic life of the Animal; including those functions by which the germ is prepared, by which it is nourished until it can be left to its own powers, by which its continued development is effected until the fabric characteristic of the adult has been built up, and by which the normal constitution is maintained through a lengthened period, so long as the necessary materials are supplied, and no

check or hindrance is interposed, by external influences, to that regular sequence of changes, on which the continuance of its powers depends. In this survey it will have been perceived, that the essential parts of these operations are, in Animals as in Plants, completely independent of the influence of that which constitutes the peculiar endowment of Animals; namely, the Nervous System.

a. The Reduction of the food in the Stomach, by the solvent power of the gastric fluid, is a purely chemical operation, with which the Nervous System has nothing whatever to do, excepting that it perhaps accelerates the process, by stimulating the Muscular coat of the stomach to that peculiar series of contractions, which keeps the contents of the cavity in continual movement, and favors the action of the solvent upon it.

b. In the process of Absorption, by which the nutritive materials, with other substances, are introduced into the vessels, the Nervous System has no participation; this being a purely vegetative operation, partly dependent upon the simple physical conditions which produce Endosmose, and partly on a process of cell-growth.

c. The Assimilation of the new material, effected, as we have seen reason to believe, by another set of independent cells, can receive but little influence from the Nervous System, and is obviously capable of taking place without its aid.

d. The Circulation of the Blood, again, though dependent in part upon the impulsive power of a Muscular organ, the heart, is not on that account brought into closer dependence upon the Nervous System; for we have seen that the contractions of the heart result from its own inherent powers, so as to continue after it has been completely detached from the body; and that the capillary power, which is the chief agent in the movement of the blood in the lower animals, and which exerts an important subsidiary action in the higher, is the result of the exercise of certain affinities between the blood and the surrounding tissues, in which the Nervous System can have no immediate concern.

e. The act of Nutrition, in which every tissue draws from the circulating blood the materials for its own continued growth and development, and by which it incorporates these with its own substance, is but a continuance of the same kind of operation as that which takes place in the early development of the embryo, long anteriorly to the first appearance of the nervous system,-namely, a process of cell-development and metamorphosis, which must be, from its very nature, independent of Nervous agency.

f. The same may be said of the Secreting operation in general; for this essentially consists in the separation of certain products from the blood, by cells situated upon free surfaces; which thus remove those products from the interior of the fabric.

g. And the interchange of oxygen and carbonic acid, which takes place between the atmosphere and the venous blood, when brought into mutual relation in the lungs, and which is the essential part of the function of Respiration, is an operation of a merely physical character, with which the Nervous system can have no direct concern.

h. Finally, the development of the "sperm-germ-cells" in the one

sex, and of the ova containing germ-cells in the other, the subsequent fertilization of the latter by the former, and the changes consequent upon that act, together making up the function of Generation, may be all regarded as modifications of the ordinary Nutritive processes; and are effected, like these, by the inherent powers of the parts concerned in them, at the expense of the materials supplied by the blood, without any direct dependence upon the Nervous system.

841. Still, although the various processes, which make up the essential part of the nutritive operations, in Animals as in Plants, are no more dependent on any peculiar influence derived from a Nervous system, in the former, than they are in the latter, it must be evident, from the details already given, that there must be in Animals various accessory changes, which are requisite for the continuance of the former, and which can only be effected by the peculiar powers with which Animals are endowed. Thus, to commence with Digestion: this preliminary process, which the nature of the food of the plant renders unnecessary for its maintenance, can only be accomplished by the introduction of the food into a cavity or sac, in which it may be submitted to the action of the solvent fluid. The operation of grasping and swallowing the food, wherever it is performed, is accomplished through the agency of the Nervous system; and if it be checked by the loss of Nervous power, the Digestive process must cease for want of material.-So, again, although interchange of gaseous ingredients between the atmosphere and the circulating fluid may take place with sufficient energy in Plants and the lower Animals, through the mere exposure of the general surface to the atmosphere, yet we find that, in all the higher Animals, certain movements are requisite, for the continual renewal of the air or water which are in contact with one side of the respiratory surface, and of the blood which is in relation with the other: for the direction of which movements a Nervous system is requisite. In the excretory processes, moreover, the removal of the effete matters from the body can only be accomplished, in the higher Animals, by certain combined movements; the object of which is, to take up the products that are separated by the action of the proper secreting cells, and to carry them to the exterior of the body, there to be set free; and these combined movements can only be effected by the agency of the Nervous system.-Lastly, in the act of Reproduction, the arrangement of the sexual organs in Animals requires that a certain set of movements should be adapted to bring together the contents of the "sperm-cells" of the male, and of the "germcells" of the female; and also for the expulsion of the ovum from the body of the latter, in a state of more or less advanced development. For these movements a special arrangement is made, in the construction of the Nervous system, and in the application of its peculiar powers.

842. Thus we see that, although the Organic functions of the Animal are essentially independent of the Nervous System, this system affords the conditions which are requisite for their continued maintenance; being the instrument whereby the muscles are called into action for the performance of the various combined actions, that constitute the mechanism (so to speak) by which the Vegetative part of the fabric is combined with the Animal portion of the organism. We are not to suppose,

however, that every movement which takes place in the Animal body is dependent upon the Nervous System; for we have seen that the Muscular tissue may be employed to perform contractions excited by stimuli applied to itself, and that it may thus execute a set of movements in which the nervous system has no direct participation. And it is desirable that the Student should observe, that these are, in all instances, those most directly connected with the Vegetative functions, and, at the same time, those of the simplest and most straightforward character. Thus, the peristaltic movement, by which the alimentary and fecal matters are propelled along the Intestinal tube, results from the direct excitement of the contractility of its muscular walls, and is entirely independent of Nervous agency; and this movement is accomplished by the successive contraction of the different fasciculi surrounding the tube, which take up (as it were) each others' action (§ 352). So again, the successive contractions and dilatations of the cavities of the Heart, which perform so important a part in the Circulation of the blood, are the result of the properties inherent in that organ; the muscular fibres of which are excited to a peculiar rhythmical and consentaneous contraction, by the flow of blood into the cavities when dilating. Moreover, in the Excretory ducts of various glands, we find a Muscular coat, by which the fluids secreted in the glands are propelled towards their outlet on the exterior of the body, or on one of its free internal surfaces.

843. In these instances, then, we observe that the simple Contractility of Muscular structure, excited by direct stimulation, is applied to effect the movements most closely connected with the Organic functions. With the processes, therefore, which take place in the penetralia of the system, the Nervous System has no direct concern. Its office is to guard the portals for entrance and exit; and to fill those chambers, which admit the new materials from the external world; or to empty the receptacles, which collect from the interior of the system the effete matters that are to be cast out from it. And we find that, for these offices, the Nervous system is employed in its very simplest mode of operation;-that which does not involve Sensation, Intelligence, Will, or even Instinct (in the proper sense of that term), but which may take place independently of all consciousness,-by the simple reflexion of an impression, conveyed to a ganglionic centre by one set of fibres proceeding towards it from the circumference, along another set which passes from it to the mus cles, and calls them into operation (§ 394). This reflex function, therefore, is the simplest application of the Nervous System in the Animal body. We shall presently see reason to believe, that a very large proportion of the movements of many of the lower animals are of this reflex character; and that they are not necessarily accompanied by sensation, although this may usually be aroused by the same cause which produces them. As we rise, however, in the scale of Animal existence, we find the reflex movements forming a smaller and smaller proportion of the whole; until, in Man, they constitute so limited a part of the entire series of movements of which the Nervous system is the agent, that their very existence has been overlooked.

844. But the main purpose of the Nervous System is to serve as the

instrument of the Psychical powers, which are the distinguishing attribute of the Animal. It has been already pointed out, that the possession of Consciousness (or of the capability of receiving sensations), and the power of executing Spontaneous Movements (that is, movements which are not immediately dependent upon external stimuli), constitute the essential features in which the Animal differs from the Plant. All the other differences in structure, that respectively characterize these two classes of living beings, are subordinate to this one leading distinction, the presence of a Nervous system, and of its peculiar attributes in the one, and its absence in the other. Now when we attempt to analyze those peculiar attributes, we may resolve them, like the properties of the material body, into different groups. We find that the first excitement of all mental changes, whether these involve the action of the feelings or of the reason, depends upon sensations; which are produced by impressions made upon the nerves of certain parts of the body, and are conveyed by these to a particular ganglionic centre, which is termed the sensorium,-being the part in which Sensation, or the capability of feeling external impressions, especially resides.

845. Now there are numerous actions, especially among the lower Animals, which seem to be as far removed from the influence of the Will, and as little directed by Intelligence, as the Reflex movements themselves; but which, nevertheless, depend upon sensation for their excitement. The sensation may immediately direct the movement, and may call the muscular apparatus into action in such a manner, as, without any calculation of consequences, any intentional adaptation of means to ends, any exertion of the reason, or any employment of a discriminating Will, to produce an action, or train of actions, as directly and obviously adapted to the well-being of the individual, as we have seen those of the reflex character to be. Of this we have an excellent example in the act of Sneezing; the purpose of which is obviously to expel from the nasal passages those irritating matters, the sense of whose presence excites the complicated assemblage of muscular movements concerned in the operation. This class of actions may be appropriately termed the Consensual; and under it we may include most of those purely instinctive actions of the lower animals, which, being prompted by sensations, cannot be assigned to the reflex group. These seem to make up, with the reflex, nearly the whole of the Animal functions in many tribes; but they are found to be gradually brought under the domination of the Intelligence and Will, as we rise towards Man, in whom these faculties are most strongly developed, so as to keep the Consensual as well as the Reflex actions quite in subordination to the more elevated purposes of his existence.-Closely allied, however, to these, are the purely Emotional movements; in which the sensation excites a mental feeling or impulse, that reacts upon the muscular system without giving rise to any distinct idea, and consequently without having called the intellect and will into exercise. In fact, these emotional movements are often performed in opposition to the

This term, derived from the Greek fux", is used to designate the sensorial and mental endowments of Animals, in the most comprehensive acceptation of those terms.