matter belonging to each. Thus in bones the osseous substance is elaborated, in cartilage cartilaginous matter, in muscle muscular fibre, and so on with every organ in the body, as the nervous system, the glandular, the membranous, and the cuticular. Every tissue of the body has the blood distributed to it in a manner adapted to its organization and to the function it has to exercise, so as to supply it with a greater or less quantity, with different degrees of velocity and with different degrees of force. While it passes through these exceedingly minute tubes, which are invisible to the naked eye, the matter belonging to each is separated, just as we find the bile separated from the blood of the vena portæ in the liver, the urine from the vessels of the kidneys, or the saliva from the salivary glands. The blood containing, already formed, a great number of the materials which compose the majority of the tissues, such as fibrin, albumen, muco-extractive phosphates of lime and magnesia, and various other salts, it may be supposed that simple separation alone takes place. This indeed may be the case to a certain extent, but it will not explain, after all, how the vessels are endowed with the power whereby they are enabled to select from the general mass of blood that particular matter which is required. On the other hand, several substances are found in the animal textures which cannot be recognised in the blood, as gelatin, the nervous matter, and even fat; for the oil found in the blood is probably derived by absorption from the adipose cells. It must therefore be admitted that the vessels of those textures which contain substances not recognisable in the blood possess the power of forming them. At the same time we cannot acknowledge them to be endowed with any thing like creative power. The elements of which these animal products are formed must exist in the materials from which they are prepared, and must ultimately be derived from without. In whichever way the proximate principles of the different tissues are derived, whe

ther by the separation of matter already in the blood, or by the formation from it of the elements of which they are composed, we have afforded us in the admirable perfection with which the processes are conducted, and their excellent adaptation to each individual organ and condition of that organ, proof of the supreme power, wisdom, and prescience, which have arranged and contrived them.

The separation and depositation of the peculiar matter belonging to each individual tissue has been termed nutrition, instead of secretion. The term nutrition has been applied to the separation of solid matter from the blood, while secretion is applied to that of fluids, and in a still more restricted sense to the separation of fluids by glands. It appears to us that there is not only no advantage, but even a disadvantage, in using different terms. to indicate the states in which the different substances derived from the blood may be separated from it, for it is allowed that the earthy matter of bones is as distinctly a secretion as when it is deposited on parts which in their healthy condition are free from it. The nutrition or nourishment of the different organs of the body depends as much upon the exercise of the function of absorption as upon secretion; that is to say, it is as necessary that the old be removed as that fresh materials be deposited; and since the term interstitial absorption has been applied to the removal of the textures, in like manner we apply the term interstitial secretion to the depositation of new matter in these textures; and by the term nutrition we understand the result of the action of both.

From the first rudimental state in the womb till life is extinguished in old age, the weight and bulk of the body is continually changing; its organs undergo alterations in consistence, colour, elasticity, and even chemical constitution. The materials which compose the body of the adult man did not enter into the construction of the infant; nor do the materials of the robust man continue to

extreme old age. The matter which forms the body which we place in the grave did not constitute the same body a few years antecedently. We are constantly throwing off old particles and adopting new; and many of those which enter into the composition of our frame to-day will not belong to us to-morrow. We cannot even be placed in the same room with other individuals for any length of time without mutually interchanging particles with each other. The emanations from one body are absorbed and adopted by another and in like manner, of those particles which are thrown off one person, some are taken up by another, and become a part of his constitution.

The rapidity with which these changes are effected depends on a great variety of causes: on age, constitution, the state of health, and the texture of the individual parts. In the earlier periods of life nutrition goes on rapidly, speedily producing a complete renewal of parts; but as life advances the changes occur more slowly. To fix the limits within which complete alteration is effected in every part is therefore impossible. With those who have attempted it, the period of seven years is a favourite one, very likely in part from some mysterious attachment to the number seven. The soft parts, and those organs in which depositation and absorption are carried on with great activity, will be completely renewed several times for once of those parts which are denser in their texture, and in which the circulation is more subdued. Sailors, soldiers, and several savage tribes, are in the habit of impressing the skin with figures, in which they trace and insert colouring matters, which remain during their lives; and by the use of the nitrate of silver the skin has been rendered permanently blue, which would appear to disprove the renewal of the skin; but it is possible that the absorbents refuse the colouring matter employed in tattooing; and as to the colour produced by the internal use of the nitrate of silver, it has frequently completely disappeared after having remained for some years.

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CHAPTER VII.

NERVOUS SYSTEM.

Introductory Remarks-Hairy Scalp-Skull formed of separate Bones-Advantages of this Construction-Consists of two Plates-Advantages of-Membranes of the Brain, and the Purposes they serve-Divisions of the Brain-Presents one Continuous Surface, without the slightest trace of Division-Dr Spurzheim's Demonstration of-Enormous Quantity of Water collected in-Microscopic Observations of Bauer-Of Ehrenberg-Structure described-Evolution of the Nervous System in size and complexity, accompanied with Parallel Advances in the Higher Instincts and Sagacity-Development of the Brain in the Foetus keeps pace with that of the System in general-Weight of the Brain in proportion to that of the Body-Size in proportion to the rest of the Nervous System-Brain the Seat of the Mind-Results of Experiments on-Spinal Marrow, its Structure and Development-Views and Experiments of Sir Charles Bell-Classification of the Nerves-Nerves of the Specific Senses-Announce the properties of External Bodies-Motiferous Nerves-Convey the Commands of the Will-Respiratory Nerves-Indicate the Instinctive Feelings, Passions, and Emotions of the Mind-Their Universal Distribution-Regular Doublerooted Nerves-One Root transmits the Mandates of the Will-The other communicates Impressions received-Ganglionic Nerves-Immediate Seat of Individual Vitality of the different Organs of the Body-Their General Connections-Universal Distribution-Four great Ganglionic Nervous Centres.

THE nervous system, to which our attention is now to be directed, furnishes the material instrument of thought, the means by which we become acquainted with the external world around us, the channels by which the commands of the will, the impressions of instinct and of passion, are conveyed to the instruments by which they are displayed, and the cords of connection which associate the various organs of the body into one harmonious whole. Forming as it does the bond of union between the intellectual and physical conditions of the constitution of man,

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it is equally interesting and important whether we especially devote our attention to sciences which have for their consideration the one or other of these two states of human existence.

Neither the limits nor object of the present work permit us to enter upon the examination of many subjects of the utmost importance connected with the nervous system; others involve so much controversy and contrariety of opinion, that to embark in them we should be necessarily led far beyond the boundaries within which we at present wish to confine ourselves. In treating of this department of our subject, we shall therefore altogether avoid metaphysical disquisition, and as much as possible controverted points, but shall endeavour to give as clear and complete, at the same time as concise an account of the nervous system as we can, taking care to advert to the functions dependent upon it.

We shall consider this system under the following divisions, and in the following order: 1st, The brain, comprising the cerebrum, cerebellum, and medulla oblongata. 2dly, The spinal cord ; and 3dly, The nerves. The last we shall classify under five orders:-1. Nerves of special sense. 2. Nerves of simple voluntary motion. 3. Nerves of respiratory or instinctive movements. 4. The regular or double-rooted nerves. And 5. The ganglionic nerves.

The Brain, constituting the most important organ in the whole body, and being, from the delicacy of its texture, incapable of resisting external influence, is carefully and admirably protected. The coverings of the brain are the scalp, the skull, and certain membranes.

The hair of the head, furnishing as it were a covering of felt, serves as a protection, by deadening blows, and preventing injury from pressure. Besides, being a bad conductor of heat, it preserves an uniformity of temperature; by favouring exhalation, it contributes to the same effect; and being impregnated with an oily matter from the sebaceous follicles, it is enabled to resist moisture.