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7. We must next attend to the internal structure of the brain, considered as made up of the two kinds of matter, the grey and the white. The distribution and arrangement of those two kinds of matter throw light upon the mode of action, or the peculiar kind of activity that distinguishes the brain.

'White Part of the Encephalon.-The white matter of the encephalon consists of tubular fibres. The general direction which they follow is best seen in a brain that has been hardened by immersion in spirits, although it is true that we do not then trace the single fibres, but only the fine bundles and fibrous lamella which they form by their aggregation.'

The fibres of the cerebrum, though exceedingly complicated in their arrangement, and forming many different collections, may be referred to three principal systems, according to the general course which they take, viz.-1. Ascending or peduncular fibres, which pass up from the medulla oblongata to the hemispheres, and constitute the two crura or peduncles of the cerebrum. They increase in number as they ascend through the pons, and still further in passing through the optic thalami and striated bodies, beyond which they spread in all directions into the hemispheres. These were named by Gall the diverging fibres. 2. Transverse or commissural fibres, which connect the two hemispheres together. 3. Longitudinal or collateral fibres, which, keeping on the same side of the middle line, connect more or less distant parts of the same hemisphere together.'

'Grey Matter of the Encephalon.-Considering the imputed physiological importance of the grey nervous substance, it may be well to mention connectedly the different positions in which it is found in the several parts of the encephalon.'

By far the larger amount is situated upon the convoluted surface of the cerebrum and the laminated surface of the cerebellum, forming, in each case, the external cortical layer of cineritious matter.'

I omit a portion of the connected account of the spread of the grey matter in the parts in the interior and base of the

DISTRIBUTION OF GREY MATTER.

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brain, as including a number of terms that the reader has not been prepared for in the present sketch of the nervous system. We must rest satisfied with perusing, in addition to the above, the account of the distribution of grey substance in the larger portions, and in the parts already in some degree known to us.

'In the crura cerebri, the grey matter is collected into a dark mass; below this it is continuous with that of the pons and medulla oblongata, and through them with the spinal cord.' Thus, though the crura cerebri are, in the main, connexions of white matter between the hemispheres and the parts below, yet, like the medulla oblongata and spinal cord, they contain in the interior a portion of the grey matter, and are to that extent centres and junctions, as well as conductors.

In the centre of each of the corpora quadrigemina, grey matter is also found, and it occurs in the pineal gland (and in the corpora geniculata). These last bodies appear to be appendages of the large masses of grey matter, situated in the interior of the cerebrum, named the optic thalami; which again are succeeded by the still larger collections of this substance, and indeed the largest situated within the brain,viz., the corpora striata.'

8. Plan of Structure indicated by the above arrangement of white and grey substance.-It would appear, thus, that the cerebro-spinal centre, or the brain and spinal cord taken together, is an aggregate of distinct nervous masses or parts, each made up of a mixture of white and grey matter. The grey matter is the vesicular substance, consisting of cells or corpuscles; the white matter is the fibrous substance, being made up of fibres bundled together. The grey matter is a terminus; to it the fibrous collections tend, or from it commence. The fibrous matter contained within any of the cerebral masses is placed there as a means of communicating with some portion or other of the layers, or other collections, of grey substance.

Assuming that one class of nerve fibres (the sensory or incarrying)—those distributed to senses, viscera, &c.—are

employed in conveying influence from without inwards; and the other class (motory or outcarrying)—distributed to muscles, in conveying influence from within outwards,-we find that both classes are usually mixed together in the same ramifying branches, and in the common stem of white matter in the spinal cord. Let us imagine, however, the two kinds separated; the sensory nerves all emerging from the centres on one hand, and the motory nerves emerging apart on the other. We can then express the plan of the brain thus:The sensory or incarrying fibres arising from the extremities enter the cord, proceed a certain way there, and begin to drop into corpuscles; from these corpuscles fresh fibres arise and proceed, some onwards and some laterally, to other cells; and so on. Thus, in the spinal cord, medulla oblongata, pons Varolii, &c.-up to the cerebral hemispheres, there is a repeated system of fibres passing into cells, and new fibres emerging, and going on to other cells; giving birth to an endless system of cross communications, like the railway network of England. Adverting now to the enormous connecting mass of fibres-ascending, diverging, and transverse-that make up the white substance of the brain, we must consider how the multiplication has been effected. There is only one conceivable process, when we consider that the entire mass is in communication, through cells, with the diminutive mass of the spinal cord. The process

is this. For one fibre coming up from the sense organs and dropping into a cell, two, three, four, or more must emerge; each of these again, proceeding onwards to a new cell, and there replaced by three, four, &c., new fibres; and so on, until the expansion or multiplication is completed. Within the spinal cord, where there is no increase of bulk, the mul tiplying process is not begun; but in the upper course of the cord, where it enters the brain, there is an arithmetical necessity for the multiplication. We can hardly avoid the supposition that the corpora striata and the thalami optici, through which the great stem of the brain diffuses itself (by the ascending fibres) in the white matter of the hemi

PLAN OF THE BRAIN.

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spheres, are the principal seats of the multiplying corpuscles. For every fibre carrying impressions up from the senses, and every fibre carrying out stimulus to the moving organs, there must be perhaps ten thousand, perhaps a hundred thousand, traversing the brain, involving a great and rapid multiplication in the corpuscles of the grey substance.*

It will be necessary, in speaking of certain functions closely allied to the mind, that some allusion be made to the portion of the nervous organization called the Sympathetic System, consisting of numerous ganglia, or little knots, together with nerve cords, and united by numerous nerve cords or branches to the cerebro-spinal system.

The sympathetic system consists of two knotted or ganglionated cords or strings, running, inside the trunk, from the neck to the pelvis, one on each side of the spine. The upper end is connected with groups of ganglia in the head and face; and, in the trunk, there are detached interlacements of ganglia, or plexuses having connexion with the great viscera in the chest and abdomen.

The knots, or Ganglia, are the centres or grey masses of the system, being made up of nerve corpuscles of a particular kind (having usually a single projection or tail). They exercise the usual functions of the corpuscles, in forwarding, diverting, reflecting, and concatenating nervous currents. The Cords are, as in the cerebro-spinal system, made up of nerve fibres, but these are of a peculiar sort, described as soft, granular, fl ittish (as opposed to tubular) fibres, without any surrounding sheaths or investments, and containing many dark nuclei; they are called the gelatinous, and the non-medullated fibres.

United with fibres from the cerebro-spinal system, these branches of the sympathetic are distributed over the whole body. Thus, as regards the head, they are found in the iris and the blood-vessels of the eye, in a muscle of the tympanum, in the nose, the palate, and the salivary glands. The great plexus of the chest (the cardiac) sends fibres to the heart, the great blood-vessels, and the lungs; from the aorta, nerves are continued to the arteries throughout the body. The abdominal plexus (called the solar plexus) supplies the stomach, intestines, liver, kidneys, and other abdominal viscera ; each organ having a small plexus of its own. A still lower plexus contributes fibres to the parts contained in the pelvis. As all the ramifications contain a certain number of cerebro-spinal fibres, so it is believed that the cranial and spinal nerves contain everywhere some sympathetic fibres.

It is presumed from analogy, and from the functions exerted by the sympathetic system, that the fibres are of the two classes-incarrying and outcarrying. The incarrying nerves would receive stimulation from the peripheral surfaces; the outcarrying would convey motor stimuli to muscular fibres. This last function is the one most clearly manifested. The muscular fibres stimulated by the sympathetic nerves are almost all involuntary muscles, as the iris, the heart, the muscular coat of the blood-vessels, the

OF THE CEREBRO-SPINAL NERVES.

9. By the cerebro-spinal nerves are meant the connexions of the cerebro-spinal centre with the different parts of the body. These connexions consist of ramifications of nerve cords, threads, or bundles, arising in the central masses, and distributed like the blood-vessels, by subdividing and spreading themselves over the various organs and tissues, thereby establishing a connexion between the brain and the remotest extremities.

'These nerves are formed of the nerve fibres already described, collected together and bound up in membranous sheaths. A larger or smaller number of fibres inclosed in a tubular sheath form a small round cord, usually named a funiculus; if a nerve be very small, it may consist of but one such cord, but in larger nerves several funiculi are united together into one or more larger bundles, which, being wrapped up in a common membranous covering, constitute the nerve (Fig. 5). Accordingly, in dissecting a nerve, we first come to an outward covering, formed of cellular tissue,

intestines, &c. All these parts are primarily governed by the sympathetic system, with inore or less interference from the cerebro-spinal centres, through the fibres intermingling with sympathetic fibres.

The sympathetic system presides over the viscera, which are the organs of the nutritive or vegetative life. It sustains the rhythmical action of the heart, and of the intestines. The fibres distributed to the small arteries everywhere maintain these vessels in a state of permanent contraction, the release from which, by extraneous influence, produces local congestion and the allied results. These fibres and their function, receive the designation vasi-motor.

The fibres of the sympathetic are not the medium of sensation. When pain arises in parts mainly supplied by them, as the intestines, it must be attributed to the irritation of the intermingled fibres of the cerebro-spinal class.

Many of the so-called reflex functions are due to the operation of the sympathetic nerves and ganglia. The extreme contrast to the proper voluntary actions is presented by the movements due to this system-witness the heart, the intestines, and the vasi-motor compression of the blood-vessels. Indeed, the absence of sensation and the absence of voluntary control are essentially the same fact.

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