When we take into account the amount of grey matter in the brain and spinal cord, which grey matter is made up of these bodies, mingled with fibres, the total number of corpuscles occurring in the nervous substance would have to be reckoned by millions.

With regard to the corpuscles existing in the grey matter of the convolutions of the brain, Dr. Lionel Beale gives the following conclusions as the result of his observations of the brain in man, in the sheep, the cat, and the dog :

1. The numerous nerve cells of the grey matter are all connected with, or give origin to, at least two fibres.

2. These fibres, wide near their origin, gradually diminish in thickness till they are not more than 1000 of an inch in diameter.

3. It is probable that the cells of the grey matter of the convolutions are connected together; but, in the adult, the cells are not often connected with those cells situated nearest them.

4. There is no reason for supposing that the nerve cells, here or elsewhere, influence any nerve fibres save those that are structurally continuous with them.' (Proceedings of the Royal Society, Vol. XII., p. 673.)

Both the nerve fibres and the nerve cells or corpuscles are largely supplied with blood, a circumstance indicating great activity. The grey matter, which is constituted by the presence of the corpuscles, is usually spoken of as the seat of central nervous energy, and hence the grey masses are called the nervous centres. The supposition is, that these masses originate or re-inforce nervous power, which is then transmitted through the nerve fibres from one part of the system to the other. It is, however, shown by experiments that the nerve fibres themselves generate force; for the currents passing through them are augmented in their progress.

A second function of the corpuscles throws light on the plan and workings of the brain. They are the Grand Junctions, or Crossings, where the fibres communicate with one

of the medulla oblongata; n the nucleus of a cell,- (a, c, and d, after Hannover). From QUAIN'S Anatomy.

another, and establish a vast system of lateral and forward connexions, necessary to the co-ordinating and concatenating of movements and sensations, in the bodily mechanism associated with mind. The fibres ascending through the spinal cord to the brain, pass into cells, some lower and others higher; new fibres proceed from these cells both laterally and onwards, and communicate with other cells and fibres in an exceedingly complicated arrangement. The spread and expansion of the white nervous substance, in the hemispheres of the brain, supposes, of necessity, that the fibres rising from below enter cells in the ganglia at the base of the brain, and that these cells send out in the upward direction a much greater number than what is received from beneath; and so on, till the multiplication attained in the hemispheres is reached.

THE NERVOUS CENTRES.

5. In the collective mass made up of the brain and spinal

FIG. 2.*

B

A

b

a

cord, and denominated the cerebrospinal axis or centre, the following parts stand distinct from each other, although mutually connected by bundles of nerve fibres.

I. The SPINAL CORD, contained in the back bone, and sending out two pairs of nerves from between every two vertebræ, one pair to each side of the body. The Cord consists of a column of white fibrous matter with a grey portion enclosed. In a cross section, the grey matter is seen to form two crescents, with the horns turned outwards, and connected in the middle of their convexities by a cross band.

• Plans in outline, showing the front, A, and the sides, B, of the spinal cord with the fissures upon it; also sections of the grey and white matter, and the roots of the spinal nerves. a, a, Anterior fissure. p, p, Posterior

This includes the entire

II. The ENCEPHALON or BRAIN. contents of the cavity of the skull, or cranium. The spinal cord is continued up into it. The brain is itself an aggregate of distinguishable masses of mixed grey and white matter. Each of these masses is looked upon either as a distinct centre, or as communicating between the centres. In proportion as the grey vesicular matter prevails, the mass has the characters of a centre and a grand junction; according as the white fibrous substance prevails, the part serves as a medium of conduction or communication solely. Of these various masses, some have a preponderance of grey, others of white matter. None are purely of one kind.

The mere mechanical arrangement of the brain is extremely complex, and there are different modes of classifying and grouping the various portions. The division adopted by human Anatomists is into four parts (a different arrangement has been proposed, founded on Comparative Anatomy). Those four parts are the Cerebrum, the Cerebellum, the Pons Varolii,

FIG. 3.*

B

fissure. b, Posterior, and c, Anterior horn of grey matter. e, Grey commissure. r, Anterior, and s, Posterior roots of a spinal nerve.'-QUAIN, Vol. II. p. 438.

* A plan in outline, showing, in a lateral view, the parts of the encephalon separated somewhat from each other. A, Cerebrum. e, Fissure of

THE MEDULLA OBLONGATA.

19

and the Medulla Oblongata. The cerebrum, which is the highest and by far the largest part of the human encephalon, occupies the upper and larger portion of the cranial cavity.' 'The cerebellum is placed beneath the hinder part of the cerebrum, by which it is completely overlapped.' The pons Varolii is in the base of the brain near the entrance of the spinal cord, and connects together the three other parts,— the cerebrum, the cerebellum, and the medulla oblongata. The medulla oblongata connects the spinal cord with the brain.

6. In giving a more detailed description of those four parts, it will be convenient to take them in an inverse order, beginning from below, or where the brain joins the spinal cord.

(1.) The Medulla Oblongata.-This portion is continuous below with the spinal cord, of which it seems an expansion; lying wholly within the cranial cavity, its upper end passes into the pons Varolii. See Figs. 3 and 4, D.

'It is of a pyramidal form, having its broad extremity turned upwards, from which it tapers to its point of connexion with the spinal cord; it is expanded laterally at its upper part. Its length from the pons to the lower extremity of the pyramids is about an inch and a quarter; its greatest breadth is about three quarters of an inch; and its thickness from before backwards about half an inch.

In form and general anatomical characters, the medulla oblongata very much resembles the cord, of which it is a prolongation upwards to the brain. It is not our purpose here to enter into the minute anatomy of the part, or to set forth the points of difference between it and the cord; we need only observe that in it the white and grey constituents of the cord are both increased in size and altered in arrangement. The grey matter especially becomes more abundant, and additional deposits occur. The medulla oblongata has thus more of the character of an independent centre of nervous

Sylvins, which separates the anterior and middle lobes. B, Cerebellum. C, Pons Varolii. D, Medulla oblongata. a, Peduncles of cerebrum; b, Superior; c, Middle; and d, Inferior peduncles of cerebellum.'-QUAIN.

action, as well as of a grand junction, than belongs to the cord. It gives origin to nerves of a very special and important nature.

(2.) The Pons Varolii, or annular protuberance (tuber annulare). (See Figs. 3 and 4 c.) This is a comparatively small portion of the encephalon, which occupies a central position on its under surface, above and in front of the medulla oblongata, below and behind the crura cerebri a, and between the middle crura of the cerebellum e, with all which parts it is connected.' By the term 'crura cerebri,' introduced in this description, is meant the 'legs' or roots of the cerebrum, or the two bundles of nerves that unite it with the parts below. The crura of the cerebellum express in like manner the several connexions of that centre with the other centres. On account of the intermediate and connecting position of the pons, it has also been called the middle-brain (meso-cephalon). From its embracing, as in a ring, the medulla oblongata and stems of the cerebrum, it has derived the name of annular protuberance; the other name, 'pons,' or bridge, expresses the same circumstance.

The substance of the pons Varolii consists of transverse and longitudinal white fibres, interspersed with a quantity of diffused grey matter. The transverse fibres, with a few exceptions, enter the cerebellum under the name of the middle crura or peduncles, and form a commissural (or connecting) system for its two hemispheres. The longitudinal fibres are those which ascend from the medulla oblongata into the crura cerebri, augmented, it would seem, by others which arise within the pons from the grey matter scattered through it.' The pons is thus mainly a grand junction between the medulla oblongata and spinal cord below, the cerebrum above, and the cerebellum behind. The existence of a considerable amount of the grey or vesicular matter proves that simple conduction or communication is not the sole function of this part of the brain.

(3.) The cerebrum or brain proper (Figs. 3 and 4, A), as already mentioned, is the highest, and by far the largest