blood-vessels. The nervous substance has long been distinguished into two kinds, obviously differing from each other in colour, and therefore named the white, and the grey, or cineritious (ash-coloured).

'When subjected to the microscope, the nervous substance is seen to consist of two different structural elements, viz., fibres, and cells or vesicles. The fibres are found universally in the nervous cords, and they also constitute the greater part of the nervous centres; the cells or vesicles, on the other hand, are confined in a great measure to the latter, and do not exist in the nerves properly so called, unless it be at their peripheral expansions in some of the organs of special sense; they are contained in the grey portion of the brain, spinal cord, and ganglia, which grey substance is, in fact, made up of these vesicles intermixed in many parts with fibres, and with a variable quantity of granular or amorphous matter.'—Introduction, p. clxxxviii.

The author goes on to describe the nerve fibres as of two distinct kinds, and the nerve cells as consisting of several varieties, but to us it must suffice to know that the nerve cords or connexions are bundles of separate fibres, and that the nerve centres are aggregates of cells or vesicles mixed with fibres.

The mode of connexion of nerve threads with the central vesicles is not uniform. In one class of cases the vesicles are pear-shaped, and send out tails that are the commencement of fibres, so that the cells are, as it were, swellings or expansions of the fibres, having granular or solid nuclei enclosed in them. According to this plan, in any nerve cord coming from the extremities of the body to the brain, the separate fibres would end each in a swollen mass or vesicle, and the total of these vesicles would be the grey matter of the brain. To this grey matter, with its infinitude of cells, all the nerves tend, or from it they issue. These two elements of nerve cell and nerve fibre are the sole ingredients peculiar to the brain. The blood vessels are common to it with every other organ; whilst the membranes or sheaths that surround the cords and enclose the brain serve partly for

NERVE FIBRES AND NERVE VESICLES.

15

protection and insulation, and partly for containing and distributing the blood vessels.

FIG. 1.*

d

To form an estimate of the multitude of nerve fibres entering into the ramifying cords, it is necessary to be made aware of the size of the ultimate filaments. Their size differs considerably even in the same nerve, but much more in different parts of the nervous system; some being as small as the 1, and others upwards of 15 of an inch in diameter; and the same fibre may change its size in different parts of its course.' Thus it would appear that a nerve branch, like the main trunk supplying the arm, might contain hundreds of thousands of separate fibres. The optic nerve of one of the eyes might contain as many as a million of fibres.

The nerve vesicles also 'differ greatly from one another in size; some being scarcely larger than a human blood corpuscle,† others of an inch or upwards in diameter.' We may, therefore, speak in somewhat similar terms respecting the

a and b from the

'Nucleated nerve-cells magnified 170 diameters. cortical grey matter of the cerebellum; c and d from the spongy grey matter of the medulla oblongata. n the nucleus of a cell,-(a, c, and d, after Hannover).' From QUAIN's Anatomy, p. cxcvii.

The magnitude of the red corpuscles of the human blood differs somewhat even in the same drop of blood, and it has been variously assigned by authors; but the prevalent size may be stated at from 6 to 3200 of an inch in diameter, and about one-fourth of that in thickness."

countless millions of nerve cells existing in the grey substance of a single convolution of the brain. The minuteness of the fibres and vesicles of the nerve substance is not without importance, for we are to consider that each fibre carries forward its own distinct impression without affecting, or being affected by, the impressions passing along the other fibres that run side by side with it in the same bundle. In the act of perceiving the objects about us, this distinctness enables us to hold in our minds all the parts of a complicated scene, each in the proper place, without mingling or confusion; and in the command of our muscular movements it gives the means of singling out specific muscles to be acted on while all the others are left quiescent.

FIG. 2.*

OF THE NERVOUS CENTRES.

5. In the collective mass made up of the brain and spinal cord, and denominated the cerebrospinal axis or centre, the following parts stand distinct from each other, although mutually connected by bundles of nerve fibres.

B

A

a

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 fissure. b, Posterior, and c, Anterior horn of grey matter. e, Grey commissure. r, Anterior, and s, Posterior roots of a spinal nerve.'-QUAIN, p. 676.

PARTS OF THE BRAIN.

17

II. The ENCEPHALON or BRAIN. This includes the entire 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 character of a centre; 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 exceedingly 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.*

A plan in outline, showing, in a lateral view, the parts of the encephalon separated somewhat from each other. A, Cerebrum. e, Fissure of Sylvius, 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, p. 681.

с

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, cerebellum, and 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.'-Quain, p. 683.

In form and general Anatomical characters the medulla oblongata very much resembles the cord, of which it is 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; suffice it to 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 action than belongs to the cord. It gives origin to several nerves of a very special and important nature.

(2.) The Pons Varolii, or annular protuberance (tuber