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.'

ness.

The nerve fibres are principally of the class termed white, or tubular nerve-fibres. They are of microscopic minuteIn thickness, they range from the to the Too of an inch; the medium or average being of an inch. When in the fresh condition, they are homogeneous and transparent, but after separation from the body they acquire a double outline or contour, and are apt also to assume a varicose or beaded appearance. The inference as to their ultimate structure, from these changes, is that each tube consists of (1) an outer structureless membrane, (2) an interior surrounding layer of transparent fatty matter, and (3) a central core or cylinder, which is not fatty, but albuminous in composition. The central band or axis appears alone, or divested of the two envelopes, both in the central connexions of the fibres, and in the ultimate ramifications in the extremities of the body; being, therefore, the essential part of the structure. In thickness, it does not exceed the Tooooo of an inch.

These tubular nerve-fibres are finest on the superficial ayers of the brain, and in the nerves of special sense; they are largest in the motor nerves. From the foregoing statements of their size, we may judge of the immense multiplication of the nervous elements. Estimates have been made of the number of fibres in individual nerves. The third cerebral nerve (the common motor of the eyes) is supposed to have as many as fifteen thousand fibres; the small root of the fifth (governing mastication) nine or ten thousand; the nerve of the tongue five thousand; these being all motor nerves, which have the largest fibres. It would be interesting to estimate the probable number of fibres of the nerve of sight, which, besides being a sensitive nerve, is much thicker than any of those just quoted; there cannot probably be less than one hundred thousand fibres, and there may be many more.

The number of nerve fibres forming the white substance of the brain must be counted by hundreds of millions.

In the grey substance of the nerve centres, the nerve fibres are supposed to be continuous with the cells or vesicles. At their other extremity in the organs of sense, in the muscles, and in the body generally, their mode of termination appears to be varied. Sometimes they end in loops, sometimes in meshes of network; not unfrequently sub-dividing into minuter nerves (besides dropping their two investing sheaths). In other cases, they seem, according to the majority of Anatomists, to end free in fine points, or else in little swellings of various structure.

It is important to note that each fibre is continued unbroken and independent from the central nervous masses to the peripheral extremity; there are no loose ends; and although the nervous cords frequently unite, as well as subdivide, in their course, the ultimate fibres are never fused with one another.

The nerve cells, vesicles, or ganglionic corpuscles, are little bodies, of a variety of forms; being round, oval, pear-shaped, tailed, and star-like or radiated. They consist of pulpy matter, with an eccentric roundish body or nucleus, enclosing one or more still smaller nuclei, surrounded by coloured granules. They vary in size from 0 to 3000 of an inch in diameter.

FIG. 1.

.....

a and b from the

* Nucleated nerve-corpuscles magnified 170 diameters. cortical grey matter of the cerebellum; c and d from the spongy grey matter

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 10000ʊ 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.

FIG. 2.*

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

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.*

fissure. b, Posterior, and c, Anterior horn of grey matter. e, Grey commis sure. 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