at an early period of their development, form one continuous tube. The walls of the tube so formed become ultimately much thickened and exhibit two kinds of texture, which, from their color, are distinguished as the gray and the white. In the case of so much of the tube as lies in the spinal canal and is afterwards termed spinal cord, the development proceeds very regularly; white matter is deposited. on the outer wall of the cylinder, and gray matter on the inner wall, until it appears solid. A minute canal, however, the central canal of the spinal cord, continues to traverse its whole extent throughout life, and is the remains of the original hollow of the tube. Towards the lower part of the cord in birds there is even a space called the sinus rhomboidalis, where the cylinder is never completed, and the central canal is open on the dorsal aspect. Now, however different the brain may be in the adult condition from the spinal cord, it is extremely interesting to note that it is the anterior portion of the same cylinder, but that the cylinder undergoes some bendings, its walls are greatly thickened in some places and imperfect in others, and the continuation of the central canal is in some places greatly dilated, and in others contracted.

As respects texture, there is much in common between the brain and spinal cord. They are similar in appearance, and both consist of true nerve tissues, with a fine reticulum of supporting substance in which those more important elements are embedded. The proper nerve tissues are two in number, nerve fibres and nerve corpuscles: the nerve fibres are long threads which have the property of transmitting along their course a certain change of condition which constitutes nervous influence, and which, it may be mentioned, is a purely physical action, not electrical, but involving in its operation electrical changes. Nerve fibres transmit this influence, but have no power of originating, directing, or modifying it: they are simply conductors, and such nerve fibres are the essential elements in all the nerves throughout

the body. Nerve corpuscles are bodies of which it is only necessary to say that they present a variable number of poles or branches, and there is no reasonable doubt that these poles are in direct continuity with nerve fibres. According to circumstances little understood, these corpuscles have the property of modifying impressions or nervous influence, and of directing them into different channels with which their poles communicate. Now the white substance of the brain and spinal cord contains only nerve fibres without any nerve corpuscles, these latter being found exclusively in the gray substance. It is quite plain, therefore, and universally recognized, that the white substance is only useful as containing channels of communication between different parts of the gray, and also between the gray substance and the muscles and sensitive parts throughout the body. But even the gray substance is not always or even generally capable of being affected directly by the consciousness; and in the case of the spinal cord, it is very certain that consciousness resides in no part of it, either white or gray. The spinal cord is the centre with which are connected the nerves of the muscles and integuments of the greater part of the body, and in the ordinary actions of the body what usually happens is this, that impressions made by the contact of external objects on the terminations of sensory nerves in the integument are transmitted by them to the nerve corpuscles of the cord, and, through series of these, conducted to the parts of the brain, which are in immediate connection with consciousness; while also, when the mind wills certain movements of the body, the stimulus proceeds from those parts of the brain, and, by some altogether unknown mechanism, is ultimately so distributed that there extend from the gray matter of the cord impressions along the nerves so adjusted as to produce precisely that amount of contraction of muscles, of whose existence the mind is utterly ignorant, which is necessary to effect the required result. But it is always the same kind of stimulus, the nervous influence, wherever

it issues from, which acts upon the cord. Thus, for example, when the cord near its upper part is severed from the brain by an injury, there is loss of all sensation and voluntary motion in the parts supplied by it below the place of lesion, the consciousness being no longer in communication with those parts; but irritation of the integument still sends a current as before to the spinal cord, and this being distributed by the corpuscles of the gray matter, and descending again by the motor nerves, causes involuntary contraction of muscles. This is probably the simplest possible example of the phenomenon termed by physiologists reflex nervous

action.

We have ventured on this extremely cursory and general survey of the spinal cord, the simplest portion of the cerebrospinal axis, in order that the general reader may form some conception of the kind of mechanism which extends through the more obscure and intricate portion, the brain. To explain fully the extremely complex structure of the brain would require much greater detail than is allowable in an article like this, but a general idea of the most important facts will best be arrived at by pursuing the account of its early development, which we have already begun.

The cylinder which we have traced in the embryo, so far as the spinal cord is concerned, is immediately on its closure, expanded in its cranial part into a series of three primordial vesicles, and immediately afterwards two little hollow buds, called the hemisphere vesicles, project laterally from the foremost of the series. Without tracing the history of the primordial vesicles, it is sufficient for our present purpose to point out that the cerebellum is originally a part of the hindermost, projecting upwards as a hollow pouch, and that it is quite certain, from the experiments on lower animals, that no consciousness whatever resides in any of the parts developed from that vesicle; also it is equally certain that not more than the very feeblest consciousness resides in those parts into which the walls of the two other primordial vesi

cles are developed. These parts are devoted to the carrying on of obscure functions connected with the sensibility and movements of the body strictly comparable with the functions of the spinal cord, and entirely of a physical description: the organs of the mental faculties are the developed hemisphere vesicles, and these only. The hemisphere vesicles rapidly enlarge and extend backwards over and around the other parts of the brain, so as to reach to the cerebellum behind, come in contact with the whole roof and sides of the skull and a large part of its floor, and press one against the other in the middle line of the whole length of the skull for an average depth of a couple of inches; and early in embryonic life they are already much the most bulky parts of the brain.

The gray matter which lines the whole length of the cerebrospinal cylinder fails to be developed in the hemisphere vesicles, except at one part placed at the neck of the vesicle, and called by anatomists the corpus striatum, but of which we know nothing in respect of function, and can only note that it is traversed by the whole mass of fibres joining the hemisphere vesicles with the cord and cerebellum. The whole of the rest of the hemisphere vesicle, or, as it is termed, the cerebral hemisphere, consists of an enormous mass of white matter, with a superadded layer of gray matter on the outside. The cerebellum has the same peculiarity of having its gray matter on the surface, and it is curious to note that both the gray matter on the cerebellum and that on the cerebrum, while differing one from the other in minute structure, differ still more from the gray matter which is found elsewhere, and the function of which is, as we have seen, in a general way, well understood. Also the cerebellum and cerebral hemispheres resemble each other in being thrown into numerous elevations and depressions, in order to expose a larger extent to the vascular membrane on their surface, which sends its minute branches into them. These circumstances might plead a little for the doctrine that

the cerebellum is connecting with a psychical faculty, whatever that might be, but its totally different source of origin is clearly opposed to such a notion; and we are not left merely to speculate on the subject, for both disease in the human subject, and experiment on animals, teach us that when the cerebellum is destroyed, the power of combining movements so as to regulate and guide them is lost, the limbs being still capable of being moved, but walking and handling being impossible. Thus it is certain that the function of the cerebellum is totally different from what the phrenologists hold it to be.

Examining the cerebral hemispheres in different animals, and proceeding from the lower to the higher forms, a progress in development is found, similar to the progress made in embryonic life. Thus in fishes they are represented by very small parts in the fore part of the brain; in birds they have not extended sufficiently backwards to be in contact with the cerebellum, and their bulk is due almost entirely to the corpora striata; in rodent animals their surface is smooth; and, as one passes to the higher groups of mammals, more and more complicated convolutions of the surface are met with; while in man by far the greatest complexity is found.

Whatever the particular cerebral changes may be which accompany and are necessary for thought, there can be no question that they occur in the gray matter, and that the white matter is only useful by bringing the different parts of the gray matter into communication one with another, an end which it accomplishes very thoroughly by its complicated commissures and countless bundles of fibres taking all directions. Judging, then, from comparative anatomy, and even on phrenological principles, one would expect that, among men, the greater the amount of gray matter of a given quality the more effective would the hemisphere be for the exercise of the mental faculties; and this, there is good reason to consider, is to some extent actually the case. But