changes it is the office of that filament to transmit. The efferent nerves take their origin in the nervous centres, and terminate, without the intervention of grey structure, by exceedingly minute loops in the substance of the muscles; calling these organs into activity by virtue of a force which they convey, and which travels along the nerve until the looped extremity is attained.

The whole of the nerves, under ordinary circumstances, serve only, as has been said, to communicate between the nervous centres and their own extremities; but there are conditions which permit an afferent nerve to convey to the centres, and an efferent nerve to the muscle in which it terminates, some impression that has been made upon it in its course. But then, in the case of the afferent nerve, the impression is always referred to, or supposed to come from, the extremity. This is well shewn by pressure upon what is called the "funny bone;" that is, upon the ulnar nerve as it passes over the bone of the arm by the elbow joint. The nerve terminates by several branches in the little finger, and on the inner side of the ring finger; and to these parts the pain produced by pinching it at the elbow is referred; that is, in these parts it is said to be felt. It is also a matter of common observation, that persons whose limbs have been removed by the surgeon, still experience sensations as if in the missing members. A few years ago, when veterans with wooden legs were more common than a long period of peace permits them to be now,'

1 Written in 1853.

they might constantly be heard to complain of pains in the feet which they had left behind them in an engagement pains that must have been due to pressure or other causes acting upon the truncated nerve of the stump; but which, like every change that nerve could convey, was felt as if proceeding from the parts in which its missing extremity had once terminated. An efferent nerve, if hurt or irritated in its course, calls the muscles dependent upon it into activity, just as if an impulse had been conveyed along it from the centres; but such an occurrence is so rare, except as a consequence of experiment or accident, that it may be altogether disregarded in a brief description like the present.

The nervous centres consist of four distinct series or sets of organs, called respectively the Cerebrum, the Sensorium, the Cerebellum, and the Spinal Cord. The whole of the first three series, and the upper portion of the cord, are contained within the skull; the rest of the cord within the cavity formed by the back bones. The centres consist in great part of grey matter (the essential of each in its individual capacity), surrounded by, or including, white tissue, which serves to maintain close union and constant communication between them.

The Cerebrum, or (as it will hereafter be styled in these pages) the Brain, forms the crowning portion of the nervous apparatus, and by its high development in mankind, places a mental barrier between the human race and the lower animals; as complete as that furnished by the hand in point of physical structure. It supplies the material instrument of the Will; and is the seat of all the intellectual operations, as

Thought, Memory, Imagination, Judgment, and the like. It neither receives nor transmits nerves; but is intimately united with the Sensorium, upon which it is dependent for all the stimuli that call it into activity, and also for its power to react upon the body. This much being first stated, a more minute description of its powers and offices must be deferred until the relations of the several centres to each other, and to the nerves connected with them, have been clearly laid down and explained.

The Sensorium, coming next in order of position, and placed between the base of the brain and the summit of the cord, has received its name from being the seat or instrument of all sensation. Of this compound body, one portion ministers to the sense of vision, another to the sense of hearing, and another to the sense of smell. To one of its divisions we are indebted for that general sensibility of the surface from which we derive all ideas of contact or temperature, and all impressions of physical pain. Through another we become acquainted with, or feel, the operations of the mind, which, as will be seen hereafter, do not necessarily involve a consciousness of themselves; but may, on the contrary, be perfectly performed without our knowledge. The sensorium receives, also, the afferent nerves from the organs of special sense; and is connected, by its tubular tissue, both with the brain and the cord; but, as no efferent nerves proceed from it, it has no direct outward communication with other parts of the frame.

The Cerebellum, or Little Brain, is chiefly employed in the regulation and direction of motor impulses, so

as to ensure the accomplishment of any desired action, by calling the proper muscles, and no others, into play. Its function may be regarded as a matter of detail rather than of principle; and will admit of being explained more clearly in the sequel.

The Spinal Cord is concerned solely in the production of motion; and into it are inserted all the afferent and efferent nerves of the body, excepting only those of the former class which proceed from the organs of special sense, and which have been already said to terminate in the sensorium. Like the sensorium, the cord must be regarded as consisting of segments or portions; each of which possesses a power of independent action, receiving and giving forth afferent and efferent nerves; but which are continuous in point of structure, and capable of free communication with each other.

The typical mode of activity of the complicated apparatus whose chief divisions have been thus enumerated, may be described as follows: the independent action of one segment of the spinal cord being selected as an illustration. Some external object or circumstance alters the condition of the grey matter at the extremity of an afferent nerve, producing an effect which is technically called an "impression." This impression is conveyed by the nerve to the grey matter of the centre in which it terminates, where a second effect, technically called a "change," is produced. The change in the centre is then conveyed back, or reflected, along the afferent nerve proper to the part in which it took place, to some muscle or muscles, which are called into action by its influence.

In the above description there are two things especially worthy of notice: first, that it does not involve any idea either of sensation, intelligence, or will, on the part of the animal; and, secondly, that we are compelled to recognize the manifestation of a power or force, which expends itself in producing movement. It is necessary to add, that the whole process may be accomplished with extreme, or almost inconceivable, velocity.1

With regard to the absence of sensation, intelligence, and will, it may at once be stated that the ideal of nervous activity, as taken from an extended survey of animated nature, is, that it should be automatic or instinctive in its character; that is, that each link in the chain of operations should follow as the inevitable result of the one preceding it; and that all of them should take place in consequence of "laws written upon the nervous pulp."

Hence, if we look only to those actions necessary to the preservation of animal life, the spinal cord and its nerves must be regarded as the essentials of the nervous system; because all the simple movements addressed to this purpose, are performed through its agency. But the function of the cord is to excite motion only, and, therefore, the actions in which it alone is concerned, cannot be controlled by the will; and are not attended by sensation. The act of swallowing may be taken as a perfect example of them. It is accomplished by the contraction of a muscular tube, which extends from the mouth to the stomach,

1 See note, p. 55.