ments of knowledge. The nervous system is, however, of a two-fold character. A part of it is employed in giving energy to those processes by which life is sustained. These have their appropriate centres either in the spinal marrow, or in the different ganglia. Thus the heart, arteries and lungs, have their appropriate system of nerves, with their proper centre. The digestive apparatus has its own nervous system. These are all parts of the general arrangement of brain, spinal marrow and nerves, but their functions are performed without volition or thought. Hence many of the lower animals, which have no need of thought, have no other nervous apparatus. The brain may be removed from some of the cold-blooded animals without, for a considerable period, producing death. In such cases sensation will produce motion, the arterial and digestive processes will continue for a while uninterrupted. Thus a common tortoise will live for several days after its head has been cut off. Thus we also perform these various functions without any intervention of the will. We digest our food, we breathe, our hearts pulsate, without any care of our own; and these functions are performed as well when we sleep as when we wake, nay, they proceed frequently for a while with entire regularity when consciousness has been suspended by injury of the brain.

As this part of the nervous system has nothing to do with thought and volition, we may dismiss it from our consideration, and proceed to consider that other portion of it which stands in so intimate connection with the thinking principle.

The organism which we use for this purpose consists of the brain and nerves. The part of the brain specially concerned in thought is the outer portion, called the cerebrum. From the brain proceed two classes of nerves, which have been appropriately termed afferent and efferent. The affe

rent nerves connect the various organs of sense with the brain, and thus convey to it impressions* from without. When an image from an external object is formed on the retina of the eye, a change is produced along the course of the optic nerve, which terminates in the brain, and the result is a change in the state of the mind which we call seeing. When the vibrations of the air fall upon the ear, another change is produced on the auditory nerve, which is continued until it reaches the brain, and the result is a change in the state of the mind which we call hearing. The other, or the efferent class of nerves, proceed from the brain outwardly, and terminate in the muscles. By these the volitions of the mind are conveyed to our material organs, and the will of the mind is accomplished in action. The process just now mentioned is here reversed. The volition. of the mind acts upon the brain, the change is communicated through the nerves to the muscles, and terminates in external action. Thus the brain is the physical centre to which all impressions producing knowledge tend, and from which all volitions tending to action proceed.

The proof of these truths is very simple. If the connection betwen the organ of sense and the brain be interrupted by cutting, tying or injuring the nerve, perception immediately ceases. If, in the same manner, the connection between the brain and the voluntary muscles be interrupted, the limbs do not obey the will. Sometimes, by disease, the nerves of feeling alone are paralyzed, and then, while the power of voluntary motion remains, the patient loses entirely the sense of touch, and will burn or scald himself without consciousness of injury. At other times, while the

* I of course use the word impression here, in a general sense, to convey the idea of a change produced, and not of literal impression or change of material form.

nerves of sensation are unaffected, the nerves of volition are paralyzed. In this case, feeling and the other senses are unimpaired, but the patient, loses the power of locomotion. Sometimes an effect of this kind is produced by the mere pressure upon a nerve. Sometimes, after sitting for a long time in one position, on attempting to rise we have found one of our feet "asleep." We had lost the power of moving it, and all sensation for the time had ceased. It seemed more like a foreign body than a part of ourselves. Longcontinued pressure on the nerve had interrupted the communication between the brain and the extremities of the nerves. As soon as this communication was reestablished, the limb resumed its ordinary functions.*

These remarks respecting the nerves apply with somewhat increased emphasis to the brain. If by injury to the skull the brain becomes compressed, all intelligent connection between us and the external world ceases. So long as the cause remains unremoved, the patient in such a case continues in a state of entire unconsciousness. The powers of volition and sensation are suspended. If the brain becomes inflamed, all mental action becomes intensely painful, the

* Sometimes this communication is so entirely suspended that a limb in this state, when touched by the other parts of the body, appears like a foreign substance. An instance of this kind, which many years since occurred to the author himself, may serve to illustrate this subject. He awoke one night after a sound sleep, and was not agreeably surprised to find a cold hand lying heavily on his breast. He was the sole occupant of the room, and he knew not how any one could have entered it. It was so dark that he could perceive nothing. He, however, kept hold of the hand, and, as it did not move, was somewhat relieved by tracing it up to his own shoulder. He had lain in an awkward position, so that he had pressed upon the nerve until all sensation had ceased. Probably many stories of apparitions and nightly visitations may be accounted for by supposing a similar cause.

perceptions are false or exaggerated, and the volitions assume the violence of frenzy.*

It may illustrate the relation which the nervous system sustains to the other parts of our material structure, to suppose the brain, nerves and organs of sense separated from the rest of the body, and to exist by themselves, without loss of life. In such a case, all our intellectual connections with the external world could be maintained. We could see, and hear, and feel, and taste, and smell, and remember, and imagine, and reason. All that we should lose would be the power of voluntary motion, and the conveniences which result from it. If, then, we should put this nervous system into connection with the bones, muscles, and those viscera which are necessary for their sustentation, we should have our present organization just as we actually find it. We see, then, that the other parts of our system are not necessary to our power of knowing, but mainly to our power of acting.

5. Of sensation and perception.

I have said that when our senses, under normal conditions, are brought into relation to the objects around us, the result is a state or act of the mind which we call knowing. A new idea or a new knowledge is given to the mind. This knowledge is of two kinds. In one case it is a simple

* Sometimes, however, astonishing lesions of the brain occur without either causing destruction of life or even any permanent injury. A case was a few years since published in the daily papers, under the authority of several eminent physicians, more remarkable than any with which I had been previously acquainted. A man was engaged in blasting rocks, and as he stood over his work, and was, I think, drawing the primingwire, the charge exploded, and drove through his head an iron rod of some two or three feet in length. The rod came out through the top of his head, and was found covered with blood and brain. He nevertheless walked home without assistance, and under ordinary medical care recovered in a few weeks.

near to me.

knowledge, connected with no external thing. Thus, suppose that I had never yet received any impression from the external world. In profound darkness a rose is brought I am at once conscious of a new state of mind. I have a knowledge, something which I can reflect upon, which we call smell. This knowledge, however, exists solely in my mind. I refer it to nothing, for I know nothing to which I can refer it. This simplest form of knowledge is called sensation.

But there is another form of knowledge given us through the medium of our senses. In some cases we not only obtain a new idea, or a knowledge of a quality, but we know, also, that this quality is predicated of some object existing without us. We know that there is a not me, and that this is one of its attributes. Suppose, as in the other case, I am endowed with the sense of sight, and in daylight the rose is placed before me. I know that there is an external object numerically distinct from myself, and that it is endowed with a particular form and color. This act is called perception.

These two forms of knowledge are united in the sense of touch, and may be clearly distinguished by a little reflection. The illustration of Dr. Reid is as follows: "If a man runs his head with violence against a pillar, the attention of the mind is turned entirely to the painful feeling, and, to speak in common language, he feels nothing in the stone, but he feels a violent pain in his head.” "When he leans his head gently against the pillar, he will tell you he feels nothing in his head, but feels hardness in the stone."Reid's Inquiry, chap. 5, sec. 2. So I prick a person with the point of a needle; a new knowledge is created in his mind, which he denominates pain. I draw the needle lightly over his finger, and I ask him what it is; he replies, the point of a needle. So, if I place my fingers