Perfect power of speech, that is, of expressing our thoughts in suitable language, depends upon the due relation between the centre of volition and that of intellectual action. The latter centre may have full power to frame the thought ; but, unless it can prompt the will to a certain mode of sustained action, the organs of speech cannot be brought into play. A loss of the power of speech is frequently a precursor of inore extensive derangement of sensation and motion. In some cases the intellect seems clear, but the patient is utterly unable to ex press his thoughts ; and in others there is more or less of mental confusion. The want of con sent between the centre of intellectual action and of volition is equally apparent in cases of this description, from the inability of the patients to commit their thoughts to writing.
The hemispheres of the brain, as has been already stated, are insensible to pain from me chanical division or irritation ; in wounds of the cranium in the human subject, pieces of the brain which had protruded have been removed without the knowledge of the patient. Never theless, pain is felt in certain lesions of the brain, even when seated in the substance of the hemispheres, or in the optic thalami or corpora striata. This results from the morbid irritation extending to other parts with which nerves are connected, as the medulla oblongata; or in which nerves are distributed, as the membranes. The nearer a cerebral lesion is to the membranes or to the medulla oblongata, the more likely is it to excite pain. Headaches, of whatever na lure, must be referred to irritation, either at their centres or at their periphery, of those nerves which are developed in the dura mater or in the scalp. The branches of the fifth pair, of the occipital nerve, and the auricular branch of the cervical plexus, are those most frequently affected.
Certain sensations are referred to the head which may occur from a morbid state, or may be produced by changes of position in the body. Such are vertigo, a sense of fullness, or of a weight in the head, a feeling of a tight cord round the head. These are, no doubt, truly subjective, arising .from altered states in the distribution or in the quality of the blood sent to the brain. A sensation of a. rushing of blood to the head is often consequent upon excessive hemorrhage, or accompanies a state of extreme debility from any cause. This is, doubtless, owing in great part to tbe feeble tone of the arteries, resisting imperfectly the flow of blood to the head, and allowing it to impress the nervous matter too much. It is well known, that, by turning round quickly on one's own axis, the sense of vertigo may be produced ; a confused feeling in the head, and an inability to maintain the balance of the body, accompa nied by an appearance as if external objects were revolving. If the eyes be kept shut, the
uneasy feeling of the head will take place, but no true vertigo. To obtain this feeling per fectly, the eyes must be open, and objects pre sented to them. And Purkinje has shewn that the direction in which external objects appear to revolve is influenced by the position of the boxly and of the head while turning round, and by the position of it afterwards, when the expe rimenter has ceased to move round. If the experimenter have kept his head in the vertical position while moving round, and afterwards when standing still, the objects appear to re volve in the horizontal direction. If the head be held with the occiput upwards while turning round, and then erect when standing still, the objects seem to rotate in a vertical plane, like a wheel placed vertically revolving round its axis.* It is highly probable that these sensa tions, as well as those which arise spontaneously, are due to some irregular distribution of blood to various parts of the brain. A sense of gid diness frequently precedes fainting, and is at tributable to the temporary deficiency in thiii supply of blood to the head. If the horizone position be immediately adopted, or the bo be laid with the head inclined downwards, t faint may be prevented. The sense of gid ness which is experienced upon rising from t horizontal position after illness, is doubtless the same kind. Anwmie patients experie this feeling of giddiness even in the horizon position ; and both it and the headache a delirium, which accompany this state of blood lessness, may be relieved by placing the patient on an inclined plane with the head downwards.
The mind possesses a remarkable power of exciting and of exalting painful sensations in rarious parts of the body. If the attention be directed very strongly, and for some time, to any part, it may become the seat of pain, for which the most effective remedy is to engage the thoughts as much as possible on some other object. In many instances, where pain has been excited by a physical cause, there can be no doubt it has been continued long after the cessation of its exciting cause, by the attention of the patient having been directed to it. It is probable, that in such cases the perceiving parts of the brain (so to speak) become habituated to a certain condition of the centre of sensation, produced by the original exciting cause of the pain. And, on the other hand, pain, at first excited by the mind, may be rendered perma nent by habit; a certain physical alteration in some part of the centre of sensation being in duced by the frequent repetition of the mental act in reference to a particular part of the body.