723c 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



them serves to establish a communication be- 

 tween the centre of intellectual action, and the 

 centres of volition and sensation. It is through 

 this connection that the intellect and the will 

 are capable of mutually affecting each other, 

 the intellect ptompting or exciting the will ; 

 and the will, on the other hand, controlling or 

 applying the powers of the intellect. The 

 faculty of attention, and, therefore, in a certain 

 degree, that of memory, are dependent on the 

 influence of the centre of volition upon the 

 centre of intellectual action. Every one is sen- 

 sible of a power which he possesses of fixing 

 his attention on any given subject, as distinct 

 as that by which he can contract any particular 

 muscle. The association of the intellectual 

 centre with that of sensation is necessary to en- 

 sure the full perception of sensitive impressions. 

 The experience of each individual can supply 

 him with numberless instances in which, while 

 the mind was employed upon some other ob- 

 ject of interest, an impression was made upon 

 some one of the organs of sense, and indistinctly 

 J'elt, but not fully perceived. When the mind 

 has become disengaged, the fact that an impres- 

 sion had been made is recalled, without any 

 ability to recollect its precise nature. And in 

 many lunatics the centre of intellectual action 

 is so impaired as to destroy or greatly reduce 

 the power of perception, whilst there is abun- 

 dant evidence to shew that the affections of the 

 organs of sense make a sufficient impression on 

 the centre of sensation, although in such cases 

 this centre may likewise participate in the general 

 hebetude. 



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 more 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- 



ture, 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 the 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 appeal- 

 to revolve is influenced by the position of the 

 body 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 the 

 supply of blood to the head. ' If the horizontal 

 position be immediately adopted, or the body 

 be laid with the head inclined downwards, the 

 faint may be prevented. The sense of giddi- 

 ness which is experienced upon rising from the 

 horizontal position after illness, is doubtless of 

 the same kind. Anaemic patients experience 

 this feeling of giddiness even in the horizontal 

 position ; and both it and the headache and 

 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 

 various parts of the body. If the attention be 



* Miiller's Physiology, by Raly, vol. i. p. 848. 



