FUNCTIONS OF THE CEREBRUM. 207 



animals, it is desirable to obtain a general notion of its structure. Three 

 principal sets of fibres may be distinguished in the white or medullary sub- 

 stance, of which the great mass of it is composed. These are the Ascending 

 fibres, which proceed from the sensory tract, and diverge from the Thalami 

 optici to the periphery, the Descending fibres, which converge from the peri- 

 phery towards the Corpora Striata, and then pass downwards into the motor 

 tract ; and the Commissural fibres, which establish the connection between 

 the various parts of the periphery and of the substance of the brain. 

 It is on the very large proportion which the commissural fibres bear to 

 the rest, that the bulk of the brain of Man and of the higher animals chiefly 

 depends ; and it is easy to conceive, that this condition has an important rela- 

 tion with mental operations, whatever be our view of the functions of dif-* 

 ferent parts of the Brain. The different relative distribution of the gray 

 and white matter in the Cerebrum, from that which is elsewhere presented 

 to us in ganglionic masses, naturally suggests the inquiry, how far this corre- 

 sponds with what has been stated of their probable functions. The amount of 

 ordinary vascular action in the gray substance, as compared with that which 

 takes place in the white, is an important circumstance in favour of the view, 

 that it is the part in which all changes originate, and that the fibrous portion, 

 like the trunk of the nerve, serves only to conduct or transmit the influence 

 of those changes. This position derives additional support from the effects of 

 disease. It has been frequently remarked that, if we compare those cases of 

 cerebral disease in which there is delirium, with those in which it does not 

 occur, we shall find that it is most common in cases in which there is an 

 inflammatory affection of the surface, or of the membranes, extending from, 

 them into it ; whilst in deep-seated inflammation, the most important symptoms 

 are those which result from sympathetic affections of the muscular system. 

 It has been even proposed to establish a diagnosis between inflammation of the 

 membranes (especially of the arachnoid), and inflammation of the substance of 

 the brain, upon this general fact ; but it is to be remembered that (to use the 

 words of Lallemand) " it is impossible that the arachnoid should be inflamed, 

 without the surface of the brain in contact with it being also affected ; but its 

 tissue not being altered, there merely results from this vicinity exaltation in 

 its functions." All the cases, therefore, which have been referred to in sup- 

 port of this diagnosis, really tend to establish the proposition, that the superficial 

 portion of the Cerebrum is the part really affected. It is absurd to suppose 

 that inflammation of the membranes, without any abnormal condition of the 

 Brain itself, can seriously affect the mental operations. It has been further 

 remarked, that arachnitis of the convexity of the Brain is more frequently 

 attended with delirium and other symptoms of excitement than similar in- 

 flammation affecting the base, in which coma supervenes earlier, with little or 

 no previous disturbance of the intellect ; this, too, corresponds with the doc- 

 trine just referred to ; since the influence of any effusion aboirt the origins of 

 the nerves and the Medulla Oblongata, is well known to be prejudicial to their 

 functions as conductors, even entirely suspending them; whilst, from the infe- 

 rior vascularity of these parts, they are not so liable to manifest symptoms of 

 excitement, from the contiguity of an inflamed membrane. In fact, inflamma- 

 tion of the white substance of the Brain is itself attended rather with a state 

 of torpor, or of partial suspension of its usual operations, than with excitement ; 

 irregular convulsive actions are not unfrequently seen as a result of it ; but 

 these are often manifested, when the power of the will over the muscles, is 

 destroyed. It may not be difficult to account for this difference of symptoms, 

 by reflecting, that a large proportion of the medullary substance of the Brain 

 consists of a sort of extremely delicate areolar tissue, by which the fibres are 

 connected together, and through which the blood-vessels are distributed ; and 

 it is probably in this that the principal changes take place, of which the early 



