It cannot be said that the brain is incom pressible. That only is incompressible, the particles of which will not admit of being more closely packed together under the influence of pressure. That the bmin is not a substance of this kind is proved by the fact that, while it is always undergoing a certain degree of pressure as essential to the integrity of its functions, a slight increase of that pressure is sufficient to produce such an amount of physical change in it as at once to interfere with its healthy action. Too much blood distributed among its elements, and too much serum effused upon its surface, are equally capable of producing such an effect.
Majendie's experiments, described in a for mer part of this article, show that the brain and spinal cord are surrounded by fluid, the pressure of which must antagonise that which is exerted through the bloodvessels. The re moval of this fluid disturbs the functions of these centres, apparently by allowing the vessels to become too full. The pressure exerted by the former may be called the fluid pressure frorn without the bmin; that by the blood, the pres sure from within. As long as these two are balanced, the brain enjoys a healthy state of function, supposing its texture to be normal. If either prevail, more or less of disturbance will ensue. Their relative quantities, if not in just proportion, will bear an inverse ratio to each other. If there be much blood, the sur rounding fluid will be totally, or in a great measure, deficient; if the brain be anmmic, the quantity of surrounding fluid will be large.
The existence of these two antagonizing forces may be taken as a proof that either of them may prevail ; and, therefore, from the presence of the cerebro-spinal fluid, we may infer that the actual quantity of circulating blood in the brain is liable to variation.
The cerebro-spinal fluid is a valuable regu lator of vascular fullness within the cranium, and a protector of the brain against too much pressure from within. So long as it exists in normal quantity it resists the entrance of more than a certain proportion of blood into t vessels. Under the influence of an unusu force of the heart an undue quantity of bl may be forced into the bmin, the effects which will be, first, the displacement of a or of the whole surrounding fluid; and, second] the compression of the brain.
When the brain receives too little blood, requisite degree of pressure will be maintain and the healthy cerebral action preserved, if tl surrounding fluid do not increase too rapid] But if the brain be deprived of its due propo tion of blood by some sudden depression of th heart's power, there is neither time nor source for the pouring out of a new fluid, and a state of syncope or of delirium will ensue. Such
seems to be the explanation of those cases of delirium which succeed to hzemorrhages, large bleedings, or the sudden lighting up of inflam mation in the pericardium or within the heart. In nearly all these cases, however, it is important to notice that the blood is more or less damaged in quality, deficient in some of its staminal principles, or charged with some morbid matter; and this vitiated state of the vital fluid has no doubt a considerable share in the production of the morbid phenomena.* Of the encephalic nerves.—There are no com mon characters possessed by these nerves, such as have been enumerated at a preceding page for the spinal nerves. They are, however, dis posed in pairs, and are quite symmetrical. NVith the exception of the olfactoly, optic, and third pair, they are all connected with the mesocephale or medulla oblong,ata.
The arrangement of these nerves originally proposed by Willis has been so long adopted in this country and on the continent that no advantage would arise from abandoning it, un less some other of an unexceptionable nature could be substituted for it. It has, therefore, been followed in this work, and the anatomy and physiology of the encephalic nerves have been described in articles prefixed by their numerical titles, in all cases except the olfactory and optic, and the eighth pair of nerves.-1 Twelve pairs of nerves are found in con nection with the base of the encephalon. Five pairs have been so classed by Willis as to form two in his arrangement, three pairs being al lotted to his eighth pair of nerves, and two to his seventh. Willis's arrangement, therefore, comprises nine pairs of nerves, which he enumerates, beginning at the anterior and pas sing to the posterior part of the base of the brain. These are the first pair or o.lfbctory nerves; the second pair or optic ; the third pair, motores oadorum ; the fourth pair,. pa thetici ; the fifth pair; the sixth pair, abdu centes oculi ; the seventh pair, including the portio mollis or auditory nerve, and the portio dura or facial nem; the eighth pair, including the glosso-pharyngeal, the pneumo-gastric, and the spinal accessory ; the ninth pair or hypo glossal. The first cervical nerve or the sub-occi pital was considered by Willis as an encephalic nerve and counted as the tenth pair.