The cerebral parts arc separated from one another at certain places, and the intervals form cavities which are termed ventricles. Of these ventricles there are commonly enumerated four, all of which are in communication with each other. By far the largest of these are the two great cavities called the lateral ventricles, which arc situated in the interior of the hemispheres of the cerebrum. Commencing in the fors part of the anterior lobes, these cavities proceed backwards in a direction parallel to each other through the middle into the posterior lobes. Their figure is winding and exceedingly irregular, and they are separated from each other by a tender mass of medullary matter termed the septum luciduin. (Fig. iii. 5.) They are lined throughout by a fine transparent membrane, which secretes a fluid that keeps them moist, gives them a bright polished appearance, and prevents them from uniting. This membrane is the pia mater, which is con , tinned from the exterior surface of the brain into these interior cavities ; and some anatomists describe the araelinoid membrane as accompany ! the pia mater in all its course through the ventricles.
The middle or third ventricle is a vertical fissure between the two large convex eminences called the thalami optici (fig. iii. 4), situated in the middle and back part of the lateral ventricles. The fourth ventricle, called also ventricle of the cerebellum, is a cavity of con , siderable extent situated between the cerebrum, tuber annulare, and the medulla oblongata.
For a detailed account of the course of the fibres the reader is referred to the work of Drs. Gall and Spurzheim, entitled 'Recherches sus le Systeme Nerveux en gdn4ral, et sus eelui du Cerveau en par ticulier, in which the direction of the cerebral fibres in not only minutely and exactly described, but illustrated by excellent drawings as large as the object... The course of the fibres that compose the I pyramids (fig. ii. 8 and fig. iv. 1) is as follows :—Iramediately before their entrance into the tuber annulare the pyramids aro a little con tracted. (Fig. ii. 8.) As soon as they enter this mass the pyramids aro divided into innumerable bundles of fibres (fig. iv. 2), which are covered by a thick layer of transverse fibres (fig. iv. 2) that come from the cerebellum. (Fig.iv. 8.) These fibres of the pyramids, thus increased in number, ascend, and receive at every point of their course fresh acces sions, until at their exit (from the tuber) forward and outward they form at least two-thirds of the cruet cerebri, as is seen at fig. iv. 3. Followed in their course forwards from fig. iv. 3, they are manifestly increased at every point by the accession of infinite numbers of fibres. (Fig. iv. 4.) At the point (fig. iv. 5) the fibres, now exceedingly numerous, manifestly assume a diverging course, proceeding in every direction forwards, upwards, laterally, and backwards. (Fig. iv. 5, 6, 7.) At length the radiating fibres, crossing and interlacing each other in all directions, form an expansion or tissue, and, being folded in various ways and covered with gray matter, constitute the convolu tions. (Fig. iv. 5, 6, 7, 7.) Thus the pyramids, progressively increased
and developed, form a large portion of the anterior and middle lobes of the cerebrum. If the corpora olivaria (fig. ii. 9) were traced in like manner, they would be found to form the posterior lobes of the cere brum ; and the origin and source of the fibres constituting the main bulk of the cerebellum can be demonstrated with the same clearness and exactness.
From the preceding account of the structure of the brain, which shows it to be an exceedingly complex organ, it might have been inferred from analogy that it would receive a large supply of blood ; but the quantity actually sent to it is far greater than any analogy could have led us to suppose. Haller made a calculation, from which he concluded that one-fifth of all the blood sent out of the left ven tricle of the heart is carried to the head, yet the weight of the brain in the human subject is not more than one-fortieth of that of the whole body. Even if this estimate, which is generally thought too large, be reduced to one-tenth, according to the idea of Moore, it will still leave a very great over-proportion. There is no part of the structure of the brain mere curious than the various contrivances connected with the circulation through the bead, which have for their object the prevention of this prodigious quantity of blood from pro ducing any injurious effects upon the tender cerebral substance, whether by its pressure or by its unequal distribution, in consequence of its stagnating in the vessels, or of its being too violently propelled against them. Many conjectures have been formed respecting the object of furnishing this organ with such an extraordinary quantity of blood, but nothing is really known of the use to which it is applied, though it may be admitted, to give a degree of plausibility to the opinion, that the brain has some analogy to a secreting organ. Without doubt one use both of the ventricles and the convolutions is to afford a more extended surface by which the blood-vessels may enter the cerebral substance at a greater number of points, and consequently in small quantity at any one point, while at the same time they are more firmly supported in their passage by the greater quantity of investing mem brane with which they are supplied. [Nzavous SYSTEM.] The brain of the vertebrate animals differs considerably from that of man, and more in proportion to their low position in the scale of development. The most obvious distinction between the brain of man and that of the other mammalia is its diminished size in most of the latter. The moment the skull-cap is raised, the difference between the full rounded appearance of the former and the com• pressed flattened shape of the latter cannot fail to be observed. The convexity of the middle lobes is strangely lessened, and the posterior lobe is in a manner lost in quadrupeds. If the brain is now removed from the cranial cavity, the difference iu bulk between that of mar and the inferior animals is strikingly displayed. The brain of the os scarcely weighs a pound : the average weight of the brain of the human being is more than 2i pounds.