Near the origin of each pair of nerves we are informed that a brown substance is observable; and in the cerebellum, confessedly the most important part of the contents of the skull, it penetrates so deeply as to form by much the larger portion of it. Nature, also, seems to have supplied it with its blood, by means so refined as to prevent almost the possibility of its being wholly destitute; for not only does it receive arteries from the external surface, but other vessels pass through the base of the skull and penetrate the medullary substance to prevent any deficiency from accident. It is these arteries only, in our opinion, which we see in the medulla. It has been doubted whether the whole of this substance is vascular: the minutest injections do not penetrate every part; nor, perhaps, were the whole vascular, could this be expected.

The medullary substance is a pulpy mass; though probably, could our sight be sufficiently assisted, we should find it fibrous, since this structure appears where the nerves are sent off, when they assume their coat from the pia mater. Various communications are observable in the medulla, from the front to the hinder part, and from side to side. In a negro the medulla is yellowish, and sometimes a blackish yellow, though in the European of a pure white. In the former the yellow hue disappears by the access of the air. The corpora striata, and the thalami nervorum opticorum, which are in an European of a flesh colour, approaching a cineritious, are in the negro of a dusky brown, like the bark of a tree.

The ventricles of the brain are four in number. Two are on each side of an oblong form, projecting in what are styled horns; and it seems as if the medulla, growing more dense in the corpus callosum and septum luci-dum, had proportionally contracted, leaving these cavities. In health they are probably distended by an halitus; but after death their parietes collapse, and a small portion of fluid is only discoverable. Two, as we nave said, are on each side; the third is more forward and in front, below the fornix, or that portion of the medulla which forms the base, and in part, the sides of the lateral ventricles. It is a sulcus or slit of very inconsiderable dimensions, and scarcely merits the name of a cavity, but from it passes a canal of soft cineritious matter, styled the infundibulum, to the pineal gland. The fourth ventricle is in a perpendicular direction, anterior to the cerebellum. It is probable that all the ventricles communicate. Dr. Monro has, with much anxiety as a discovery of importance, claimed the honour of having first described an opening between the lateral ventricles, and consequently a communication between the three former. Yet it seems that this communication which we have ourselves often traced, is not always found.

When we contemplate the ventricles, we seem to think that, like the chambers in the Egyptian pyramid, they are so disproportioned to the bulk that they can probably be of little advantage, and seem, as we have hinted, to be accidental. A more attentive examination, however, shows the fallacy of this conclusion; and they appear formed by design, and with a judgment so subtle or refined, as to elude our research. The pia mater from the basis of the skull, is conveyed as a lining round the parietes of each; and if the cineritious matter is an important organ, the pia mater, from which it originates, must be equally so. Again: the base of the brain is diversified by numerous projections, evidently designed to increase the surface, and afford a larger scope for the cineritious matter which we find in different parts, and chiefly about the origin of the nerves, which spring from the base. This cineritious substance is also found in many parts of the different ventricles; and in these too, we find plexuses of vessels so minutely convoluted, as probably to subserve some important purpose. Let us add, that any suppuration or any disorder on the base of the brain, soon produces the most alarming and fatal symptoms; while some spoonfulls of the medullary substance of the hemispheres may be discharged from a wound, without apparent injury to the intellectual faculties. We forgot to mention that the commissurae, the medullary cords, which unite the different parts of the brain, are by far more common at the base of the skull than in any other part; nor is a circumstance wholly to be overlooked, that the infundibulum, which"passes from the ventricles, terminates in the pineal gland: an organ probably of considerable importance, though its office is yet unknown. The in fundibulum is not indeed hollow through its whole length, yet it seems to be occasionally so, as some authors have described the aperture as continued to the gland. Its uniform direction, and the small space occupied by the loosely textured medulla, seem to show the probability of some communication.

Anatomists have described, with great precision, all the minuter projections, cavities scarcely discernible, medullary cords of connection (commissurae), and numerous sulci. To follow them would be useless; for this work is not designed to teach the minuter branches of anatomy, and the reader may think that we have already been unnecessarily minute. Yet we thought it right to give a particular outline of the anatomy of the brain; and we think we have not trespassed in descriptions which will not admit of some application.

The projecting and upper parts of the skull are filled with the two hemispheres of the cerebrum which rest on a membrane, a prolongation of the dura mater, styled tentorium. Below it is the cerebellum, whose connections we shall next describe. From each side of the brain, near the middle, medullary processes arise; and passing downwards and backwards, form what are styled the crura cerebri: these unite at an acute angle, and form what is styled the pons varolii or tuber annulare. From hence, what are styled the crura cerebelli 3 E 2 arise; or to the tuber they descend. A prolongation of the latter forms the medulla oblongata; which, when it escapes from the head, is styled the medulla spinalis. In the whole of this part of the brain, the striated or cineritious matter is freely united with the medullary; and at the union of the crura there are some protuberances, which, from their shape and colour, have obtained the names of corpora pyramidalia and olivaria.