Figure 2. represents the base of the brain taken out of the skull. This figure has been copied from a different brain from that represented in figure 1. It is longer and narrower. The division into hemispheres does not descend to the base. Anatomists, for the sake of giving precision to descriptions, divide the brain into three lobes, called the anterior, middle, and posterior. The parts before AA are called the frontal or anterior lobe; the parts behind CC, the posterior lobe; and the parts between them, the middle lobe. Anatomists draw lines from AA and CC directly up the sides, and over the upper surface of the brain, till they meet at the top, and include in the different lobes the whole parts so mapped out; but the lines are imaginary, and like those of latitude and longitude on a globe, are introduced merely to indicate the localities of the parts. The convolutions before AA lie chiefly on the bones which form the roofs of the sockets of the eye-balls. The convolutions between A and C lie chiefly above the ear. DD is the cerebellum.

E is the medulla oblongata, which during life descends almost perpendicularly from the brain, and joins with the top of the spinal mar

row.

The BRAIN is a mass of soft matter, not homogeneous, but presenting different appearances. Part of it is white in colour, fibrous or striated in texture, arranged in lines distantly resembling the outer surface of a cockle-shell. This is generally named medullary substance, and abounds most in the interior. The other matter is of a grey colour, and has no fibrous appearance. It is called cineritious, from the similarity of its hue to that of ashes, and sometimes cortical, from its supposed resemblance to bark. It forms the outer part of the brain. The cineritious substance does not blend gradually with the white medullary matter, but, on the contrary, the line of distinction is abrupt. The cineritious seems to have a greater proportion of blood circulating in it than the medullary. There is no fat or adipose substance within the skull, although it pervades every other part of the body.

The brain consists of two hemispheres, separated by a strong membrane, called the Falciform process of the dura mater. Each hemisphere is divided into three lobes, the anterior, middle, and posterior. The cerebellum is distinct from, but connected with, the brain. Mr BELL observes*, that "whatever we observe on one side has a corresponding part on the other; and an exact resemblance and symmetry is preserved in all the lateral divisions of the brain. And so, if we take the proof of anatomy, we must admit, that, as the nerves are double, and the organs of sense double, so is the brain double; and every sensation

• Anatomy of the Brain, ii. 381.

There is a gene

+ This statement of Mr BELL is not rigidly correct. ral correspondence between the parts on the opposite sides of the brain, but not "an exact symmetry," in the strict sense of these words. The approximation to symmetry is about as great as between the blood vessels in the right and left arms.

conveyed to the brain is conveyed to the two lateral parts, and the operations performed must be done in both lateral portions at the same moment."

The two hemispheres, and of course the organs of each side, are brought into communication and co-operation by fibres running transversely; these are called the corpus callosum, and the Anterior and Posterior commissures. The cerebellum and brain are only slightly and indirectly

connected.

The greater part of the brain is destitute of sensibility : It may be pierced or cut without the patient being aware, from any feeling of pain, that it is suffering injury. Mr BELL mentions, that he had his finger deep in the anterior lobes of the brain, when the patient, being at the same time acutely sensible, and capable of expressing himself, complained only of the integument." So far from thinking the parts of the brain which are insensible, to be parts inferior in function (as every part has its use), Mr BELL states, that, even from this, he should be led to imagine that they had a higher office, namely, that they were more allied to intellectual operations. The wide difference of function betwixt a part destined to receive impressions, and a part which is the seat of thought, is in accordance with the presence of sensibility in some parts of the brain, and its absence in others.

The external substance of the brain is arranged in convolutions or folds. The convolutions appear intended for the purpose of increasing the superficial extent of the brain, with the least possible enlargement of its absolute size; an arrangement analogous to that employed in the eye of the eagle and falcon, in which the retina does not form a continuous line, as in man and quadrupeds, but is presented in folds to the rays of light, whereby the intensity of vision is increased in proportion to the extent of nervous surface exposed to their influence. The rolling up of the substance of the brain in folds in a similar manner, strong

ly indicates that extent of surface is highly important to its functions. In certain low classes of the inferior animals, there are no convolutions. As we ascend in the scale of beings, they increase," and in man above all other animals, are the convolutions numerous, and the sulci (or furrows) deep, and, consequently, the cineritious mass great, and its extension of surface far beyond that of all other creatures.”—Bell's Anat. ii. 386.

sures.

The cineritious matter is extended over all the upper, lateral, and over part of the inferior surfaces of the brain: the white or medullary matter lies within it, and in some places in intimate combination with it. Medullary fibres run from the convolutions of the brain upon one side to the convolutions on the other. These are called commis"Unless," says Mr BELL," the cineritious masses were important organs, why should there be commissures or nerves forming a distinct system, arising and terminating in nothing? But if we take them as commissures, i. e. bonds of union betwixt the corresponding sides of the great organ of the mind, we at once perceive how careful nature is to unite the two lateral organs together, and out of two organs to make one more perFECT."-P. 386.

Each side of the brain, and also the cerebellum, are supplied with separate arteries conveying the blood to them; but the sinuses or canals, by means of which the blood is returned to the heart, are common to them all.

The CEREBELLUM is composed of the same nervous matter with the brain, and presents both cineritious and medullary matter; but, in form and internal arrangement, it is quite unlike the brain. The cerebellum is separated from the brain by a strong membrane, called the tentorium: in animals which leap, as the cat and tiger, the separation is produced by a thin plate of bone. Its fibres, however, originate in that part of the medulla oblongata called the corpora restiformia, from which also the organs of several feelings or propensities arise; so that the brain and cere

bellum, although separated by the tentorium, are both connected with the medulla oblongata, and through it with each other.

The MEDULLA OBLONGATA is sometimes spoken of as one of the three great divisions of the brain. It is, in fact, the part from which the fibrous matter of the brain and cerebellum proceeds, and it forms, as it were, the capital of the column of the spinal marrow.

OF THE INTEGUMENTS OF THE BRAIN.

THE brain is formed before the bones which invest it. The ossification of the bones of the skull is a gradual process. The brain already formed is invested with strong membranes, and betwixt the coats of the outer membrane the points of ossification commence, which process is not completed until the ninth year.

During life, the brain is embraced in its whole peripheral extent by a very thin transparent and delicate membrane called the pia mater, which sinks down into its furrows, and serves to convey the bloodvessels to its different parts. Immediately above the pia mater, is an extremely thin membrane, named the tunica arachnoidea, on account of its extreme tenuity, resembling a spider's web. It covers the surface of the brain uniformly without passing into its folds or cavities. It secretes matter, to lubricate the surfaces of the pia and dura mater. The dura mater is also a thin but strong opaque membrane lining and strongly adhering to the inner surface of the skull, and which embraces the outer surface of the brain above the membrane last mentioned. When in health it does not pos sess sensibility, and has been pricked without causing pain. All these membranes are pliant in the highest degree, and accommodate themselves precisely to the figure of

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