Moreover the component fibres of the dura mater in certain parts of its course separate into layers, which are so disposed as to leave spaces between them, for the most part of a triangular form. These triangular spaces, which are commonly termed sinuses, are lined by a smooth membrane perfectly analogous to that which lines the veins in the other parts of the body, and these sinuses perform the office of veins, returning the blood from all the parts of the brain to the neck. Nothing analogous to this structure occurs in any other part of the venous system. In almost every other part of the body the pressure of surrounding parts is a most important aid to these vessels in enabling them to carry on the circulation of the blood; but in the brain the venous tubes are guarded from pressure, the dense dura mater being for this purpose stretched so tensely over them that the weight of the surrounding parts is completely taken off them.
The smooth surface of the brain which is exposed on the reflection of the duns miter, is formed by its second investing membrane which is named the Tunica Arachnoidea, from the extreme tenderness and delicacy of its tissue, which gives it a resemblance to n spider's web. This thin colourless and transparent membrane is spread uniformly over the surface of the brain, covering all the eminences termed convolutions (fig. 1. 2, 2), but not insinuating itself between any of the depressions between the convolutions. (Fig. iv. 7.) On account of its extreme tenuity and its close adhesion to the membrane beneath it, it cannot be easily separated from the latter ; but there are situations at the basis where the arachnoid membrane, as it passes between opposite parts of the brain, can be seen distinct from the subjacent tunic.
The third investing membrane, the Pia Mater, derives its name like the former from the tenderness and delicacy of its tissue ; but unlike the tunics arachnoidea, in which not a single blood-vessel has hitherto been described, the pin mater is exceedingly vascular. The blood-vessels with which every part of this delicate membrane is covered are the nutrient arteries of the brain ; before they pene trate the brain these vessels divide, subdivide, and ramify to an extreme degree of minuteness upon the external surface of this membrane, so that the blood does not enter the tender cerebral substance with too great force. When a portion of the pia mater is gently raised from the brain, these blood-vessels appear as exceedingly fine delicate threads, which on account of the elasticity with which they are endowed are capable of elongation as they are drawn out of the cerebral substance. As the pia mater contains and supports the nutrient vessels of the brain, this membrane is not only spread a9 a general envelope over its entire surface, but it penetrates between all its convolutions, and lines every cavity which is formed in it.
It has been stated that the large portion of the cerebral mass, termed the cerebrum, occupies the whole of the upper part of the cavity of the cranium. The cerebrum is divided into two equal lateral halves termed hemispheres (fig. i. 2, 2), which have an ovoid figure somewhat resembling an egg cut longitudinally into two equal parts. The hemispheres are separated from each other by the membrane already described, the falx cerebri (fig. i. 3); and their inner sides in apposition with the falx are flattened, while their upper and outer surfaces are convex, being accurately adapted to the concavity formed by the inner surface of the bones of the cranium.
Each hemisphere is subdivided into an anterior, a middle, and a posterior lobe, but it is only on the under surface of the brain that these lobes are accurately defined. (Fig. ii. 1, 2, 3.) The anterior and
middle lobes are separated from each other by a deep fissure named the fissura Sylvia (fig. ii. 4), which extends obliquely backwards from the basis to n considerable depth between the convolutions; but the middle distinguished from the posterior lobe, not by a fissure but by a superficial excavation on the under surface of the posterior lobe. (Fig. ii. 5.) The anterior lobes rest upon the orbital plates of the frontal bone; the middle lobes are lodged in the temporal foes= formed by the sphenoid and temporal bones, while the posterior lobes are supported upon the tentorium.
The whole of the external convex surface of the hemispheres is divided into numerous eminences termed convolutions, which run in 1, Cut edge of the bones of the cranium ; 2, superior convex surface of the two hemispheres of the cerebrum, with their convolutions ; 3, separation between the two hemispheres of the cerebrum, occupied by the falx marl, different directions, and are of different sizes and lengths in different parts of the hemisphere. (Fig. i. 2, 2.) The depressions or fissures between the convolutions, termed clefts, or sulci, generally penetrate, the consistence of the brain to the depth of about an inch or en inch and a half. (Fig. iv. 7.) The greater number of these pursue a • zigzag course, but some run longitudinally, others obliquely ; some communicate with each other, while others termionte separately in the substance of the brain. (Fig. iv. 7.) The nervous matter constituting the cerebrum is composed of two distinct substances, which differ from each other materially both in their colour and conaistence. (Fig. iv. 7.) The outer substance is sometimes termed ciueritioue, from its being of a grayish-brown colour ; at other times cortical, from its surrounding the inner part of the brain, as the bark the inner par't's of the tree ; by some it is also called glandular, and by others secretory, from the supposition that its nature is that of a gland, and that it secretes a peculiar fluid. It is of a softer consistence than the inner part, and leaves by desiccation a smaller quantity of solid residuum. It is composed almost entirely of blood-vessels connected and sustained by exceed ingly fine cellular membrane. its structure is uniform throughout, presenting no appearance whatever of a fibrous texture. It gives to the entire surface of the cerebrum an external covering, generally about the tenth of an inch in thickness. (Fig. iv. 7.) The bluer substance, termed white or medullary (fig. iv. 7), is firmer in consistence and larger in quantity than the gray matter ; and when an incision is made into it, its surface is spotted with red points, the cut orifices of its vessels, which vary in number and else according as they may be more or less distended with blood. It is now universally agreed that this part of the brain is composed of fibres. When examined in its recent and most perfect state, especially after it has been artificially hardened and condensed by the action of heat or certain chemical substances, if it be carefully scraped with a blunt Instrument these fibree become perfectly distinct and are of consider able magnitude, with furrows between them, which for the most part are placed in such a direction AS to converge towards the base of the brain. (Fig. iv. 6, 5,4.) The fibres do not merely unite, forming what are called commisaurea, but they actually cross each other and pass into the opposite rides of the body. This decussatiou of the medullary fibres has been demonstrated in the most satisfactory manner by Drs. Gall and Spurzheirn.