Cerebral Hemispheres

Cerebral HemispheresExternal Configuration. - When viewed together the cerebral hemispheres have somewhat the appearance of the upper half of a sphere, but the antero-posterior diameter is greater than the transverse (Fig. 57). They present a large convex outer surface, which is adapted to the shape of the vault and sides of the skull, and a very irregularly flattened inferior surface. The greater part of the inferior surface is moulded on the irregularities of the bones forming the base of the skull, but its back part rests on a shelf of dura mater which intervenes between it and the cerebellum.

The hemispheres are separated from one another in the middle line by a deep cleft known as the great longitudinal fissure. The separation is complete in front and behind but only extends for about half the depth in the middle, the two hemispheres being joined to one another below this level by a large band of transverse fibres - the corpus callosum. Each hemisphere presents, therefore, three surfaces - outer, mesial, and inferior.

The extreme anterior end of the hemisphere is the frontal pole, the posterior end is the occipital pole, while on the side the temporal pole projects forwards as the anterior extremity of a subdivision of the hemisphere termed the temporal lobe.

The surfaces of the hemispheres are marked by series of convolutions or gyri, separated by fissures or sulci of variable depths. The surface area is thus greatly ine.reasedwithout a corresponding increase of the bulk of the hemispheres.

The arrangement of the convolutions and fissures, which may be referred to as the brain pattern, bears no direct relationship to the location of the different functions of the cerebral hemispheres; it varies very greatly in different animals, and its intricacy is no measure of the degree of nervous organisation of the animal.

The brain pattern of man, however, is very intricate, and, further, it varies, within limits, in different individuals; there is a certain fairly definite ground plan common to all, but the individual embellishments are as varied as are the features in individual faces.

The more constant fissures are utilised in the description of the functional areas on the hemispheres, and may therefore be briefly indicated.

Outer Surface. - The surface is divided, for convenience of reference, into lobes; these have no physiological significance. The fissures which demarcate them are termed interlobar; they are, the fissure of Sylvius, the fissure of Rolando, the external parieto-occipital fissure, and the circular sulcus of Reil (Fig. 58).

FISSURE OF SYLVIUS. - The fissure of Sylvius is a deep cleft which lies chiefly on the outer surface of the hemisphere. It begins on the under surface near the optic chiasma by a short stem which turns round the margin and divides into three limbs. Two are short - one (anterior) horizontal, the other (ascending) vertical; the third, about three inches in length, extends almost horizontally backwards and terminates in an upturned end. When the lips of the various parts of the Sylvian fissure are forced apart a sunken portion of the surface, termed the island of Reil, is exposed.

FISSURE OF ROLANDO. - The fissure of Rolando, or central fissure, notches the upper margin of the hemisphere about half an inch behind its middle. From this point it runs downwards and forwards on the outer surface, and terminates just above the posterior limb and behind the ascending limb of the Sylvian fissure.

EXTERNAL PARIETO-OCCIPITAL FISSURE. - The main portion of the parieto­occipital fissure is on the mesial surface of the hemisphere, but a small portion about half an inch in length, the external parieto-occipital fissure, extends on to the outer surface two inches in front of the occipital pole.

CIRCULAR SuLCUS OF REIL. - This surrounds the island of Reil and marks it off from the rest of the outer surface of the hemisphere.

For mapping out the lobes on the outer surface of the hemisphere a line is taken from the external parieto-occipital fissure to a small notch on the inferior riiargin ; the posterior limb of the fissure of Sylvius is prolonged back to meet, this line ; the fissure of Rolando is continued down to meet the fissure of Sylvius. Five lobes are thus demarcated : (1) the frontal lobe in front of the Rolandic fissure ; (2) the parietal lobe between the Rolandic fissure and the external parieto-occipital fissure ; (3) the occipital lobe behind the external parieto-occipital fissure; (4) the temporal lobe below the posterior limb of the Sylvian fissure; (5) the island of Reil.

The frontal, parietal, occipital, and temporal lobes are all described as con­tinuous on to the mesial surface of the hemisphere, but no practical use is made of the description, so it may be neglected.

FRONTAL LOBE - There are three fissures dividing this area into four convolutions. One, the precentral, lies parallel to the fissure of Rolando ; the convolution between these two fissures is termed the precentral or ascend­ing frontal. The other two fissures, known respectively as the superior and inferior frontal run at right angles to the precentral. They intervene between the superior, middle, and inferior frontal convolutions. The posterior part of the inferior frontal convolution of the left side has a special interest; it is known as Broca's convolution-=after Broca, who described it as the centre for articulate speech.

PARIETAL LOBE. - Parallel to and behind the Rolandic fissure is the post­central fissure demarcating the postcentral or ascending parietal convolution. From near the middle of this fissure a second fissure extends backwards at right angles, the combined fissures forming a |-- shaped fissure termed the intraparietal. The horizontal limb separates the superior from the inferior parietal convolution. In the latter are the upturned ends of the Sylvian fissure and the first temporal fissure; the areas around these ends are known respectively as the supra-marginal gyrus and the angular gyrus.

OCCIPITAL LOBE. - Two fissures cross the surface of this lobe. The upper, placed a short distance behind the external parieto-occipital and usually continuous with the horizontal limb of the intraparietal fissure, is termed the transverse occipital fissure. The other - the lateral occipital fissure - runs horizontally across the lobe and divides it into superior and inferior occipital convolutions.

TEMPORAL LOBE. - This is traversed by two fissures parallel to one another and to the posterior limb of the Sylvian fissure; they separate off the superior, middle, and inferior temporal convolutions.

ISLAND OF REIL. - The island of Reil is a submerged somewhat triangular portion of brain surface which is completely concealed from view by the overlapping of the adjacent lobes (Fig. 59). The covering portions are separated from one another by the limbs of the fissure of Sylvius into a series of opercula; they are - (1) the orbital operculum below the anterior limb of the Sylvian fissure; (2) the frontal, between the anterior and ascending limbs; (3) the fronto-parietal, between the ascending and posterior limbs; and (4) the temporal, below the posterior limb.

Mesial Surface. - The mesial surface of the hemisphere cannot be fully exposed until the hemispheres have been separated from one another, and from the rest of the brain. The most prominent landmark on it is the cut surface of the corpus callosum which is shaped like an arch with its convexity upwards; the anterior end of the arch is curved on itself to form the genu or knee which is prolonged into a thin pointed extremity, the rostrum; the posterior end is sharply folded on itself, and presents a thickened pad, the splenium (Fig. 26). The chief fissures on the mesial surface are the calloso-marginal, internal parieto-occipital, calcarine; collateral, and dentate (Fig. 60).

THE CALLOSO-MARGINAL FISSURE lies practically parallel to the anterior two-thirds of the corpus callosum. It begins below the rostrum, curves round in front of the genu, and extends backwards to end by turning up to the superior margin just behind the fissure of Rolando. Between it and the corpus callosum is the callosal convolution; above and in front of it is the marginal convolution.

THE INTERNAL PARIETO-OCCIPITAL FISSURE is continuous with the external parieto-occipital of the outer surface. It runs downwards and forwards to join the calcarine fissure. Between it and the upturned end of the calloso­marginal fissure is an area termed the quadrate lobe or precuneus.

THE CALCARINE FISSURE begins immediately above the occipital pole, and extends almost horizontally forwards to just behind the splenium. It is joined near its anterior third by the internal parieto-occipital fissure, and the two fissures mark off a triangular area known as the cuneus. Between the anterior portion of the calcarine fissure and the corpus callosum is a narrow convolution, the isthmus.

THE COLLATERAL FISSURE reaches from the under surface of the occipital lobe towards the temporal pole. The lingual convolution separates its posterior part from the calcarine fissure.

THE DENTATE FISSURE extends forwards from the splenium of the corpus callosum to a hooked surface prominence called the uncus. Between the dentate fissure and the collateral fissure is the hippocampal convolution which is continued forwards into the uncus and is in continuity behind with the isthmus. The callosal convolution, the isthmus, and the hippocampal convolution form one continuous arch, which is designated the limbic lobe.

Inferior Surface. - The front part of the inferior surface rests over the orbit (the bony capsule containing the eyeball) and is hence termed the orbital surface. It presents, close to the middle line, an antero-posterior fissure - olfactory sulcus - in which lies the olfactory tract. The rest of the orbital surface is divided by an H-shaped fissure. Further back, on the,inferior surface, a long interrupted fissure - the occipito-temporal - extends between the occipital and temporal poles.